The Starter System   
first posted Jul/2007 - R. Kwas,  changes on-going (Comments Added)

Dec/2016 Added G. Bender material and link.

Troubleshooting Solenoid Function
Solenoid Current Waveforms
Solenoid Non-Retraction
Bosch Starter Solenoid
Bosch Starter at LeMans '88
Permanent Magnet Starter Notes


General, and Terminology:  The electric starting assembly fitted to vintage Volvos, commonly referred to a "Starter", is really an Assembly combination of two important parts, consisting of a Solenoid and a one Horsepower electric Starting Motor (SM).  The Bosch Starter design is a quality, series wound, (field coil is in series with armature) DC motor.  The SM spur gear has 8 teeth, and the flywheel ring-gear has 80, so the torque advantage the SM has over the engine (10:1) is significant.  Later permanent magnet/planetary-geared types are fit/form/function interchangeable...the SMs on these are noticeably smaller in diameter, due to the absence of bulky field coils...they also sound different, due to the higher RPMs of the actual motor and planetary gears, but these are also highly reliable as their forbearers from what I've seen.  Avoid mechanical shocks, like hammer strikes, to these, as this can demagnetize the ultra-powerful field magnets...then it’s no longer a starter but a good doorstop or buoy anchor...the factory is probably the only one equipped to remagnetize the assembly at that point.  

Starter Assembly, showing the earlier style Solenoid. 
The newer Sol has a different form but is the same functionally.  Source:  GPC's Site

Normal Function, Electrical and Mechanical:   One important function of the Sol is as an electrical contactor to switch the significant starting currents.  It does this with two dedicated High Current Contacts (HCC).   As power is applied to the Sol coil, the ferrous armature is moved due to the magnetic field created.  This causes the attached bridging contact to complete a connection across terminal 30 (terminal to which battery cable is connected), and an unnumbered output terminal connected directly to the SM by way of a short hop-over cable (the hop-over cable (5) Is often without insulation, but a rubber grommet keeps it from making contact with the SM case as it enters).  Later versions also had an additional small disconnect terminal, by which the fuel injection system could also be imformed when the engine was being cranked.  Individuals who have, or intend to convert their IGN system to a solid state module (Pertronics or Allison/Crane) can also connect one input of the diode "ORing" cable to this point in order to get power while cranking.  This is required because terminal 54 of IGN Switch (the other input to the ORing cable), which would normally be supplying solid state IGN module, is disconnected from power during cranking (LINK to IGNITION and Armored Cable article), in order to shed other loads, which allows the battery to have the least additional load and give the best chance of starting. *By “OR-tying” these two power sources, it is assured that the IGNition electronics are continuously supplied.


Detailed Starter System diagram (but simplified Solenoid, see below for fully detailed, correct drawing!), showing control and high current paths,
assembly internals, and the way the SwEm Start Switch Upgrade comes into play. 

**  What's the big deal about the Braid?  The there is nothing sacred about a braid other than its fine wire strands are better able to cope with the movement and vibration without being damaged. 


In-Situ:  A well charged battery and clean tight connections along the (high) current path are a prerequisite (ALWAYS, when troubleshooting electrical equipment, but more-so when troubleshooting high-current electrical equipment) ...and this includes the chassis-to-engine strap which also carries the substantial starting current as can be seen in the Diagram above.  Poor/loose connections cause voltage drops under the substantial load current...then things don't work so good...and the electrical current seeks the easiest alternate path...which might be through the throttle linkage or chock cables...then you can get some really weird secondary effects... but all bad! 

Bypass the IGN switch and apply power directly to energize Sol, and in turn SM.  A "remote starter switch" connected between the big bolt, terminal 30, and the smaller quick disconnect control (terminal 50) works well for this...a much less elegant (but equally effective in a pinch) technique uses any conductive tool between these two adjacent terminals (like a big screwdriver...but I wouldn't use my about the right tool for the job)...this makes big sparks and one needs to be ready for the engine movement when starter engages...  A remote start switch is preferable because it allows for the troubleshooting steps which follow, without big sparks in the engine compartment (big sparks scare onlookers, pets, and can ignite stuff!).  A remote starter switch with its spring loaded electrical clips isn't bothered by the rockin' anda rollin' of the engine either. 

