W124 M104 E320 not starting.

[Martyn_n]

I am having trouble with my '95 E320, I tried staring it for the first time in a few weeks (Battery was fully charged first) and whilst it turns over fast enough it makes no effort to fire.
I have spent many hours trying to get to the bottom of it but have drawn a blank as to the root cause.
I have checked the incoming voltage to the ECU from the two outputs of the OVP and they switch on correctly so I think I can rule the OVP out (It's quite new) (I believe the third switched output is used by the ABS only)
The wiring loom is in good condition.
The ECU does not try to run the fuel pumps, I've checked and the ECU outputs a constant 8v to the relay. The pumps run if you bypass or false feed the relay.
All the injectors and coil packs have the positive sitting on them, just nothing from the ECU.

Does anyone know if the ECU checks the health of sensors before priming the fuel system by energising the fuel pump relay or does it do this immediately upon the ECU waking? If it does a health check I need to check the various sensors but I fear that it looks as though the ECU has died for some reason.

I have spent the last few weeks working in and around the front suspension which has involved me messing around with the brake pipes under the bonnet but I cannot see that I have disturbed anything that could cause me to brick the ECU. I'm gutted as I've spent a small fortune on welding and suspension parts and when I put it all back together this happens!!

Any advice would be appreciated.

 

[grober]

ARC WELDING?--I take it the ECU was removed during this process ?
other possibility is an aftermarket alarm malfunction

 

[Martyn_n]

ARC WELDING?--I take it the ECU was removed during this process ?
other possibility is an aftermarket alarm malfunction

Thanks for the input. I've started trying to get my head around the alarm wiring, it's a right bodge job! ECU seems to be getting all the power feeds it needs so I can't see it being this but I will definitely look.
As for the welding, the car has been started a number of times since this was carried out so I guess I can rule that out.

 

[grober]

Problem with aftermarket alarms is there is no industry standard for their connection They usually come with two isolating relays which act on certain components vital for the car starting. E.g. fuel pump relay coil, starter solenoid or the OVP relay coil which disables the ECU a.k.a. the ignition. I'm afraid if it's a faulty alarm you simply have to retro trace /engineer where it cuts into the car normal electronics/wiring and restore the factory default wiring /connections. Then throw it away and fit a new one if you feel the necessity.

 

[Martyn_n]

I had this alarm fitted when I first bought the car about ten years ago. It was to replace the original, dealer fit piranha alarm that didn't unlock the car. It looks as though they have spliced it into the original alarm wires which are in turn spliced into the car's wiring. It'll take time, and I don't think its the issue here, but I will get my head around it.
If I could find out if the ECU immediately outputs a fuel pump relay feed when energised, rather than waiting to prove the health of other components I think I can quite quickly diagnose a faulty ECU (Which may be the final nail in this car's coffin!) even if I cannot explain why.

 

[ivan1983]

You mention that the fuel pumps run when you bridge the relay, I take it that you tried to start the car with the relay bridged?

 

[Martyn_n]

You mention that the fuel pumps run when you bridge the relay, I take it that you tried to start the car with the relay bridged?

Yep, tried that and no difference. Thanks for the input.

 

[grober]

http://www.autolib.diakom.ru/CAR/Mercedes-Benz/1995/E320/SYSTEM%20WIRING%20DIAGRAMS/

 

[Martyn_n]

Thanks Graeme, they are much easier to read than the ones I have been using so far. Just have to wait until the rain passes before getting back under the bonnet.
I think I must have disturbed a dodgy alarm joint when I was installing the new brake pipes so I'll have to re-trace my steps in the areas I did most rummaging!

 

[Martyn_n]

Does anyone know where I may find terminal X4/25? I cannot find its location listed in any documents I have used so far.

 

[wrdcc01]

What circuits go through it?

 

[Martyn_n]

What circuits go through it?

It's where the air flow meter gets its positive feed from the OVP, Also the point where the OVP "Clean" voltage goes towards the ECU. I haven't had time to look but I am convinced it's going to be an in-line coupler in the vicinity of the ECU. I think all the documents I use only show terminals that are physically fixed to the chassis which is why I cannot find it listed. Once I confirm this (or otherwise) I'll update the thread in case someone else does a search for "X4/25" (I doubt it will be "trending" on any search engines!)

 

[grober]

Connections

www.benzworld.org

 

[wrdcc01]

From looking at the engine control diagram I think the connector in question is one of the two large ones that plug into the ECU. I don't recall coming across a connector inbetween, when I added air con' which meant tinkering with the OVP and fuel relay. One main plug goes direct to the coils and injectors and the other direct to the other bits and bobs with the power being delivered from the battery distribution point on the inside of the front bulkhead under the fuse box entrance. The alarm maybe cutting ECU power at the connector near the battery which is mounted on the partition. Violet white which sends power to the ECU when the park/neutral switch is closed. (also supplies power to starter solenoid.

 

[wrdcc01]

Just noticed a connector symbol on the red/blue wire that connects the AFM, OVP and ECU. I guess that's the one you're after. Maybe behind the battery.

 

[Martyn_n]

From looking at the engine control diagram I think the connector in question is one of the two large ones that plug into the ECU. I don't recall coming across a connector inbetween, when I added air con' which meant tinkering with the OVP and fuel relay. One main plug goes direct to the coils and injectors and the other direct to the other bits and bobs with the power being delivered from the battery distribution point on the inside of the front bulkhead under the fuse box entrance. The alarm maybe cutting ECU power at the connector near the battery which is mounted on the partition. Violet white which sends power to the ECU when the park/neutral switch is closed. (also supplies power to starter solenoid.

I hadn't noticed the Violet / white connection, I think I can rule it out however as the starter runs and the fuel pump relay should pick before this connection becomes live and it doesn't. Thanks so much for taking the time to look into this.

 

[optimusprime]

If you have bodge on the alarm like you say,, you could hunt this down for ever and a day . Ask me why ,,i went thorough the same thing myself after pulling the winter covers of few years ago. I diid all my tricks of the trade on it with no joy. Even had to call an auto electric guy to come and check it out .I had no spark at all . It took him all of 10 min to find it - Alarm imobiliser

 

[Martyn_n]

If you have bodge on the alarm like you say,, you could hunt this down for ever and a day . Ask me why ,,i went thorough the same thing myself after pulling the winter covers of few years ago. I diid all my tricks of the trade on it with no joy. Even had to call an auto electric guy to come and check it out .I had no spark at all . It took him all of 10 min to find it - Alarm imob

I've just been looking at the alarm wiring. Starter immobilisation circuit has not actually been connected to the new alarm, they simply unplugged it from the old one, pushed the spade connectors onto either side of a fuse and wrapped it in tape! Not much to go wrong there. I just need to find if there is another circuit they have messed with.

 

[Martyn_n]

I sent an email to the alarm manufacturer for advice on which wires were the immobiliser circuits. A really helpful chap from Chorley called me straight back and described how best to rule out the alarm. Excellent service from Scorpion.
It doesn't help me though as it is now clear that the fitter didn't wire in any immobilisation circuits!
Please don't use "In Car Services" Leicester, they appear to be less than diligent!

 

[Martyn_n]

Just noticed a connector symbol on the red/blue wire that connects the AFM, OVP and ECU. I guess that's the one you're after. Maybe behind the battery.

It's located between the battery / ecu and the bulkhead towards the centre of the car. And not the cause of my problem!