any wiring gurus around, MB W202 immobiliser playing up

[berendd]

Gave my mate a hand to change the HG on his 1995 C180, all done now, but the car won't start. Starter motor turns but it won't start. lights in the mirror (red and green) flash alternately indicating an immobiliser issue. The vehicle is NOT fitted with a dealer alarm option (usual issue with these).

Reading errors with Carsoft gives an immobiliser issue, I verified this by measuring the voltage on pin 12 on the ECU and this should be 4-7 volts when the car is open, but it stays on 2,2 volts both open and closed. This means the ECU is not released as i understood.

How can this be fixed, if needed by taking out the immobiliser completely, car is not worth stealing anyway. Is there anything else in the car involved in the immobiliser proces?

I swapped in a different ECU+immobiliser from a working car but other than that the central locking now works with the key out of the other car, it still won't start. The ignition lock does not have a transponder ring.

Measured the OVP relay, and replaced it to be sure but it did not help

Any ideas? maybe a wiring diagram of this system?

attached pics of carsoft errors and the immobiliser

[berendd]

Measured the circuits of the immobiliser unit yesterday and found that the inputs are all fine, 12V and Mass where they should be... but the CAN-H is 12V instead of the normal 5ish Volts.

This could either mean that the circuit is shorted to 12Volts or that the circuit is broken so the module increases the voltage up to 12volts to try and communicate with the other ECU's on the bus, as i understand.

So will dig into the CAN wiring again to find any anormalities..

any suggestions where to look? or different views?

[berendd]

Yesterday I sat down and thought over the things we allready knew, carsoft pointed to a short in the wiring and measuring the CAN confirmed a short circuit. And I overlooked the things we did to the car since Jan, besides changing the HG.

When we removed all the ancilliaries we found the wiring looking like in the attached image, you see the wiring at the coils, which maked it a miracle the car still ran at all.


luckily i had a breaker 1996 C180 in the yard that could provide a good loom, both the loom for the coils and the engine wiring loom where transferred. However the donor car was converted to run on LPG (gas) and the injector wiring was hacked into, I resoldered this and measured everything carefully. As there are no CAN wires near the injectors I did not suspect this wiring loom at all. However i totally forgot about the plug connection (near the ABS unit) I took from the old loom and soldered onto the new one.

Took the plug apart and the measured continuity into the CAN wiring, and pronto the fault was found, the blue and green wire where switched around so the CAN wiring now ran to the coolant sensor, no wonder it wasnt working. After switchting them around and turning the key, the car started immidiately and ran perfectly, RESULT!!!

After this succes I made a testdrive to confirm the engine was doing good and went to bed.

Need to check the plug more as i suspect another wire swap in it as the coolant temperature wasnt doing anything yesterday. If anyone can provide a picture of the pin layout of this plug on a 1994 C 180, it will be appreciated. Btw i need the engine wiring harnass side, the other side wasnt touched.



Hope anyone can use this solution in the futere, hence the report back.

[Tepper]

Nice diagnosis, thanks for sharing.