Thread: Continuity Bosch starter motor solenoid

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.

Originally Posted by bee utey

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.

Originally Posted by workingonit

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.

Originally Posted by bee utey

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!

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.

Originally Posted by bee utey

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

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.

The following may explain it better:

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

Originally Posted by Old Farang

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.