Temp gauge not working, tried the old tricks

Post Mon Mar 08, 2021 1:34 am

Hi , Working with a 1990 325i. Temp gauge does not seem to function. Worked through all the threads and measured resistance changes to the infamous brown plug / temp sender and had signal connectivity up to Pin 26 on the C1 ( blue) connector on back of gauge cluster. Pulled cluster and verified grounds. Nut is tight, and I measure connectivity from the temp gauge ground nut to the fuel gauge ground nut. Seems like pin 26 does not go to temp gauge. The trace on the circuit board from 26 doesnt go anywhere near the temp gauge. Ohm'd all pins in C1 and pin 6 goes to the ground nut and measures short. Measuring the top pin of the gauge seems to connect over to pin 3 on C5. Lower pin seems to go to pin 6 on C5. If I pull the gauge and connect top pin to positive of a battery, and lower pin to negative, the gauge will pin hot. What am I missing? All help appreciated!
here is the schematic I have been working with: https://www.e30zone.net/e30wiki/images/ ... luster.pdf

Post Wed Mar 10, 2021 1:16 am

follow up, saw a post where the signal for temp actually goes into the Si board on pin 7 and back out on Si board pin 3 up to temp gauge. somewhere there was a video of someone soldering a jumper onto the main cluster board from pin 7 to 3 and removing the Si board. Can one just remove the Si board and bypass it? Thanks

Post Wed Mar 10, 2021 2:30 pm

Have to disagree with you. Pin A26 of the instrument cluster has a copper track going directly to one of the temp. gauge pins, via a single wire link
Temp. gauge gets its power from the SI board, so won't work without it.

Post Fri Mar 12, 2021 2:40 pm

Hi Brian, visually and electrically I don’t have that trace on my board. If you happen to have your cluster out and can take a pic, I would love to compare it to mine. I did place the jumper as described on the si video and my gauge does now work but I lost the efficiency gauge function, but at least I know my gauge is ok and signals are good.
Saw your post for 30%. Gauge deflection. Did you try pulling the gauge and connect ends to a 9v battery and check for full range motion?