W124 coupe no start.

[laverdakeith]

A little help if anyone can please. I have a 124 220 coupe 1993. A while back I had a no start.... so...
Initial investigation revealed a very crumby engine loom which I have now repaired. Still no start, Thought it might be an alarm issue, so removed that entirely and reinstated the factory wiring. Still nothing, engine cranks but no fuel pump or spark, I have checked crank trigger and all seems OK.
Starting to think the ecu has become damaged by all of the shorts that must have been in the engine harness but got to admit I'm just guessing now.
Any ideas most welcome as would a link to a wiring diagram, searched the Google net far and wide for a diagram but so far nowt...
Thanks in anticipation.

Keith

 

[optimusprime]

Keith First chyeck all fuses in the fuse box . Find your conveniance relay one with fuse on the top green in colour ...
Your fuel relay is right behind the battery. You need to remove the battery and the black plastic cover in order to retrieve it. It is located and mounted by the side of your ABS module or HFM module.
The relay is Green one with a fuse on the top...You could plug your aux fan relay, stage I (BLUE) or stage II (GREEN) (at the rear side of your fuse box right in front of your windshield) to test. Make sure you use the correct fuse amperage while testing.
Good luck.

 

[laverdakeith]

Keith First chyeck all fuses in the fuse box . Find your conveniance relay one with fuse on the top green in colour ...
Your fuel relay is right behind the battery. You need to remove the battery and the black plastic cover in order to retrieve it. It is located and mounted by the side of your ABS module or HFM module.
The relay is Green one with a fuse on the top...You could plug your aux fan relay, stage I (BLUE) or stage II (GREEN) (at the rear side of your fuse box right in front of your windshield) to test. Make sure you use the correct fuse amperage while testing.
Good luck.

Thanks for reply. All fuses check out OK as does pump relay. Don't seem to have a convinience relay, only other square type relay is a cream coloured one in the back right hand corner of the relay box behind the fuse box. Checked crank position sensor and have AC coming from that. As stated I have carefully rebuilt the entire engine loom including coil sub loom and servo throttle unit using new conductors. The old looms were in a shocking state and obviously had been for some time, car was always a bit rough at idle but always started and ran. I have volts here and there around the engine so something is getting through but without a wiring diagram I'm a bit in the dark. Is it possible the multiple dead shorts that must have been present in the old loom have damaged the ECU ? I did have to have the ECU rebuilt in one of my old 280 wagons after engine harness replacement on that.

thanks.

Keith

 

[grober]

You may have to rebuild/replace the OVP RELAY which powers the ECU the ABS unit and the fast idle unit. Here is a couple videos to show what's involved and what the OVP relay looks like.
https://www.youtube.com/results?search_query=REBUILDING+OVP

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[optimusprime]

All here but quality is very poor Mercedes W124 Wiring Diagrams

 

[laverdakeith]

Thanks guys. had the OVP apart and is as new inside, fuel pump relay the same..everything looks to be in great condition, apart from the crumbly wires which are now all gone, I can operate the fuel pump relay manually and the pump runs but still no spark on cranking ???
We were to be lending the car to a niece this weekend for her bridesmaids transport, looks like they are on the bus unless any other suggestions..
Tried the link to the diagrams, not sure if it is my old eyes but they seem illegible. I bought a cd rom workshop manual for one of our other cars, does anyone supply simmilar for late w124's ?

thanks.

Keith

 

[grober]

The M111 four cylinder is basically the same as the M104. It has no distributor and a twin coil lost spark system firing 1 and 4 and 2 and 3 together. The coils are normally live at 12 V and are fired by power transistors in the ECU earthing them causing a fall in the magnetic field of the coil primarys. Its the HFM type system and the ECU power transistors can be replaced at reasonable expense if that is the problem.
The two coils are hidden away under the inlet manifold and so difficult to spot.

 

[laverdakeith]

I have 12volts at the coils but still no spark. Long time back very occasionally when starting the car I would get no fuel pump. If I got out and locked the door unlocked and tried again it would go. This was long before I replaced the engine wiring and removed the alarm., I kind of assumed it was an alarm problem however when recently removing the (Scorpion) alarm in my quest to get the thing going again I could find nothing connecting the alarm circuitry with the fuel pump, it seemed to immobilise by disabling cranking.
Is my next course of action sending the ECU away ? Any recommendations for ECU menders ? I'm in the UK.

thanks.

Keith

 

[ToeKnee]

Most of the coils that I've dealt with are permanent live but switched through earth.

 

[grober]

I have 12volts at the coils but still no spark. Long time back very occasionally when starting the car I would get no fuel pump. If I got out and locked the door unlocked and tried again it would go. This was long before I replaced the engine wiring and removed the alarm., I kind of assumed it was an alarm problem however when recently removing the (Scorpion) alarm in my quest to get the thing going again I could find nothing connecting the alarm circuitry with the fuel pump, it seemed to immobilise by disabling cranking.
Is my next course of action sending the ECU away ? Any recommendations for ECU menders ? I'm in the UK.

thanks.

Keith

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