Distinguishing between mechanical and electrical failure in ignition diagnosis

Question

After a month and a half of not driving the car, yesterday I tried to turn it on, however the only sound that I heard was the rotation of the starter, which is not engaging the flywheel (the engine is not turning over). Additionally, there is not a clicking sound when I turn the key to ignition.

I've tried multiple times and the result is still the same and I've checked the the battery voltage: 12.2 V, which is not 12.6 V, but there were some large temperature variations that explain possible discharge.

In my opinion, the most probable cause is the starter solenoid or some electrical component related to it, like a relay or fuse.

What testing procedure could I follow to detect and identify the problem given the above "symptoms"?

More specifically, how can I test whether there is a electrical (relay, fuse) or mechanical (solenoid, shift lever) problem?

I was not able to find a schematic, but this what the starter looks like externally; above is an old one and below is a new one.

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The car is a 1997 BMW E39 520i

Answer 1

If you are hearing the starter turn (typical starter whir), the most probable problem would be the one way clutch has gone out. It's the part of the starter with the teeth on it which engages the starter ring gear. If the solenoid has gone out, the starter motor will not spin, because the solenoid is the part which forces a large copper washer into the posts which transfers electricity to the starter motor. Without it working, there is no connection for the starter motor to be energized.

The only real way to test this is to take the starter off of the engine and check the one way clutch. What you'd be looking for is to just turn the gear on the one way clutch in both directions. If it does spin in both directions (without spinning the motor along with it), then it's bad. If it only spins freely in one direction, that is not your issue.

While the starter is off of the engine, you can bench test it to ensure the starter is working as you'd expect. You just need a pair of jumper cable which are attached to the battery. Attach your negative lead (ground/earth) to the starter housing. The place the positive to the main solenoid stud (labeled battery connection in your illustration). You'll then need to take a screwdriver and jump the connection between the solenoid stud and the "S" connection on the solenoid (this will be the small terminal between and outside of the two larger terminals on the back of the solenoid). If the one way clutch pops out and spins, it should be in good shape (as long as the one way clutch turns out okay). When placing the positive jumper to the post, you don't want to run it for too long ... No more than a few seconds. You only want it there long enough to observe operation.

Answer 2

If the picture you have attached is similar to the starter on you car, then the power to the starter motor is only delivered if the solenoid operates, so the solenoid must be working.

However, the shift lever labelled in the picture, that pushes the pinion forward to engage with the flywheel may be broken, that would explain your problem.

Your problem could also be explained by the pinion being siezed on the shaft it slides on, where all the motion of the shift lever is being absorbed by the spring behind the pinion.

It is also possible that the teeth have been stripped off the pinion or the flywheel. If the flywheel teeth are damaged, you could try nudging the car forward a little with it in gear so that the flywheel moves different teeth next to the starter pinion.

My hunch would be that the starter pinion has siezed on the shaft, so that it is not being pushed toward the flywheel. You may be able to tap the rear end of the main starter motor shaft with a hammer to free it off, otherwise you will need to remove the starter and lubricate the shaft.