Additional Safety Reminder:  Gearbox should be in neutral, handbrake applied, and IGN OFF (prevents starting).  Lunging cars scare wives and neighbors ("...maybe you should let that nice mechanic at Joe's Garage fix your car, dear")!

Since the Sol is directly connected to SM, this should normally result in a clack, and the SM immediately turning over the engine, as both work correctly...but if there is no clack, SOL is not being activated due to an external electrical problem (not getting power from IGN Switch), or an actual SOL failure exists (rare but not impossible!). 

A clack not accompanied by the SM turning the engine over on the other hand suggests a HCC problem OR a problem with the SM itself.  A simple way to determine which one to suspect is to measure if voltage is being routed through the high-current-contacts and being applied to the SM itself.  A voltmeter on the short uninsulated cable stub which hops from the SOL (output terminal), to the SM while energizing SOL will tell a lot...if battery voltage is measured, but no action from SM results, the SOL is working as expected, but the SM is suspect.  If on the other hand, SOL clacks when activated but no voltage is detected, the HCC is suspect.

X-Ray view of the Starter Assembly from the green factory manual, showing high starting
and low IGNition switch current paths, also the mechanical function of the pivot linkage...
Sol pulls, pinion gear is driven in direction away from SM to engage Flywheel Ringgear. 
Note that two coils of Solenoid are shown, see additional info on Solenoid below!:  


Removed from Vehicle:  Operation of a starter can be checked off a vehicle by establishing similar operating connection as when in the vehicle.  A set of battery jumper cables may be used, but smaller gauge test wires 16-14ga. will do fine, since the SM will not be operating under load, and will therefore not be drawing typical high operating currents which necessitate really heavy wire found in the vehicle.  A negative connection must also be established from starter housing to battery negative.  Be aware:  The high starting torque is capable of twisting it out of ones hands if you are not ready for held in a vice or simply on the ground with a foot securing it is a good idea!  When activating, the Solenoid should clack, apply power to SM, and spur gear should spin and also advance, along its shaft, and away from the motor body (to engage ring gear when installed), when released, the spur gear should return to its unenergized position.  Any excessive slop should be investigated and repaired as necessary. 

Note:  Do not apply power for more than a "touch-test" as an unloaded series-wound motor can "run away" and self-destruct! 

The two through-bolts which hold the endplates can be removed in order to gain access to the internals and to check the Brushes (22).  If length of Brushes is such that Brush-Springs (28) are down to the holders, Brushes should be replaced.  PN for springs is:  906983


Normal Function, Mechanical:  In addition to the electrical non-magic happening when the SOL is engaged, it must also perform a very important other mechanical function, that is, to connect the SM to the flywheel.  It does this by a simple pivot linkage.  As the SOL armature is pulled to connect the HCC, it also moves one end of a lever.  The other end of this lever simultaneously engages the SM driven gear with the flywheel ringgear.  After the engine has started and the flywheel's ringgear speed exceeds the SM spur gear, an over-run mechanism, which only allows the SM spur gear to engage the ring gear when flywheel speed is slower than SM speed, disengages the SM's spur gear from the flywheel ring gear.  

Mechanical Problems:

High Mileage Issues:  With the high margin build into all the Starter components, the assemblies are quite reliable, but, eventually, even a brick wears out.  If a starter has more that 200,000mi. on it, and along with it an unknown number of prolonged cranking sessions which were necessary to bring the engine to life in frigid ski area parking lot for instance, a removal and inspection might be a good idea! 

Typical symptoms of wearout problems:

Brushes wear, so starters then have been known to develop "dead spots".  (This is where MG drivers would apply several calibrated blows from their (required on-board equipment) brass hammers...often with success...Lucas equipment often works like that...even when new!...your result with Bosch equipment is likely to be quite different and more rewarding...see above!).  

HCCs carbon up and become intermittent (see above) a pretty rare case, I had a contact actually become detached from its mount, and bridge between battery terminals and chassis...YEAOW!...this made for an instant engine shutdown due to voltage collapse, smokeshow from under the hood as oil and dirt burned off the ground strap at the chassis, also the temp gauge heading instantly for the sky, as electrical current coursed along every possible path (including the metallic capillary tube of the temp sensing system, and probably also the throttle linkage for all I know)...and me scrambling, trying to get under the hood to disconnect the battery..."steppin' anda fetchin' like my hair was on fire and my a** was aketchin' " (with apologies to Charlie Daniels). 

