Starting problems - 1985 W123 200 (petrol)

[tsa]

I could need some input in troubleshooting why my MB doesn't start . . .

Last year it was standing for a while whilst I was sorting out the clutch hydraulics, and when ready to roll there was an intermittent non-starting problem that has gradually got worse, to the extent is doesn't start at all now. Turning the ignition key goes as normal, in the "run" position the charge and brake failure warning lights light up, and in the "start" position (engaging starter motor) the two dash lights dim - indicating that enough current flows to lower the voltage to the instrument cluster. But the starter motor doesn't spin up, there is no clicking from the starter engaging it to turn over the engine.

Googling has indicated three possible causes; 1) faulty ignition switch 2) corroded/poor connections at fuses, and 3) sticking solenoid on the starter.

In case of 1) - I thought this would not make the instrument warning lights dim when the key is turned to engage the starter? Is there a way to 'jump' the starter circuit (aka 'theft hot wiring') to check if this is the problem? If so, which wires should be used to attach a temp press button?

2) replaced some corroded fuses, twisted the rest to improve connectivity; no improvement. I should mention I haven't seen any other electrical drop-out signs

3) suggested cure is tapping solenoid / starter with a hammer to allow sticking solenoid to move. No effect. If this is the problem, should the starter motor spin up?

Diagnostic suggestions welcome!

Thanks -

Tor

 

[vijilants]

Put +ve power directly from the battery to the solenoid to see if it engages.

If it does engage you have a wiring / ignition switch issue. Note that on a W123 I had back in the past I had the same issue but intermittant and found the wire to the solenoid was broken inside the insulation.

If it doesn't engage, the starter is faulty.

 

[Stocious]

Is the engine earth strap still there

 

[tsa]

Time for an update -
@vijilants - sound advice, many thanks. 12 V+ to the solenoid connector from the ignition switch resulted in a spark (short circuit style). Checked and found continuity between the solenoid connector and earth, suggesting faulty soleniod (?). Connected a test lamp between wire from ign. switch to solenoid and earth; this lit up when I turned the key to the start position, indicating OK ignition switch.
@Stocious - yes, earth strap OK

Got the starter motor out today, and saw that the pinion gear is stuck in the 'extended' position (see attached picture) . Not sure what happened first here, solenoid failure or starter sticking?

I don't have easy access to replacement starter motors near me, so wonder if it is realistic to try a DIY refurbishment? It is probably easier to get a new solenoid than the whole unit.

 

[Ted]

If the engine has been running with the pinion stuck out like that, then the starter will be toast.
The armature will have been spinning at thousands of rpm due to the gear ratio between the ring gear and pinion.