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Starting & Starter Problems
PLUS:  starter fitment problems

Bosch and Valeo Starter motors.  Overhaul.
 Converting from a Bosch to a Valeo (and vice-versa).
Starter relays, starter solenoids, notes, advice, etc.
DENSO starter install, photo and notes.

Copyright, 2012, R. Fleischer


article 16-A


For more information:

Starters, alternators, plus all sorts of electrical parts for them and lots more; really good prices too!   Try here first!      Have one good recommendation, no personal experience.
here is a shortcut:
They also have discounted alternators for BMW particular, K bikes.

Problems with starting BMW Airhead motorcycles can often be traced to a bad battery or its connections, but there are other possibilities:

1.  Poor contacts on the starter relay under the fuel tank.  This can be sometimes be repaired by opening
     the can and burnishing the contacts.  It is possible for you to hear this relay's faint click, and yet the
     contacts are not making electrical connection.  Very rarely, the relay has corrosion INSIDE the base.
     MORE commonly, the starter relay has faint corrosion on its EXternal male prongs, & in its socket
     (on models having a socket).  Cleaning external connections will cure the No-Start-NO-Click.  A
     permanent fix is to JOIN, very cleanly & neatly, & insulate afterwards, all the RED wires (usually 3) at
     the socket.  The reason joining the EXTERNAL red wires works is because there are two terminals on
     the relay for these wires, and the jumpering occurs INSIDE the relay, thus any problem with the
     EXTernal spade connections will act like the relay is not there.

2.  Poor solenoid (located on the starter motor) internal contacts.  A replacement is best; but I will have
     more to say later, herein, about how you might be able to repair one.

3.  The starter itself may have worn brushes, worn bush/bearing, poor drive gear assembly, or an
     armature that needs undercutting at the commutator and truing on a armature lathe.  It may also be
     heavily grunged-up with dirt/filth.   

4.  Early Valeo starters had a nasty habit of the glued-in-place field magnets coming loose and jamming
     the starter.  Some folks have rebuilt them using Saturn car starter parts.  Better to change to a
     Bosch...or, get an updated Valeo, or EnDuraLast, both of which are the easiest thing to do, and not

5.  You CAN change the original Bosch Starter to a late model improved Valeo (see 4 above).  You may
     have to do a tiny bit of metal work, described later in this article.  Be SURE to get the appropriate 8 or
     9 tooth starter!!!   Same, regarding metal work and 8 or 9 teeth, if converting to the Denso starter.

6.  A more expensive solution is to install the modified Denso starter that is available from numerous
     sources.   I am not convinced it is worth the large cost.  Distributed/Sold by Motorrad Elektrik,; and possibly available from such as Ted Porter's BEEMERSHOP, etc.

     These are modified-nose versions of the DENSO-made starter used on Toyota's, etc.  Possible
     interference problems with this starter, & need for some mild metal grinding, is shown with a photo,
     at the end of this article. A MINOR problem.

7.  On many BMW's the starter is the ground for the headlight relay coil, but this is not a problem for
     starting; as all it does is turn off the headlight during starting, yet leave the instruments & tail light on.

8. at item 4.D., has some further information on problems with the starter
      circuits.   There is more information, such as for the /5 problems, also in that long article.

WARNING!.....>>Fitting a starter motor; breaking nose castings; ETC.


Failure to properly fit ANY starter, squarely and solidly/securely, to the cradle in the motorcycles, can lead to a broken starter nose.  Note also that the Bosch original starter MUST have the forward plate affixed (remove the engine front cover to see and R/R that plate) and it must be fastened tightly.   Do NOT "assume" anything.  Never install a starter motor, any brand, without carefully checking how well it fits into the engine case cradle.  This caution applies to the original starter, and any replacement.  ANY STARTER!

The Bosch starters have a bent metal plate at the forward end, using one bolt to the timing chest wall.  This supports the starter to the timing chest wall, needed with the heavy Bosch.

Electrical problems can cause very heavy multiple impact loads on the starter motor, and then tend to be concentrated at that 'breakable' nose casting.  By electrical problems I mean anything that causes chattering when using the starter motor; such as a poor battery. 

