560 SEL Starter Circuit Fault

[doggiedog]

I have a 1987 560SEL, which has a really strange fault which I am trying to fault find, as the Auto Electricians are closed for a week or more.
I went to the supermarket, got back in the car, tried to start it, seemed like it had a flat battery, the usual groan, groan of the starter motor, Damn, turned the key off, but the Groan groan of the starter motor still continued with the key out.
Got out lifted the bonnet, the engine is still trying to turn over slowly, check the boot, Damn no tools, no one around I could borrow a shifter from, so had to let it continue on its merry way, didn't even have the phone with me, after a couple of minutes, the battery started issuing out vast quantities of vapour (steam or acid?), and eventually it stopped.

Rang the Roadside assist, eventually towing my car home.

The starter motor solenoid in this car has had issues before (not engaging properly), which I had fixed.
I initially thought the starter motor solenoid had welded itself, so i removed it, but it tested OK on the ground. I tested the starting circuit using a multimeter, and tested OK.
Bought a good second hand starter motor, and fitted it to the car, but tested the solenoid circuit again before attempting to start it (not wanting to destroy another $185 battery), and found that as soon as the ignition is switched on, there is 12V to the solenoid connection.

Next step was to test the ignition switch to see if the problem was there, removed instrument cluster, tested ignition switch, and all seems ok.

So I'm now scratching my head, deciding if it is a loom fault or something else.
According the the wiring diagrams I've downloaded (which seem to be correct), the starting circuit goes to a few other components as well, such as the ECU, Fuel Pump relay, Idle Control Circuit etc, so I'm wondering if one of these has failed in some way allowing ignition voltage into the starting circuit.
I'm wondering if anyone else has experienced this type of problem before.
I would take to a Merc Specialist, but the closest one is 2 hours away, I'm in Country Victoria, Australia


Cheers
Terry

[mbdoc]

I guess since the car is 20+ years old, I would just run a new wire from ignition switch to neutral switch & from there to starter.

[doggiedog]

When the fault first happened, I thought it would be easy to rewire the starter circuit, but looking at the drawings, the starter circuit also has input into the ECU, fuel pump relay, and Idle Control circuit as well.
My next step in the diagnosis, is to disconnect these components, to see if one of these has failed, causing the fault.
Does anyone know where the ECU is located, or what the idle control circuit consists of?
I just don't want to rewire the whole thing if the wiring is not at fault, although I suspect it probably is, especially as its heading towards 40degC (105 deg F) here at the moment.
Thanks for the reply

Terry

[doggiedog]

With Christmas/New Year shutdown, the Merc has sat in the driveway, and I have been driving around the Citroen CX (which has reminded me how much I love that car), and I am not sure if a Merc Specialist is the best way to deal with the electrical problem I an experiencing, since they are a bit thin on the ground her in country Victoria Australia.
I have because of the holiday period, tried to diagnose the fault, which has resulted in the starter motor being OK, and the ignition switch being OK.
The circuit for the starter also has an input to the ECU, so I was wondering if anyone could tell me where the ECU might be on the 560SEL.
I could pull panels off trying to find it, but if someone could tell me it would be easier.
Once I disconnect the ECU, and the Fuel Pump Relay, if the fault still exists, then the fault is either a loom fault, or in the Gearbox selector Switch.
I Think!!!!!!