Help! W202 C180 unable to figure out what's wrong


New Member
Oct 5, 2020
Hi all,

This will be a long post, but I've tried everything and now it seems like I'm just stuck. A bit about the car, it is a 1997 C180 W202 series Elegance 5 speed automatic UK edition. Bought the car in perfect working order. As soon as I got home the car started having a shaky idle and it was running rough. I've checked the spark plugs, coils, cleaned throttle valve and mostly everything related to fuel mixture was checked & cleaned but nothing changed.

Soon after I've noticed that the car's fuel consumption is way too high and black smoke is coming out of the exhaust. Decided to buy an OBD -> OBDII adaptor to see what errors I can get from the ECU. I was surprised to find only 1 code "036 Fuel adaptation (lambda) control, mixture too rich (lean stop)". At that point I replaced the O2 sensor (this model apparently has only one?) but symptoms are still there and noticed that the auto gearbox started to shift rough and in high RPM but not over 4000 under normal acceleration. Changed the gearbox oil alongside with conductor plate and plug but still symptoms were the same.

So now it was time to look at the injectors side. I've checked the resistance of the injectors and all was well. However, when checking the injectors harness voltage to ground with the ignition on (engine off) on all the plugs both pins had positive (+) voltage. One pin was reading 12v and the other pin, which I can only presume it is the ground, was reading 6.2v. Here was the point where I started doing a bit more research on this model and what sort of electrical issues it may have. And... I stopped looking online because it would require changing the entire car to solve all known electrical issues on this model :)

To give it a shot and take on a process of elimination. I've traced the continuity of the injector harness to the ECU then measured the harness resistance with the ECU unplugged. Test passed and excluded the harness. Now, knowing the pins on the ECU I've measured them with the ignition on (engine off) and found 4 pins that were giving 5.8v and a resistance to ground of 513ohms. Took the ECU off the car happy that I thought I found the culprit. Measuring the resistance with the ECU off the car everything was fine. So here we go again.

ECU back on the car and started removing plug by plug and kept measuring. Until I found this one plug...that after I remove it, there is no longer a positive (+) voltage on the ECU pins and no longer a 513ohm reading. Now, with that plug aa...well unplugged, ignition on (engine off) I measured again injector harness and found that on one pin it was 12v and the other was 0v! As it should! However, car will not start with that plug removed and alarm lights (under the rear-view mirror red and green LEDs) have an alternate blinking.

I have attached 2 pictures. One with the fuse box where the plug sits and one with the B2 plug that is in question. Does anyone know what that plug is for? And, what might be causing the voltage spike on ground?

Cheers for any helping hand! I nearly have half the car dismantled at this point :)


  • Fuse box with B2 plug.jpg
    Fuse box with B2 plug.jpg
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  • Plug B2.jpg
    Plug B2.jpg
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