If the starter motor bearings are quite bad, that can cause tilting of the armature, not good for the starter nose, and not good for the starter Bendix drive, flywheel teeth, etc. 

Other problems include a bad starter ring or fitment on the flywheel/clutch carrier, and too tight a clearance between the bendix gear and the ring gear.  These are rather rare, IMO.

If your bike kicks back (or, backfires backwards), that can be VERY hard on the starter, and the problem needs attention. 

Some early engine cases may need some filing to ensure a proper fit.  Further down this article are some of the interference problems found on starter fitment, but these are not necessarily the same fitment problems found with starters back in the seventies.

Starter relays:

See #1 above.
Some later model Airheads use a starter relay with a diode inside.  Some folks have substituted a 0-332-014-118 relay (03-32-014-118), and some HAVE used a DF005 'Blazer' relay from AutoZone stores (which has two 87 terminals and no 87a terminal).   The Bosch starter relay uses two #87 terminals, and may sub to Bosch 03 32 019 150 for 1977+ bikes.  That is a common Bosch accessory use relay.  Connector, if you need one, is 0 334 485 007, while the spring loaded terminals are 1 901 355 917.  It is certainly possible to substitute almost any common 20 or 30 ampere 12 volt relay for the starter relay on Airheads, but there are some complications, such as with the /5; and with late models with diodes in the relay, so ask ON THE AIRHEADS LIST about it.  Complications, if any, are easy to deal with, so don't be discouraged with my remarks here.  MORE information on starter relays later in this article!!
BUT....see the next paragraph:

BMW has an SI on retrofitting the Valeo starter on 1985-1988 bikes that came with Bosch starters.  BMW says that the original starter relay contacts are not up to the larger current draw of the Valeo SOLENOID COIL.  It is my belief that the original relay will usually be OK for most, but if you want to install one with larger current capacity, see this document, which I have here in pdf format for you: Valeo Starter-2383.pdf
...OR, install a common relay as a paragraph above.
There is another paragraph, later in this article on the relays problems.

Types of Bosch Starters used on BMW Airhead motorcycles:

There are at least THREE types of Bosch starters used on the Airheads as original equipment: 

Bosch starters up through 1974 were 8 tooth, rated 0.5 hp and 290 A.  The /6 bikes for 1975 and 1976 used an 8 tooth rated 0.6 hp and 320 A.  The 8 tooth starters are used ONLY with the 93 tooth flywheels; and are for practical purposes, interchangeable. 

For 1977 and later, the starter has to be 9 tooth, for use with the 111 tooth flywheels  (more properly called the clutch carrier from 1981).  The starter is, rated 0.7 hp and 320 A.
Solenoids, unconfirmed, seem to be the same as EARLY air-cooled VW.
   When overhauling a Bosch, I usually replace the Bendix, the bearings, the brushes, undercut the armature if needed and often make sure it is round and semi-polished, and a full overhaul includes replacing the solenoid assembly.  You can possibly get away with NOT doing all these things.  Your choice on what you do.   

If you divide the flywheel teeth number by the starter motor teeth number, you get the gear ratio for the starter to engine drive.   You will see that the change made the gear ratio a bit larger; which likely was to get the additional power, by effectively allowing an increase in starter rpm.

NOTE:  The "Airheads Beemer Club" had an account with Ace Houston Warehouse, a wholesaler/importer/remanufacturer, ETC. 
The Club account is or was #700.  Call Bob Spencer at 1-800-392-3332  or e-mail to  Mention account 700.  Per information I received from a fellow Airhead, the part was D6RA15, and then Valeo changed it to 432586.   This is supposedly a brand NEW starter.  There was no core charge, but they will or might pay shipping to get your old one.    I do NOT know, no one has confirmed it yes or no... if this is the latest updated Valeo with the safety clips for the magnets!!  They may still rebuild Bosch starters (??), last price, quite some time ago, was $200 and a $100 core charge and shipping (core charge refunded).   Bob Spencer thought that the Bosch starters MIGHT be put back into production at some point.   I have had NO dealings with them.  I consider the prices HIGH....and I think their stock of starters may be getting thin anyway. 
NOTE!  The above information is supplied as a courtesy, and likely way out of date now.    

