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'94 940T Wagon Died, Tachometer fluctuated rapidly, Fuel Injection Relay buzzing

posted by  walczyk on  Mon Dec 19 12:58 CST 2022
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last visit: Mon May 12 14:20 CST 2014
[RELATED]

I was driving on the highway when all of a sudden, the tachometer started going from 0 to red line, while the engine itself started to stumble. I started to hear an awful buzzing from the fuse box behind the ashtray and I pulled over where it finally did die. The relay keeps buzzing, whenever the car is in KPII, and the tachometer gets pegged to 5500rpm. I tried replacing the relay, but the problem remains. I unplugged the RPM sensor, no change. I started to look at the wiring diagrams, I can attach photos of the pages if that can help. I was having trouble understanding the diagram for the dash; in particular the input to the tachometer, there are two pins labelled with the same number, but I think the tachometer signal is coming from the ECU. My current thinking is that something is shorted and is sending voltage into the ECU, and/or the ECU is now fried. I also should try again at understanding the fuel injection wiring diagram and figure out what is making the relay go nuts. Lastly, I tried the onboard diagnostics but when I plugged in the bullet connector into socket 2 I noticed the actual red obd light to turn on dimly, and the OBD system is not working, no codes are coming out, not even 1-1-1.

Has anyone experienced this? What should I start unplugging first? Thanks for all your help as always.






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