Auto check battery symbol keeps coming on

[Zeus]

T26a/15 puts out 6.35V with a quarter tank of gas, so thats working like it should, and pin T26a/16 puts out 12V as expected, the fault isnt in the wiring, its in the autocheck panel itsself... and I actually don't see how the problem could be caused by the fuel gauge system?

I tested this with the panel that DOESN'T give the battery symbol, I'm now going to test it with the faulty board.

[Zeus]

Results with the other circuit board are the same... 12V and 6.35V

[Nuvo]

from what I can tell from the schematics, I shouldn't be getting +12V there... T26a/15 crosses the line with fuse S12, it's not connected to it, as far as I can tell, it's just connected to the fuel gauge sender G, which connects to mass, on the other side it is connected to a voltage regulator through the fuel gauge. So I cant get any power there with the cluster disconnected while measuring. But my fuel gauge is working fine so I guess the voltage regulator is OK too.

I agree with you that the purple/black wire from T26a/15 is connected to the fuel gauge sender and not to S12.

I meant to say T26a/16, not T26a/15 sorry the text is a little unclear for me. As you will see this uses a black/blue wire to S12 via C15a on the relay board.
Sorry about that.

I'll try and measure this (T26a/16 to which T5c/2 is connected) in a moment, but T5c/2 isn't an output from what I can tell on the schematic, its just +12V feed for the autocheck I think??

Yes, I agree T5c/2 is most likely to be a supply for the AC, I was also thinking about the possibility that it could be used to monitor battery health too? Hmm, I'm not sure about this.

I would have thought that T5c/1 will do the health check via the alternator regulator though as mentioned before.

thanks for the help btw

You're welcome.

[Zeus]

The blue wire for the alternator is on T26/13 on my car...
I just measured it and it gave the same signal with the working and with the faulty battery symbol circuit board, so that's working fine too I guess...

Problem must be in the circuit board itsself I guess, but I can't spot any burnt parts or anything...

[Nuvo]

The alternator connection changes from T26/13 for 3B and T26/21 for ABY.
I've noticed you have a 1991 3B so that makes sense. Here is a more relevant schematic showing this important difference.

You could try comparing the voltage at the input port of the mcu which carries the signal from the alternator. If the voltages read the same then I would suspect the mcu to be at fault. If they are different, then trace back to find the faulty component(s).