is to get your updated starter here: 

****I HAVE had dealings with John Rayski at and I CAN recommend them for Bosch starters and for Valeo starters (yes, brand-new updated Valeo's) and their own EnDuraLast.  He also stocks PARTS for starter repairs.  He has the proper Valeo starters with the updated magnets and clips; his prices are good, he offers free shipping (on some orders...and can be quite a savings on a heavy starter)....and he is knowledgeable about the use of these in our Airheads.  He is the same person who makes and sells the EnDuraLast alternator conversion for BMW airheads and for MotoGuzzi's.  He can supply you with proper spark plugs and a lot of other things, including various filters, electricals, etc. 

You can also get these updated Valeo starters from
such as Ted Porter's Beemershop 

Be sure
to use the Bosch (or other) starter with the proper number of teeth!!!

The Airhead Bosch starter uses ELECTROmagnets, not permanent magnets, for the field coils. Bosch does make a permanent magnet starter, but I have never seen one in the Western Hemisphere.

The Bosch electromagnet starter is very old-fashioned technology, reasonably rugged, reliable, but less powerful than the "permanent magnet high rpm with planetary gear type" like the Valeo.  A nice property of the Bosch field coil type is that as it 'sees' a heavier and heavier load (cold engine, thick oil, etc), it draws more and more current, producing more and more power.   Still, the ultimate power under most conditions is less than the Valeo, which spins faster, and has the planetary gear reduction drive inside.   I consider it a tossup as to which is better, considering all factors.   The Valeo parts are less rugged, but that appears not to be any big factor, even over many miles, at least with the updated type with the secured field magnets.

It has been theorized that the Valeo permanent magnet type MIGHT loose some magnetism over TIME and USAGE.   However, they have held up surprisingly well....and I have had NO reports of any loss of magnetism. I suggest ignoring this 'supposed' problem.

The Bosch starters have a bent metal plate at the forward end, using one bolt to the timing chest wall.  This supports the starter to the timing chest wall, needed with the heavy Bosch.

When installing a Valeo in place of a Bosch, you do not use the the plate nor bolt, and the Valeo installation is maybe 6 pounds lighter than the Bosch.  Do NOT throw away that mounting plate, someone, or youi, may want it.   When installing ANY starter be especially careful that the starter is properly and squarely mounted, you want to avoid breaking or otherwise cracking the nose (the casting that is the rear part over the flywheel) during operation.  Be sure the mounting cradle, nose area, etc., is CLEAN of filth and there is NO proud metal, etc.  See later on that subject in this page.  This caution on fitting INCLUDES the original starter!

The Valeo Starter, and some Bosch information:

The magnet failures on the early Valeo's is well-known.  Using epoxy to glue magnets may have its place in this world, but there were lots of failures, especially in Airheads where the starter is subject to so many repetitive heat-cool cycles from engine heat.  I do NOT like the ideas of epoxies being subjected to constant wide temperature changes, their expansion-contraction rate is vastly different than the surrounding metal. The starter in an Airhead does, remember, sit right on top of the engine, inside a cover!The constant heating and cooling cycling can crack or otherwise ruin the bond of the glued magnet.  
Valeo addressed this problem (in 2001) by changing the type of glue and reshaping the side of the magnet that fits against the starter shell.  ....and late manufacture Valeo starters are adequate and seem reliable.  BUT: 

There is an aftermarket type of Valeo housing available with a modification to help keep the magnets from moving, spring-type separating pieces (clips).
   Valeo permanent magnet well as permanent magnet starters from many other manufacturer's, even Bosch (rare, never seen one), work fine in cars.   The proper aftermarket Valeo's with the updated magnets, bonding, and clips, are available from and probably from such as Ted Porter's BEEMERSHOP.

