Continuity Bosch starter motor solenoid

[workingonit]

Should I get continuity across the S and M posts?

Bosch 0 001 218 768. Off a Tdi300.

The solenoid has three posts - Starter (connects to your ignition key and activates magnetic coils), Battery (direct line from + battery terminal) and Motor (directs current to starter motor when bridge active).

Websites seem to say there should be no continuity between the S and M post ie put a multimeter across the two and there should be no current flow (or no beep if using the continuity setting). If there is continuity (beeping) then the pull winding(?) is damaged - get new solenoid.

Fair enough you think, get a new solenoid. However, when I go to 4 other starters motors in situ on LRover vehicles, they all have continuity across S and M. Am I having a lot of bad luck or is Bosch a little different?

Personally, if the bridge had failed and I had to short the starter via the Motor post then I might find continuity to the Starter post desirable. Shorting to the M post would spin the motor and at the same time ensure the S was activated to throw the Bendix into gear.

Side note on this Bosch model. Normally the brush retainer is held in the bowl shaped back plate by two screws. Not this model. The brush holder is retained by the rubber grommet covering the motor wire coming from the solenoid.

Why am I doing this? Getting a hot-no-starter-action after short drive. Only thing that seems to get the starter to turn is hang around for half an hour for cool down, or pour water onto solenoid and have someone gently tap the solenoid. Will be checking earth straps etc.

Was also curious to see conditions of starter internals, which seem good, plenty of carbon brush left. But the continuity has me stumped. On the bench the starter works well, throws and withdraws the Bendix gear and spins motor.

Should the iron core and sleeve in the solenoid be lubed or dry? Seems dry on dismantling. Can't inspect the bridge and pads as their housing is crimped into the solenoid body.

[bee utey]

The S and M terminals should definitely have a connection. When you first twist the key a low resistance high current pull in coil is energised from the S terminal and grounded via the motor at the M terminal. As soon as the solenoid contacts close this coil cuts out. Another coil of higher resistance is earthed to the solenoid case to hold in the plunger to the contacts. This function saves the solenoid drawing continuous large currents more suited to turning over the motor.

[workingonit]

The S and M terminals should definitely have a connection.

Thanks for that confirmation bee utey.

On now to checking earth straps. And to setting up an emergency push button over-ride.

A few weeks ago I filed the points on the starter relay and checked power was present on the Starter wire at the solenoid.

[bee utey]

A few weeks ago I filed the points on the starter relay and checked power was present on the Starter wire at the solenoid.

Silver plated relay contacts don't take kindly to filing. All you do is expose the base layer which oxidises rapidly. New relay time.

[workingonit]

Silver plated relay contacts don't take kindly to filing.

Under the hand lens one contact had a huge lump while the opposing had a huge hole. It was still clicking and passing current but I took a punt that this might be the problem and filed things flat. Of course in the next outing the vehicle again failed to proceed so put paid to that idea it was the relay. No automotive supplier up here has a replacement so a wider search will ensue. Maybe time for some creative silver brazing?

Fortunately it is not my daily driver, but does have my lockers!

[workingonit]

Yesterday, reinstalled the starter motor with solenoid; removed all earth straps and inspected; sanded all contact faces both cables and chassis; closed the starter wire spade to ensure good grip on the terminal.

Regarding the solenoid steel/cast iron(?) plunger and the bridge post shaft - they seemed dry when disassembled - not being certain of what state of lubrication they should be in I applied some way oil.

Today took vehicle for a 30 minute run to heat up. Now seems happy to turn off and restart without issue when hot. When engine is off there is an 0.03 amp current draw.


The S and M terminals should definitely have a connection.

I should also clarify that the solenoid was off the starter motor when I tested it and found continuity between the S and M terminals. There was continuity between S and the solenoid body, which I understand there should be. And there was also continuity between M and the solenoid body - implying S and M are linked when they should not be, maybe? Not sure the absence of these facts may make a difference - sorry in advance bee utey.

I can understand there being continuity between the S and M when attached to the body of the starter motor itself - I would have thought current going via the S terminal would go through the coils first then to ground on the solenoid body, and on to the starter motor common earth casing if attached to the starter motor - current going via the M terminal would first go through the motor windings before going to common earth casing. This may explain why all the other engine mounted starters I tested gave continuity across S and M.

I would not have expected continuity at the terminals end of the solenoid when off the body of the starter, they all molded into place in a plastic base, which itself is crimped into the steel casing - to have continuity between S and M implies that the two are connected via the solenoid casing, or via some deliberate link inside the casing or some damage inside the casing. I am still a bit perplexed. Time will tell I guess.

One of the sites I read

Starter Solenoid: The Definitive Guide To Solve All the Solenoid Problems

(http://startersolenoid.net/starter-solenoid/)Looking at the schematic does seem to indicate a connection between current flow from S (black) and the M post (red) for slow rotation of the starter motor, to allow Bendix engagement, before full power from B kicks in to drive the motor at full force.

[Old Farang]

The following may explain it better:

http://www.aeroelectric.com/articles/strtctr.pdf

[workingonit]

The following may explain it better:

http://www.aeroelectric.com/articles/strtctr.pdf

Thanks Old Farang.

But would you believe it, I just started the vehicle to move it and now find front and rear windscreen wipers are running and can't be stopped until I turn the ignition key to the off position.

[workingonit]

Got to check the manual to be sure, but have just noticed the rear door window heater demister may be on, judging by the orange/red light in a dash mounted push button - it also will not turn off.

So have now pulled two fuses for front and rear wipers, and one fuse for the demister.