W124 M104 E320 not starting.

[Martyn_n]

No, your fault is either the ECU or that alarm system.

I could only find evidence of the alarm circuitry in the starter motor solenoid feed and the engine turns over so I have come to the conclusion that it must be the ECU. I have checked the resistance between the battery positive terminal and all of the relevant contacts of the ECU (with the OVP strapped out) and they are all really low resistance so there isn't any sign of an immobiliser open circuit within them.
Do you think there are still places that can diagnose / fix an ECU of this vintage or is it a case of sourcing one from a breakers?

 

[Martyn_n]

I'm coming to the inevitable conclusion that it is the ECU. I could only find evidence of the alarm in the starter solenoid wiring and the engine turns over ok. I have checked the resistance of the ECU feed wiring from the battery positive to the ECU plug coupler contacts (via a strapped out OVP) and it's less than 0.5 ohms for pins 27 and 39 so there is clearly no open circuit immobiliser wiring in the way.
Do you think there are places that will be able to diagnose / repair ECUs of this vintage or is it a case of finding one at a breakers?

 

[Martyn_n]

Sorry, duplicate(ish) post!

 

[carat 3.6]

I use BBA Reman for any suspect ecu's I get in the workshop. They will test it first, but if they do a repair the test fee is free.

If you do need an ecu ring one of the proper merc breakers, don't get one from ebay. Also you want to match your part number if you can, not all 320 ecu's are the same.

ASR, cruise control, and age of the car all make a difference to which part you can and cant use.

 

[wrdcc01]

What if any diagnostic flash codes did you get when/if tested? Worth picking up an ECU from a breaker. Nice not to have the bother of coding and other component matching nonsense of newer cars!

 

[grober]

What if any diagnostic flash codes did you get when/if tested? Worth picking up an ECU from a breaker. Nice not to have the bother of coding and other component matching nonsense of newer cars!

http://www.diakom.ru/CARS2/pribor/cs1000/manual/cs1000_mb.pdf

 

[Martyn_n]

I use BBA Reman for any suspect ecu's I get in the workshop. They will test it first, but if they do a repair the test fee is free.

If you do need an ecu ring one of the proper merc breakers, don't get one from ebay. Also you want to match your part number if you can, not all 320 ecu's are the same.

ASR, cruise control, and age of the car all make a difference to which part you can and cant use.

Thanks for the link, I did suspect that it may be a minefield of different part numbers depending on spec. I don't have ASR but it does have cruise and ASD which may be an odd combination.

What if any diagnostic flash codes did you get when/if tested? Worth picking up an ECU from a breaker. Nice not to have the bother of coding and other component matching nonsense of newer cars!

I haven't done this, do they tend to help much?

http://www.diakom.ru/CARS2/pribor/cs1000/manual/cs1000_mb.pdf

Thanks for the link.

 

[carat 3.6]

Thanks for the link, I did suspect that it may be a minefield of different part numbers depending on spec. I don't have ASR but it does have cruise and ASD which may be an odd combination.

ASD is a independent system so dont worry about that.

Cruise can be a problem. On a non cruise car the throttle position signals are sent directly to the ecu like any other car, but for whatever weird reason on a cruise equipped m104/m111 this signal is routed through the cruise ecu to a different set of pins on the engine ecu.

As such if you fit a non cruise ecu into a cruise equipped car, the ecu cannot see throttle signals and the engine will cut out when you try to rev it.

 

[Martyn_n]

ASD is a independent system so dont worry about that.

Cruise can be a problem. On a non cruise car the throttle position signals are sent directly to the ecu like any other car, but for whatever weird reason on a cruise equipped m104/m111 this signal is routed through the cruise ecu to a different set of pins on the engine ecu.

As such if you fit a non cruise ecu into a cruise equipped car, the ecu cannot see throttle signals and the engine will cut out when you try to rev it.

Thanks for clarifying this. Is it possible that my problem may be related to the Cruise ECU / throttle body rather than the main ECU? I am aware that the wiring can be fragile on the throttle body and it is not something I have tackled. Whilst looking into this I have noticed that the throttle body makes a whirring sound when the ignition is on (Or the OVP bypassed) It may always have done this but I don't normally have my head under the bonnet when I start the car!! I have presumed that it would still start but have a poor idle if the fault lies with the cruise ECU / TB actuator.

 

[carat 3.6]

Thanks for clarifying this. Is it possible that my problem may be related to the Cruise ECU / throttle body rather than the main ECU? I am aware that the wiring can be fragile on the throttle body and it is not something I have tackled. Whilst looking into this I have noticed that the throttle body makes a whirring sound when the ignition is on (Or the OVP bypassed) It may always have done this but I don't normally have my head under the bonnet when I start the car!! I have presumed that it would still start but have a poor idle if the fault lies with the cruise ECU / TB actuator.

If the fault was in that system the engine will still start, it might run badly but it will attempt to run. A noise from the throttle body is normal, it's normally a whine when the ignition is on but engine not running.

The tell tale with your problem is that the fuel pumps dont prime when the ignition is turned on, that means either the ecu is not getting the power/grounds it needs or the ecu itself is at fault.