In the past I have been outspoken in that I personally preferred the Bosch electromagnet type for our Airheads.  However, I am open-minded on the latest Valeo starters (especially those from Euromotoelectrics)....and think them now, if you are making a decision between overhauling your Bosch, or replacing it with a Valeo.....or; replacing a Valeo with a Bosch....I no longer have any especially strong preferences for the Bosch over the Valeo (or the aftermarket Denso, for that matter).  Typically/usually, overhauling your existing Bosch starter, assuming the armature is not going to have to be replaced, is going to be much cheaper than replacing it with a brand-new 'anything else'.  The stock Bosch starters are perfectly adequate....and almost any starter/alternator shop can overhaul one, if you do not want-to.  However, the below company (link) sells new starters at a very good price, so, perhaps, fewer folks are now rebuilding the Bosch starters.

For parts for rebuilding for both Bosch and Valeo, see, John Rayski. 
John knows the in's and out's of all of them, and has a stock of all the pertinent Bosch and Valeo parts; and has Valeo starters of the correct type in stock.  Remember that there are both 8 and 9 tooth starters.
BE SURE you have the correct number of teeth on the starter...and if you are breaking nose cones, count the flywheel teeth too....just in case something got switched. 

The Bosch is a common starter for many starter repair shops, easily worked on...but some might not have the parts for the starter used in our Airheads.     The same sort of comment could be made about the Valeo and Denso.

You may run into information leading you to think about using a Saturn housing when rebuilding an original troublesome Valeo.  Unless you are trying to save the absolute maximum amount of money, I recommend you get a new upgraded version Valeo from   Or get them to fix you up otherwise.

Repeating here: 
When installing a Valeo in place of a Bosch, you do not use the the plate nor bolt, and the Valeo installation is maybe 6 pounds lighter than the Bosch. When installing ANY starter be especially careful that the starter is properly and squarely mounted, you want to avoid breaking or otherwise cracking the nose (the casting that is the rear part over the flywheel) during operation.  Be sure the mounting cradle, nose area, etc., is CLEAN of filth and proud metal, etc.  See later on that subject in this page.  This caution on fitting INCLUDES the original starter!

NOTE! is possible to mis-fit a starter, and the nose cone does NOT break immediately. That is rather common for the broken nose situations.

Going from a Valeo to a Bosch and vice-versa:

If you decide to change your failed Valeo to a Bosch:  Brand New Bosch's are very pricey and probably not available. If you want a Bosch, and yours is not, you can try to find a cheap good used one, and rebuild it if needed. Generally you need only a set of brushes and bushings (bearings) and a solenoid assembly. If the Bendix drive is questionable, replace it.  The solenoids can often be rebuilt too but I do NOT recommend it, although I have the information in this article.  You may have to undercut the armature, farm this out or do it yourself, it is not difficult.  YOU MUST obtain and use the forward bent metal plate when using the Bosch.

Keep the Bosch 'Bendix' drive lightly lubricated. I like to use a silicone grease due to its general non-hardening and wide temperature range. But, I have some mixed feelings about the use of silicone's on fast moving parts, and maybe a lightly laced moly grease is better...I am NOT sure about this.  Starter rebuilders may have ideas.

 This is applicable to 1985-1988 models officially:
BUT....if you install a Valeo in somewhat earlier models, pay attention here too!

There is a POTENTIAL problem, not commonly known, and there is even a BMW SI (Service Information, in other words, a Bulletin) on it, and the bulletin is 12-013-9 (2389)....but, see below.   

SEE also, the later SI; this is a repeat of earlier information:  BMW has another SI on retrofitting the Valeo starter to those bikes that came with Bosch starters (applies primarily to pre-1988 bikes).  BMW thinks that the old starter relay contacts are not up to the larger current draw of the Valeo SOLENOID COIL. 
It is my belief that the original relay will almost always be OK, but if you want to install one with larger current capacity, see this document, which I have here in pdf format for you: Valeo Starter-2383.pdf

That SI does not have ALL the information...but I do, below:

If the original starter relay is 61-31-1-244-019 (that's a BMW number, not a Bosch number!), it contains a diode, and has other properties that BMW thinks do not match the Valeo starter, & the relay contacts might stick.......and the starter continue to run!  

