w124 300E-24 no sparking - help!!

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Oct 29, 2005
Amersham, Bucks
1997 W140 S600, 1980 Porsche 928 4.7 Manual
I'm normally pretty adept at sorting out mechnical/electric problems, but this time I'm stumped. I've also learnt over the years not just replace electronic parts until I can prove that they are dead first (especially when very expensive!).

Anyway, here is the history:
I go to start the car... the engine turned over put didn't start which it's never done before. So first off I repeat again listening for the fuel pump which came on ok. All the dash lights looked correct (this is the M104 engine but a 1989 model with no diagnostic light in the dash). Next I checked sparking by removing one of the plugs. Nothing. So then I move straight to the coil. No output here either. Next I remove the coil altogether and check it over. All looked fine and the resistances of the primary and secondary coils all checked out fine. I gave it a quick clean before refitting.

Next I did some research. I've got schematics, read lots of forums etc etc. I found that as this car has got a 9pin diagnostic connector I can connect pin 3 via an LED and 10K resistor to ground, then by grounding this pin 3 for 3-4 seconds a diagnostic flashing code will be visible. This sounded like it would reveal the problem. Looking at my schematics pin 3 does go straight to the ECU. Anyway, when I try it, I just get a constant dim glow from the LED, ie. no diagnostic output at all. Is this in itself enough to be confident that the ECU needs replacing?

I've also been advised to check the OVP (over voltage protection) relay with the fuse on top - this seems to be fine, certainly the fuse is ok. I tried reseating too.

Does anyone have any other ideas at all?
when you crank the engine over does your Tacho move?
Then I am quite sure that you can discount the ECU. The OVP's have a habit of going duff. Not only the fuse but the soldered connections inside it. See if you can borrow one to try.
Thanks for the advice Ian. I'll try to get hold of one in the next couple of days.
do you have an alarm sytem/immobiliser fitted?? If faulty the immobiliser may isolate the OVP coil preventing operation.
A good point Graeme, I didn't think there was, but looking under the dash on the drivers side is a security lock which implies that there is. Problem is that I don't have a key for this so can't open it up. Also I can't see any evidence of an immobiliser within the engine bay... I should have another OVP relay tomorrow, but in the meantime I can check the voltages in the relay socket to see whether the relay has a chance of operating.

I'm not sure if this could be the problem, but this went on my w123, it the Ignition Control Unit. I had the same symtoms as you descibe, with no warning, etc. I had to replace it. (with a second-hand one)

Its number 5 in the attached illustration.

Good Luck.
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I agree with Mangoman, try the Ignition control module (not cheap). I would see if you can swap it iver with someone elses to make sure its that.
A new one of those in the dealers is

£1600+vat :eek:
I'm really hoping it's not the ignition control unit for obvious reasons... If the OVP doesn't do the trick then I guess that's next on the list. Unfortunately I don't have access to another to swap with to find out - does anyone know a method of knowing for deterimining whether one of these is definitely the culprit.. i.e dead?
Actually - wouldn't i be able to read a diagnostic code to tell me that the ignition control module is dead?
The ignition module is basically an ecu that triggers the LT side of the coil just like points used to.

Disconnect the module and connect 12v to the coil +ve terminal, and a piece of wire connected to the -ve terminal. Scratch the end of the wire over an earth connection. This should cause a HT spark if the coil is good.

The ignition module will be fed from a low voltage hall effect sensor or reed switch in the ditributor (assuming it has one).
By checking the inputs of the amplifier you should be able to deduce if the output transistor is switching to ground or not.

In most ignition amplifiers the fault is the high voltage output transistor goes dead due to the voltage spikes. If the fault is the output transistor you may be able to buy one and solder it in.
The detail on the EZL looks incredibly useful - even when I do get everything running I'll be getting my scope out to check the condition of the whole ignition system - thanks very much!
I had the same problem on the 300 24v and it turned out to be the ignition pack. I also have a 300CE 24V and if you want I could let you try my ignition pack at a small price.
I'm based in London near Heathrow.

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