Pivot or over-run mechanism wears or loosens causing gear engagement or disengagement problems.  (often characterized by a noticeably different sound while cranking or releasing...your car is talking to you...are you listening?)

Starter Replacement Notes:  Later starters had threaded mounting holes to eliminate the nuts, but these were metric threads...they can be used but require drilling out the threads.


* This connection, or the diode ORing cable are not required if a SwEm Start Switch Upgrade (always a good idea!) is installed.  


Related Links:


Additional Information:  

My response to Thread Starter Playing Up
Additions not part of the original posting. 


You wouldn't expect to hear a click other than at the Ign Sw when turning ON the Ign Sw (Pos 3)...the only time Solenoid should show any signs of life is when turning key to Start position (4), or better yet pushing Momentary Start Button! That's when it gets power applied and activates...and that in-turn applies power to Starter Motor, which automatically engages Flywheel and turns over engine...

Clarification: "Starter" is the common term for the Solenoid and DC Motor assembly. (The reason I must mention this is because on some vehicles like Ford, these components are not one assembly, but separated!) When it fails to function as expected, it is good to troubleshooting the assembly by the Divide and Conquer method!

Solenoid(Sol): Its function, as a High Current Contactor (HCC), is to pass power through to DCMotor when activating it (either by turning Key to the Start position or better yet, pushing Start Button after turning ON Ign with Key) we would expect it to at least audibly KLACK! ...but we probably wont even hear that because if it passes power through to DCMotor as it should, that will turn over engine, which makes a lot more noise and which is what we will hear...if everything goes as expected...

DCMotor: It's function is to turn (with authority, which requires high current*, therefore power must be switched by a HCC), as power is applied, and by way of the bendix coupling, engage the Flywheel Ring Gear to turn the Engine over. If it does not turn when Solenoid is activated, one of two fault conditions can exist:

1. Power is not being applied through High Current Contacts (HCC). [This may be because of a worn or carbonized contacts, or other Sol issue...they are serviceable but this requires special equipment and techniques.] Or

2. Power is being applied and DCM is not turning [If power is being applied should be verified by checking for voltage at the output of Sol, when Sol is activated - Output terminal is the lower stud to which small hop-over heavy cable into DCM connects.  See Below!]

Striking the DCMotor with a hammer to shock it out of a "dead-spot" is hold-over technique from Lucas equipment, where such action is known to often is quite rare for such an action to help on Bosch equipment, and in-fact extremely BAAAAD, and absolutely VERBOTEN(!) if Starter Assy has been replaced with a more modern Perm Mag DCM, where such a strike may demag the PMs and render the Starter into an anchor for small water craft!! See also:  Permanent Magnet Starter Notes

* Note: If DCM fails to be supplied by a good power source (read stiff voltage source), which is not collapsing from a poor Battery State of Charge, or Voltage Drop somewhere along the current path, it may not have the authority to turn over engine (or even to continue to hold the Sol pulled in, resulting in "Cyclic Klacking" to coin a possibly new descriptive).  This is where while troubleshooting, one must remember which comes first...and that is power from the Battery!


Troubleshooting Solenoid Function:  

If power is measured on lower stud (output terminal of Sol) when activation power is applied to term 50, but Starter DCMotor does not activate, DCM should be suspected. 

Solenoid business end.  Upper stud is Battery Power input (30), lower stud is Output to DCMotor (not-numbered), additional Terminals are, 50 for activating Sol from Ign/Start Switch, and an additional terminals present on later (and replacement Starters one might run into) (also unnumbered, highlighted in ORANGE), slaved off the output Terminal, by which Injection system (both Thermo-Time Switch/Cold-Start Injector, and ECU) know engine is being cranked.  This terminal can also be used to disable load on engine by Air Conditioning System during cranking. 


Update:  In recent detailed reviews of the Bosch Starter Solenoid, I have come to learn that the Solenoid is not as simple as I had previously shown above. This is particularly apparent when the Sol End-cap is removed.  The Sol has two coils much like the Overdrive Sol*!  (see also:  OD Sol Notes ). 