BMW says that the proper relay is 61-36-1-391-397, which has a higher current carrying capacity, and a stronger spring to help the contacts open.  If you have to replace the -019 relay, BMW will supply the -397 relay.    It also has the diode inside. Some have substituted generic 30 ampere relays. 
NOTE that I have not actually heard of anyone having this sticking problem due to the starter-swap. I have heard of sticking relay problems, but maybe only rarely, since I began working on Airheads!  So, it is likely a rare event.  On the other hand, it is my belief that if there IS a problem with putting a Valeo into an Airhead that came with a Bosch, that the problem is much more likely to be only with 1985-1988 models; due to a change in the relay internals construction that is NOT mentioned by BMW.   It is possible that lengthy starting can tend to overheat the relay contacts.

To avoid confusion, if you have a pre-1985 Airhead, the stock relays seem to hold up just fine with the Valeo.  I have MEASURED the current draw of the Bosch and the Valeo solenoid coils under actual starter use, and they are NOT excessive, and are just about equal!  The inductive kick-back voltage is also comparable.  This solenoid coil current is the current that passes through the starter relay contacts.  I would not be inclined to replace the early stock relay, although I might open it and burnish the contacts. is my belief that the problem, if there really is one (per BMW), that it is confined to SOME of the 1985-1988 models.

When retrofitting a Valeo to a BMW Airhead that had come originally with a Bosch starter, there can be an metal interference problem, which can be very slight and hardly noticeable, more moderate.  The problem comes about because the starter nose machining is NOT EXACTLY the same between the Bosch and the Valeo starters themselves, and BMW was, perhaps, a bit more careful about machining the engine case when the Valeo was installed as the stock starter.   It is my belief that when BMW shipped their own Valeo starters on the later Airheads, that the machining on their Valeo nose might be very slightly different.  I have not made enough measurements to prove this. 

I strongly suggest that ANY time you replace or even re-install, ANY starter, that you CAREFULLY check its fitment!!  There have been instances of a starter poorly fitted, and the nose breaking!

I THINK that this particular metal interference problem will be seen only when installing a Valeo in place of a Bosch, but I am not 100% sure about that>>>>>there were instances of the Bosch having problems with broken nose cones in the seventies models....where finding the interference and using a common file, were necessary.

  So, my advice here is generic, and assumes you will check fitment of any and all starters, that includes the Bosch, the Valeo, and the Denso.

I think that anytime you remove and replace even the same starter that was in the bike....that you should check for the proper fitment!!!   This is particularly so if the nose cones have been breaking.  They should NEVER break.

While interference can happen at numerous places, I suggest THIS: 
CHECK the fitment, before you remove the old starter....and then:
After you remove the old starter, LOOK carefully at the area the aluminum nose of the old starter had recessed into, just forward of where the gear mates to the flywheel (or 'clutch carrier', 1981+) teeth.   On some Airheads, BMW may not have cleaned up excessive metal over the last 1/2" or so of the round cavity, at the top inside edge. When you try to install the starter (probably this happens only with the Valeo, but I am NOT sure about that), you may find some combination of the following:

The starter does not fit perfectly, may not want to go fully downwards at that nose area and the Valeo threaded bolt holes (or even Bosch unthreaded ones??) may not line-up perfectly.   Inspect carefully, and use a half-moon fine file, or a small electric rotary tool of some sort, to make this inside area smooth at the top, with NO metal that is 'proud' (or of lesser diameter, causing a reduced diameter).  You can feel this with a fingertip, and usually see it with your #1 eyeball.   I have personally seen this and had to remove about .020" (+-), on both sides, at that top area.   This is easy to do, and then the Valeo (Bosch?) SHOULD fit/mount correctly.  

The Valeo, as opposed to the Bosch, has threaded holes, so you use the bolts you had from the Bosch, sans nuts, from the rear. 

Note the excess material at the left, pointed at by the red lines
in this photo. Don't worry if you remove a bit too much material! 
DO NOT remove material at the bottom. Since I can not
visually see YOUR installation, check how the nose fits, very
carefully, at ALL POINTS.  Use a dentists mirror & strong light, &
I also use a feeler gauge.


How the starter circuits and associated parts really function:

The starter motor is a powerful electrically operated motor.   To obtain the type of mechanical power needed from the starter motor, in some instances upwards of 1/2 to 1 horsepower in mild temperatures and modest compression engines, even more when the engine is quite cold and has thick oil in it, the starter motor requires a quite high electrical current.  The starter motors in our Airheads are not very efficient motors, as modern electrical motors go (the Valeo is more efficient than the Bosch, however), so quite a lot of amperes might be needed. 

