W124 M104 won't start
 

Hello everyone, having issues with '92 W124 M104.092 not starting.

I have the infamous wiring harness problem it seems- I changed the plugs a couple of weeks ago and the car ran ok, but a couple of times was slightly hard to start. Then yesterday it took two tries to start, and the car felt slow (like my old 136hp diesel). Got it home and took a look at the wiring to the coils from the ECU, crumbling to bits. I'm handy enough with the soldering iron so I make a new harness up to just past the engine cover where the old wires aren't too bad- now I have no start.

Fuel pump is ok, plugs were wet after repeated tries to start. Changed to brand new, clean plugs. Starter is fine and engine turns over ok. Changed coils over for good spares I had, and back again. All electrical connections are ok and in correct order. I have 12v at the coil feed wires.

Any clues?!

I'd replace the entire wiring harness. Some mechanics won't bother diagnosing drivability problems unless the entire engine wiring harness is replaced.

It's strange that the problem is on your 1992. I thought the problem was chiefly limited to 93 to 95 models.

My 'bad'... it's a '93, keyboard slip up. Anyway, got her started again, but wiring harness replacement is on the list.

Yes, I was going to say your sig states 93. Anyways, what did you do to get it working again or did it just start up on it's own?

I'd check the wiring again. A simple continuity check from sensors / actuators to the ECU plug would be a good start.

Have you made attempts at getting some fault codes?

It was a simple case of the wiring for the coil packs being the wrong way 'round- the wiring from where it enters the top of the engine to the coil plugs was so bad I couldn't see the colours on the feed wires, so found someone online who had listed the wiring colours and coil order, which turned out to be completely wrong for my car!
I tore into the old coil harness a bit more (it's encased in moulded rubber) and managed to find some bits of colour in the wires to check the correct order.

Haven't checked for codes but seems to be running fine now.