 *  In the case of the OD Sol (which stands alone, and where power remains applied continuously, or for long periods of time anyway, power to the Pulling Coil is removed after the initial work of engaging the Sol is done), this is to keep the OD Sol from overheating.  Starter Sol typically only gets used briefly to start engine, but it must do even more work during that time, so a special sequencing provision is used to prevent overheating while it is energized.  Once engine is started, Sol is once again de-energized, and can rest and cool. 

After removal of the Sol End-cap, one finds a Heavy wire (Green), Pulling Coil, and a Lighter (Blue), Holding Coil.  Note that at the right side of picture, both coils are connected at the solder joint to Terminal 50, and the left, the light wire coil is spot-welded to chassis (at Blue) while the heavy wire is soldered to the output side of HCC. 

Improved Starter System diagram, showing full Solenoid details (electron current shown),
nergization and De-energization current paths of Solenoid are highlighted,
as well as the way the SwEm Start Switch Upgrade comes into play. 

In the case of the Starter Sol, there are also Pulling and Holding Coils (PC, HC) to prevent overheating, but the Starter Sol does not stand alone, it instead passes power to a high current load.  The function is thus a bit different than the OD Sol, but preventing both from overheating is still crucially important, and this is accomplished by sequencing the powering of the two coils, just as with the OD Sol.  I've seen several good explanations of this (not simple) sequencing (See Reference Information below), but here is my own: 

Solenoid Activation:  When Ignition Power is applied to Terminal 50, a high current (Energization, in the above Diagram) passes through heavy wire coil (and Motor, which are in series), pulling in the Sol Plunger (hence the term Pulling Coil), and a lesser current also passes through the light wire coil to (Holding Coil) chassis.  The two currents combine (note polarity dots) to create a strong magnetic field, which quickly actuates the Sol Plunger, engaging the driving gear into the Flywheel and also closing the HCC.  Upon closure of the HCC, high current through the heavy wire coil stops as there is now a direct current path from Battery to Motor, so there is no more voltage across that coil.  Low current through light wire coil continues, and this is sufficient to hold the Sol in the activated position for as long as term 50 remains energized, hence the term Holding Coil.  The actuation transition happens within a fraction of a second, so the very high current passing through Pulling Coil occurs for only that very short time limiting Sol heating.   

Solenoid De-activation:  Upon de-energization of Terminal 50, current (De-energization, in the above Diagram) is momentarily generated by the spinning Motor and flows in the opposite direction through Pulling Coil and through Holding Coil (no change of direction here!) to Chassis.  The resulting mag fields of both coils now cancel, so Sol Plunger returns under spring force.    


Solenoid Current Waveforms:

David Mc Kenzie was helpful in getting OD Sol current waveforms, reposted with his kind permission: 

44.1ms to engage (Power is applied and current comes up to a maximum of 2.1A). 
[I would have guessed a higher final current, but the measured values don't lie!]

Note initial current rate-of-rise is steeper at first, (when both Pulling AND Holding Coils powered), then drops to a less steep rate, after Internal contact for Pulling Coil opens, and current drops to only that drawn by Holding Coil.  


Solenoid Non-Retraction:  

Its a pretty rare occurrence, but I finally ran across pictures of what happens when the Starter Pinion fails to release from the Flywheel, like when power to Solenoid becomes stuck ON...and engine Starts and Bendix mechanically fails and does not retract when Flywheel speed exceeds Pinion speed!  ...not so pretty, because the Starter Pinion Gear is overwhelmed and massive mechanical carnage can this, boys and girls, is one reason why the Gorilla Spring is so darn strong! 

Picture of results of a stuck Ignition Key!!! 

Geoff Cockram pix reposted with his kind permission.