There needs to be a means of switching on and off the very large amounts of current to the starting motor.   The heavy duty starter solenoid switch, physically located on the starter, is fed by a large diameter copper-conductors-cored cable, directly from the battery. 

There is NO FUSE.

The starter solenoid itself is activated by a modest current delivered by the pushbutton switch causing contacts closure on the starter relay.  The 12 volt power travels from the starter relay via a thin BLACK wire that goes to the SPADE terminal on the starter solenoid unit. The starter relay is, therefore, a small relay that sends electricity to another much larger relay, called the solenoid (or, solenoid relay).   

The sequence for starting is:  Ignition key ON, which supplies power to the START switch on the handlebars; pressing that handlebar switch button supplies power (completes the circuit) to the coil of the starter relay located under the fuel tank.  The starter relay contacts now close, sending a modest amount of current to the solenoid switch coils, the solenoid switch unit is located on the starter motor. This current causes a very current to flow and that causes a substantial magnetic field in the solenoid PULL-IN winding coil.   This magnetic field now moves a large magnetic steel slug.  One end of the steel slug has insulated contacts or a contacting strip which is pushed towards large copper contacts and the pressure causes those contacts to be jumpered.  Once jumpered, the large battery cable at the solenoid can now pass a very large flow of electricity into the starter motor. At the same time, the Pull-In wiring inside the solenoid switch (there are TWO interconnected coils) shorts out, which here means disconnects the PULL-IN coil section, as there is a secondary STAY-IN coil that draws less current and will not overheat, that keeps the solenoid piece pulled-in and the massive switch in contacting position.   In your Airhead, the starter solenoid is strictly a mechanically & electrically operated high power electrical switch.   

As the starter begins to rotate, a mechanical device called, generically, a 'Bendix' or Bendix drive, begins spinning.  It is located on the rear of the starter motor on the starter motor shaft.   This device, via spinning forces and guide grooves, moves rearward with quite considerable force, and as it moves towards the rear it engages its teeth into the engine flywheel teeth, that causes the starter motor shaft to lock-up to the flywheel, causing the engine to rotate.  The teeth of the Bendix drive, and on the flywheel gear, are shape-designed to enable quick meshing at high levels of force.   

Later model airheads may have a Valeo starter motor which has permanent magnets instead of field coils like the Bosch has, and the Valeo may also have a planetary reduction gear inside its nose, which allows the starter to develop higher rpm and hence higher power.  There is a factory bulletin on the starter relay on 1985 and later bikes (most specifically, 1985-1988), information will be found the the Bosch-Valeo article:

/5 and /6 to 1974:   8 tooth, used with 93 tooth flywheels, starter marked, rated 0.5 horsepower and 290 amperes.
/6, 1975 and 1976: 9 tooth, used with 111 tooth flywheels, starter marked, rated 0.6 horsepower and 320 amperes.
All later starters are 9 tooth, and if BOSCH, are, rated 0.7 horsepower and 320 amperes.

Late bikes had Valeo starters installed, they are all 9 tooth.  The early ones were troublesome with glued-magnets coming loose.   See for fixes or new ones, without the problems.

Starters can be repaired.  Starters do not always fit perfectly....see
The fitting problem is slight, but can cause broken starter nose-cone's, so DO check.


Overhauling the Bosch Starter Motor and Solenoid Switch


Bosch starters are used in a variety of makes and models of vehicles. Sixties and Seventies VW's, for one example use similar starters to the ones in Airheads.  They vary some, but are mostly the same.    So, it is not difficult, even at some auto-parts stores, to get parts.  I do, however,  suggest you get them from , so you can be sure of getting the correct parts, and at very good prices.