 

[Martyn_n]

If the fault was in that system the engine will still start, it might run badly but it will attempt to run. A noise from the throttle body is normal, it's normally a whine when the ignition is on but engine not running.

The tell tale with your problem is that the fuel pumps dont prime when the ignition is turned on, that means either the ecu is not getting the power/grounds it needs or the ecu itself is at fault.

That's great thanks. Thank you so much for sharing your expertise on this.

 

[Williamwoo]

I still have the original ECU for my car. I replaced it about 3 years ago because it stopped the engine revving above 3000rpm (and I took a risk in replacing it as I didn't have the time to do any proper diagnostics at the time). I took a chance and replaced it with one I got off Ebay from some guy in Rumania of all places iirc - it solved the problem and has worked perfectly ever since.

If you want my old one, you're welcome to it and if your car then works up to 3000 rpm, it will prove that it is your ECU that is the problem. (It might even work properly and let the car rev to over 3000!). As others have mentioned, there are many different versions, depending on spec - my car is a '95 4-speed and does have cruise, but I'm afraid I don't know what ASD is. I also don't know what other factors might affect it but, if you're interested, I'll dig it out and let you know the serial number.

 

[Martyn_n]

I still have the original ECU for my car. I replaced it about 3 years ago because it stopped the engine revving above 3000rpm (and I took a risk in replacing it as I didn't have the time to do any proper diagnostics at the time). I took a chance and replaced it with one I got off Ebay from some guy in Rumania of all places iirc - it solved the problem and has worked perfectly ever since.

If you want my old one, you're welcome to it and if your car then works up to 3000 rpm, it will prove that it is your ECU that is the problem. (It might even work properly and let the car rev to over 3000!). As others have mentioned, there are many different versions, depending on spec - my car is a '95 4-speed and does have cruise, but I'm afraid I don't know what ASD is. I also don't know what other factors might affect it but, if you're interested, I'll dig it out and let you know the serial number.

Wow, that would be really helpful. I can't think of a better way to prove it's the ECU. I believe that my part number is A0165456032 (or 0261203492), but I have not removed the unit from the car yet to confirm this.

 

[wrdcc01]

Martyn did you test the fuel pump relay yet as it should be working on your 8 volt output?

 

[carat 3.6]

I still have the original ECU for my car. I replaced it about 3 years ago because it stopped the engine revving above 3000rpm (and I took a risk in replacing it as I didn't have the time to do any proper diagnostics at the time). I took a chance and replaced it with one I got off Ebay from some guy in Rumania of all places iirc - it solved the problem and has worked perfectly ever since.

If you want my old one, you're welcome to it and if your car then works up to 3000 rpm, it will prove that it is your ECU that is the problem. (It might even work properly and let the car rev to over 3000!). As others have mentioned, there are many different versions, depending on spec - my car is a '95 4-speed and does have cruise, but I'm afraid I don't know what ASD is. I also don't know what other factors might affect it but, if you're interested, I'll dig it out and let you know the serial number.

That should work fine.

As regards the 3000rpm rev limiter that is a feature of late w124's, the ecu will not let you rev the engine past that when the car isnt moving to protect the gearbox. It uses a speed signal from the abs system to disable it, allowing the engine to rev freely.

There is a 3 pin plug hidden near the battery which this circuit passes through, the pins in the plug corrode breaking the circuit to the abs, so the engine ecu engages the rev limiter.

 

[Martyn_n]

Martyn did you test the fuel pump relay yet as it should be working on your 8 volt output?

I have tried two relays in the socket and neither pick. The 8v was with no relay in the socket so I suspect that with a relay coil in circuit this is pulled down and the coil never actually has 8v on it at all.

 

[Williamwoo]

Wow, that would be really helpful. I can't think of a better way to prove it's the ECU. I believe that my part number is A0165456032 (or 0261203492), but I have not removed the unit from the car yet to confirm this.

Would you believe it - both those numbers are the same as mine!



Carat 3.6, yes I am aware that there is a rev limiter that operates when in neutral although, from memory, I think it cuts in in 'pulses'. Next time I take the car out, I'll try it (I don't like starting and running an engine for just a few moments). My issue was that I was simply prevented from going above 3000 (quite smoothly) and, in any case, replacing the ECU solved the problem without dealing with the hidden plug that you refer to. As the plug is near the battery it must also be near the ECU and I suppose I could have inadvertently nudged it and improved the contacts when replacing the ECU but as that was over 4 years/20,000 miles ago it seems unlikely that the issue would not have recurred by now.

 

[Martyn_n]

Would you believe it - both those numbers are the same as mine!

View attachment 108034

Oh, I do hope mine is definitely the same. I just need six inches of snow to thaw off the bonnet so I can remove it. Are you happy to post it in the current climate?

 

[Williamwoo]

Oh, I do hope mine is definitely the same. I just need six inches of snow to thaw off the bonnet so I can remove it. Are you happy to post it in the current climate?

Yes, happy to post, especially as we're probably at least 150 miles apart, and I'm not sure plod would call it an essential journey! Let me know your address.

 

[Martyn_n]

Yes, happy to post, especially as we're probably at least 150 miles apart, and I'm not sure plod would call it an essential journey! Let me know your address.

Great, I'll PM you when I have checked the serial numbers. I was quite looking forward to picking it up from France though!!