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