Not shown anyplace that I know of, is how to overhaul a malfunctioning solenoid switch. 
That is because most people will replace the solenoid.   A malfunctioning solenoid that has an open winding (there are two windings) is not repairable.  However, most of the time the problem is poor contacts inside; although poor contacts at the brushes of the starter can make the solenoid act like it was bad.  Below are photos of a Bosch solenoid unit as used on our Airheads.  IF THE SOLENOID CONTACTS ARE REALLY WORN OUT, IT IS BEST TO REPLACE THE ASSEMBLY, ALTHOUGH NERDS LIKE ME HAVE MADE NEW CONTACTS AND CONTACT PLATE.  Most of you will just replace a solenoid switch as a normal matter when overhauling a Bosch Starter.  But, for those of you that wish to try to fix or improve such a switch, I include the details here.  See the photo.

I have removed the terminal end cap.  To do so, you need a large soldering iron with a big tip.  Even a 35 watt iron with a big tip mass will work.  It is the MASS of the tip that is important.  A very large soldering gun (perhaps the 275 watt size) might also work OK. 

You undo the 2 screws as shown, and then put a slight to modest amount of pressure on the end cap (as if trying to remove it), whilst you heat the solder joints until they release.  Go back and forth to the soldering places; until the cap comes off.  Clean up the solder holes. It is VERY important not to mix up which wire goes to what terminal hole!!! watch them as you remove the cap.   DONT GOOF HERE!!!  The easiest way to avoid this is to mark the end cap and body before removal!

Inside you will see the flat switch contacts recessed in the cap you removed....and the mating contacts held to the solenoid with a funny 'nut'.  I usually don't bother disassembling further the plunger side. 

BEST way to clean the contacts in the cap and at the end of the plunger:  Use one of the very small rotary stainless steel (or brass) cone-shaped brushes, that you can put in your drill press or electric drill.  They remove very little material, and it takes only a few seconds at modest rpm.  An acceptable other method is with the flat end of a piece of doweling or other tool, covered in 220 grit sandpaper. 

You MUST keep the contacts FLAT and square to the assembly, so maximum contact surface will be obtained.

There are 4 thin solid copper wires that have a thin enamel coating for insulation.   The paired wires (push the insulation back a bit if you need to, to see it is a pair) go to the SPADE terminal that has a black wire to the starter relay.  You've already marked the case and cap so you won't mix them up, right?  There is one wire that is spot-welded to the metal end case; that's the ground wire for the HOLD-IN coil.   The single wire next to it is the one that goes to the starter and one of the large solenoid switch contacts.   You did note where the wires went when you removed the cap?  If you goofed and did not mark the cap and body, the information here will guide you to the proper fitment of the wires before you re-solder.

When re-assembling, you need to be sure the solenoid contacting plate fits into its mating cap depressed area portion, and the wires go to the correct terminals!  You can assemble the cambric ring and the wires to the cap, and lift the cap a wee bit, and rotate the tang at the far end of the solenoid, to line up the contacting plate with the cap cavity.   Push against the tang end, holding the cap end, on your bench top, to be sure the contacting plate actually moves into the cavity for it.  Assemble cap (align wires and pegs) to the plunger side of the solenoid unit carefully, using #1 eyeball, and test that the solenoid plunger mechanically works OK with your fingers, then screw the cap on with the two screws (locate the pegs!)...and THEN solder the terminals. Use ONLY a very hot large tip soldering iron.  Use standard rosin core electronics solder.    Be careful to assemble correctly!

The solenoid should be tested with it mounted and connected-up to the starter.   Do the testing on the repair bench, before you install the starter into the engine block.  You will need heavy gauge wires.  Connect the + terminal of the battery to the battery large threaded terminal (#30) on the solenoid.  Connect the - terminal of the battery to the starter body...such as a nose mounting ear.   If you get a small spark from this, you mis-wired the solenoid!

Hold the starter will tend to jump around as it starts!.....and, using a jumper wire or screwdriver, etc, momentarily connect the SPADE terminal to the + battery connection at the solenoid (#30).  The solenoid should clicking strongly and loudly and the starter should run.

