Reversed polarity
 

Hi, I did something really stupid and by mistake reversed polarity when installing new battery on 87 300td.....got a big spark for a moment when wrong terminals touched....


After that incident I lost completely any tachometer reading and ABS light is now on (was not before).
I checked all the fuses but could not find any burned and also not the one that would be related to the instruments.


Would maybe someone know where I should start to troubleshoot this; what would be the first most likely thing that would burn from reversed polarity and make Tacho and ABS stop working?


Thanks a lot, Robert

Reversed polarity most likely cooked/fried the tachometer and possibly the pickup, same goes for the ABS module and sensors.

You’ll probably find that the alternator winding is fried too, along with a multitude of other electrical parts (the Starter and blower motor will be fine though)

The OVP relay may have done its job and saved some of your electronics. Look for a blown fuse in it's top, or replace the OVP relay if it's cooked.

Expect some failed electronics in things like the cruise control amplifier or HVAC system. I strongly suspect someone connected a battery backwards in my SDL at some point in its life due to the unusual number of cooked electronics I've repaired in that car.

Hi, great, thanks, did not even think about OVP relay! Puled it out and yes, the fuse is burned. Changed the fuse and all is back, working perfectly!
Thanks a lot; I guess I was really lucky!

Really glad to hear about that for you.


Lesson learned (I hope) with out any serious consequences!

Glad you were lucky. Thanks to the good old MB engineers.

Dodged the bullet this time ;) .