Tachometer & fuel gauge wiring under carpet

[l0gistaja]

Evening. So I've been doing some rust repairs on my car lately and thus have removed the carpet from the car. After the carpet was removed, the car stood outside for like 2 weeks with no battery connected. Now when I tried to get her started again, the tachometer is dead. The fuel gauge is alive but the needle is always on empty (when there was no fuel in the tank, the little indicator light lit up, it's gone now that I've put some fuel in). Don't know about the speedo tho, haven't moved the car.

I made a video to explain myself better (notice the fuel gauge - just put 10 liters of fuel in).

https://www.youtube.com/watch?v=L07CEAzURGA

Could it be some wire under the carpet? Those wires were in a quite bad way when I dug them out, covered in mud and whatnot. There's also an unplugged molex connector coming from under the dash. Any ideas?

[l0gistaja]

Okay so it turned out that the fuel gauge was just pessimistic. I added 10 more liters and now it shows that I have around 15l in the tank so it's all good.
The tach is still dead tho, si batteries are at 2.8 volts. Checked the black & rd/gn wires on the blue connector too, both show good values. Could the si board be dead? One other thing I just remembered - I removed the service lights from the board. Could that cause an issue like this? Gonna put em back on tomorrow to test it out, I'll keep you updated.

Cheers

[Gregs///M]

Yes, your SI board batteries are dead. They should be at 3.2V. Once the batteries drain, the 20+ transistors on the circuit board will go into rapid switching on and off and will die also. Ever hear about people smacking the dash to make the tach and temp gauge work? That's the transistors failing to switch. You will need to have the SI board refurbished.

[XcvX]

If you refurbish/replace batteries on SI board and it still fails to correct the problem, then its worth looking for the signal feed between the dash clocks and the ECU. The ECU pumps out a square-wave signal to the dash, so you can't check it with a regular multimeter, but you can check that the signal wire is in good condition between the clocks and the ECU. This is a black wire that, on Motronic 1.3 vehicles, runs from ECU pin 6 through the C101 plug pin 7 to the dash cluster's blue plug pin 7. Most likely, there is corrosion on the C101 plug which is affecting the signal, or the C101 isn't screwed together properly.

For older vehicles, especially those using Jetronic, there is a secondary plug next to the ECU which carries the tachometer signal to the instruments known as C104.

And as much as I'd like to be this knowledgeable, I found that writeup here: http://www.e30zone.net/e30zonewiki/i...ommon_Problems

[l0gistaja]

Thanks for the responses.
I'll be getting a different cluster tomorrow. We'll see then if it's the wires or not. I would try replacing the batteries but I can't seem to find them anywhere but eBay and I don't have enough time to wait for them to ship.


Cheers

[jeffnhiscars]

Ive got an 87 eta that I bought as a non runner with wiring issues. Its running great now but I have no tach even w new SI batteries. I did swap the known good cluster from our other 87 ETA and still no tach so there's a signal feed issue somewhere.

Any thoughts.

[MCreally?]

I cut some of those wires when i cut into the rusted sections of the floor pan. i will be paying attention to this post. i am starting to wonder if they are what is making my DME connections haywire

[Jean]

Yes, your SI board batteries are dead. They should be at 3.2V. Once the batteries drain, the 20+ transistors on the circuit board will go into rapid switching on and off and will die also. Ever hear about people smacking the dash to make the tach and temp gauge work? That's the transistors failing to switch. You will need to have the SI board refurbished.

Transistors don't exactly fail from batteries being low voltage . They may not work if they are not getting enough voltage to work but they don't "fail" if you mean they go bad .