Here is another test, but just for the solenoid assembly, not mounted to the starter.  This is a sophisticated type of test.
1.  Make a clamp, of wood, for your bench vise.  You can use wood as jaw pieces, or drill a hole in a piece of wood to ~match the case diameter of the solenoid assembly, then split it down the middle.  You want to be able to clamp the solenoid into your vise, without distorting the case.
2.  Connect the CASE to the battery - terminal.  I usually just put a bare wire around the case, during the clamping.
3.  Touch-connect terminal #50, the start terminal (male spade) to the battery + terminal. This will cause the assembly to be a fairly strong electromagnet, easily seen/tested.  This tests the HOLD-IN coil.
4.  New connections:  Connect the battery + to terminal 50 which is the male spade terminal.
5.  Momentarily connect the battery - to the terminal that normally goes to the starter motor.  This connection will draw more current, than in #3, and a much stronger magnetic field.
6.  Push/pull on the solenoid to see how the magnetism feels to you.

There is a test that can be done to determine that the Pull-In coil is not energized DURING the starter motor's actual turning.  It is done by actual current testing, and isn't described here.

Another test is that you can use an ohmmeter instead of #1 to #6. Disconnect the battery.  With unit NOT attached nor connected to the starter motor:
Moderately low ohms:  terminal 50, the male spade terminal, to the case.
LOW ohms:  Terminal 50, the male spade terminal, to the solenoid large terminal that goes to the starter motor.

Overhauling the Valeo

I was going to write up how to do this, but a fairly decent article with photos is on Anton Largiader's website, so here is the direct URL:

Note:  Valeo starters can be purchased from; and they have parts for Valeo, and Bosch, and many other things.  I highly recommend them (over Ace Houston Warehouse, which is mentioned in Anton's article as well as the one you are reading, well above this area).   Euromotoelectrics Valeo's are the latest type, with the modified magnets AND magnet clips.  NOTE the information and photo, well above, on fitting Valeo's to an Airhead that previously came with a Bosch.

The Denso Starter fitment problems:

It was reported to me that the Denso aftermarket starter, such as sold by Motorrad Elektrik, etc, does not, or may not, fit perfectly.   I obtained a photo.  The installer noticed TWO problems with metal interference.
1.  Light interference with the solenoid to the starter cavity cover (rib on it).
2.  Interference at the mounting ear on the back cover.  In the photo, below, you can see that the ear has been ground down so as to give about 1-2 mm of clearance between the crankcase and the starter ear.

This may not be the only interference problem; so check CAREFULLY!

02/06/2004:  add link to /5 article; minor other clarifications
05/30/2004:  add NOTE on the starter relay changes
10/27/2005:  updated in numerous places
04/22-2006:  add red note on Valeo installation needing special relay
11/28/2006:  correct URL spelling for euromotoelectrics
11/29/2006:  Change to 16-A, and do minor editing (16-B is being written)
04/28/2007:  Fix actual hyperlink for euromotoelectrics, display had been OK, link NOT.
09/28-10/1/2009:  Update hyperlinks, clarify some details that were unclear; re-arrange article a bit for
                           additional clarity, add all the information on the Bosch Starter solenoid overhaul.
10/17/2009:   photo of the metal interference problem.  Clarifications & editing throughout article.
11/23/2009:  reference to Anton's site for the Valeo overhaul, euromotoelectrics in that same area, etc.
04/25/2010:  Add photo and information on the Denso problem
05/24/2010:  Fix typo regarding forward plate that IS used with Bosch, NOT Valeo.   Clarify minor details.
05/12/2011:  Fix typo where word alternator should be starter. Add dividing line at one place to clarify a
                    new discussion section that was confusing. Fix a couple of mis-spellings.  Remove hyperlink
                     to 'thisoldtractor site (that has removed the starter motor overhaul information).
08/04/2011:  Fix note, DF005 relay now confirmed as substitute.
03/18/2012:  Clarifications
11/14/2012:  Clean up article, separate sections needing such, add some minor details, fix whole article
                    so will display a bit better on Chrome and Firefox, and on somewhat smaller screens.
08/11/2014:  MINOR changes
09/12/2014:  cleanup
02/16/2015:  Extend, somewhat, the warnings
04/04/2015:  Add information/section "How the starter circuits and associated parts really function:".  This
                    is the same section I expanded-upon, located in article startingprobs.htm.  Expand testing.
                    Fix wrong description on wire connections inside the solenoid assembly.

  copyright, 2012, R. Fleischer

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