Reference Information:  (Reply No 5 by "Kiwi_Roy" has a very good detailed explanation!)  (Reply No 20, again by "Kiwi_Roy" has another, and also very good detailed explanation!)  Link to Kiwi_Roy Study:



Motorcycle enthusiast Gregory Bender disassembles and rebuilds a Bosch Starter from a Moto Guzzi which to me for all practical purposes looks like it came off a Volvo!  His excellent documentation article has lots of detailed pictures including of the HCC of the Solenoid!:

Here are two of the detailed pictures (republished with kind permission of Mr. Bender...I sent him a big sample of ACZP to use on the electrical connections of his scooter in appreciation!): 

Connector end of the Solenoid.  Note Terminal 50 is marked.  Upper Stud is Terminal 30 (Battery), and lower stud (not-numbered) is High Current Contact (HCC) output to DC Motor.  Visible also is solder joint for connecting one end of both Pulling Coil and Holding Coil wires to Term 50.   Removal of Solenoid Bakelite end-cap after removal of securing screws, and desoldering wires of Coils.  Note there are two solder joints, and two coils!  The Pulling Coil of heavy wire, and Holding Coil of thin conductor.  Note also, some contact erosion is evident on octagonal copper High Current Contact (HCC), attached to armature. 


Replacing Starter Motor Brushes:  

Their wire does need to be soldered on the later Starter Motors...use a 100W soldering gun to desolder the old wire, then clean the soldered area and resolder wire of the replacement Brush...a 40W electronic soldering iron will not put out sufficient heat to make a good joint.  Use a quality electronic tin-LEAD (oh my God!) solder (the no-lead ecosolder does not wet and flow nearly as well...especially on a less-than-spotless clean surface, and as far as the lead issue position is that anyone who comes and nibbles on the solderjoints in my Starter deserves what they get!...and don't start about the fumes coming off the soldering iron...those are fumes from the flux, not lead!). Reference:  Wiring Notes, Soldering Notes, Lead Poisoning?...Hogwash!


Related Story:

Bosch Starter at LeMans '88:  I'm not suggesting that they used the same Starter at LeMans, but it's definitely from house Bosch, and pretty similar!  In researching Bosch Starters, I remember the fluke Jaguar win at LeMans in '88 because the leading Porsche ran out of fuel due to a communication problem with the pits...  After running out of fuel, it was driven back to the pits ON THE STARTER AND BATTERY, making it quite possibly:  The first hybrid Porsche!  ...I think this says a lot for the operating margin  of the Bosch starter!

This is copied from:


Permanent Magnet Starter Notes:  



My stupid comments to Faceplant questions: 

"I guess that leaves out the British Starter and Generator repair techniques (developed with, and common practice, and even occasionally successful, with Lucas Equipment)! 

The Permanent magnet*/planetary gear Starter Motors work great, but don't take well to hammer demags them!...then they are only good as anchors for small watercraft. Don't do it! "

*...but it's only permanent if not allowed to relax back to non-magnetized, which a sharp energy input like a hammer strike will cause them to do...

If you are going to use the Lucas repair Technique on your Starter, make sure your AAA membership card is up to date (you'll need it!). 


FC Cristophersen and D Koehler pictures used with their kind permission.

Unknown creative Batt Neg wiring in FC C's ES...correct in D Koehler's picture for an 1800ES (and likely also an 1800E, but I'm not 100% certain!).

My Response to a question about "...proper configuration for the ground connections from battery?" [on an 1800ES]:  It does look like someone may have/HAS bypassed the chassis to engine "Ground Strap" and wired the Bat Neg directly to the engine. I agree with [other poster] Dean, as shown in his picture, there should be a short run over to the Hoodlock mount for Batt Neg, BUT this should/must be accompanied by the "Ground Strap/Braid" which hops from Frame to Engine, across Exhaust. Do not remove your current Neg cable and return your configuration to the OE factory without also checking for presence of the GS, and adding it back if not present!  Maybe someone can post a detailed picture showing the GS. GS MUST BE PRESENT as a current path for high starting current! Cheers

Edit: IF there is no GS present, all return currents from chassis to Bat/Chg Sys (ie Fuel Inj, Lamps, Instruments, etc) would be finding an alternate current path...which would be BAAD!...but FI would probably not function without it is likely present...just make absolutely certain!!


External material attributed.  Otherwise, this article is Copyright © 2007-2022.  Ronald Kwas.  The terms Volvo, Bosch, Pertronics, Allison/Crane, or Lucas are used for reference only.  I have no affiliation with any of these companies other than to try to keep their products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here. The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at, (or worshipped!) at the readers discretion.  As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles! 

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source.  If you don’t, you’re just a lazy, scum sucking plagiarist, and the Washington Post wants you!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the odd metaphor and probably wise-a** comment. 


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