Fluttering temp gauge

[Varinn]

Standard story around these parts, but I've checked the usual stuff. Gauge seems to sit steady when at highway speed right on the middle mark, it reacts to bumps in the road with a flutter. Lower speeds it goes wild but tends to sit near red, I have OBD2 diagnostics on my S52 and live data shows temps the near the same (highway vs low speed) so it's a reading problem, not an actual temp problem. Sender was replaced during motor rebuild recently (I didn't drive the car after the rebuild until recently), I suspect it might be faulty as old sender = no issue, new sender = issue.

Nut on the back of the gauge is good, plenty of solder on the connection point. The blue harness connector also appears to be totally intact. SI board is functional and I have no other gauge issues.

I pulled the connector for the sending unit and the gauge plummets and parks itself at full cold.

Jump a wire from the connector pin to a ground point and the gauge spikes to full hot. It does wiggle quite a bit at this point, but that could just be the contacts on my jumper wire not being the best, I will test again today with a better wire and ground to be sure.

With the connector plugged in I tried wiggling the harness right at the connection body all the way back to my C101 with zero noticeable gauge movement.

What should the resistance be on my sender? I'll check that next. With the way it tends to sway to the high side it has me thinking the sender or the gauge itself is bad, if it was a wiring issue most likely case would be poor connection which would cause it to indicate cold would it not?

[AndrewBird]

If you have exposed wiring somewhere, it could be shorting to ground and causing it to jump to red like you mention. I'd check for continuity between the plug and ground when it is reading in the red.

[Varinn]

If you have exposed wiring somewhere, it could be shorting to ground and causing it to jump to red like you mention. I'd check for continuity between the plug and ground when it is reading in the red.

Good thought, I'll add that to my list of things to check tonight. My only thought on that is it didn't seem to move in the slightest bit with the sender connection pulled off, it was flatlined at full cold. If i had a wiring short would it not show up even with that sort of situation?

[Varinn]

So I checked a few things this afternoon

Engine off key off
- Resistance from the sender pin to the cyl head is about 55-65 ohms at operating temp. Slowly increases as the engine cools
- resistance from backprobing the c101 pin and the cyl head is exactly the same at 55-65 ohms (wire is ok up to c101 engine side)
- resistance across the wire from c101 to the sender's connector is 0 ohms

Engine off key in position one
- ohms drop immediately by about 5 on all measured points versus key off

Engine off key position 2
- ohm readings showed no continuity at all measured points between sender, head, wire, etc. I triple checked at all points and multiple places on the head. Suspect this is part of the problem
- removed and cleaned main engine to chassis ground points
- resistance reads 0 ohms between measured points, gauge appears semi accurate with engine off and fan off. Voltage about 2.8v @ 95celsius measured through torque app

Engine on
- gauge immediately jumps 1/4 -1/2 scale higher than the EoK2 reading
- resistance remains the same at 0 ohms
- aux pusher fan running causes immediate 1/4 gauge rise in gauge reading, drops again when fan stops

[Gregs///M]

Sender should be 1000 ohms at room temp. If your Si board has never been refurbished, it will need to be to have a reliable and accurate temp gauge. Same goes for the temp gauge itself and the main cluster board.

In our refurbished clusters, our temp gauges never flicker and are always accurate. Good luck

[Varinn]

Thanks for the input, I had kinda somewhat come to the same conclusion. I was talking with an old friend of mine about this and he's a big e30 guy that builds electronics boards for a living, graciously offered to repair my board for me. I'll go through these steps first off and see if it solves the problem.

[dnguyen1963]

Thanks for the input, I had kinda somewhat come to the same conclusion. I was talking with an old friend of mine about this and he's a big e30 guy that builds electronics boards for a living, graciously offered to repair my board for me. I'll go through these steps first off and see if it solves the problem.

You'd better off going with Greg. He knows E30 gauges and clusters.

[Varinn]

My guy helped me fix it up, all is good now. Problem was a shitty ground connection between the motor and the chassis. Cable is new and was well routed, clean contacts and everything. Turns out I have powdercoat between my motor mount arms and the block that was interrupting current flow to the arms so it wasn't even properly reaching the cable (1.6v drop). I relocated the cable to the mounting holes for the ac compressor and now everything is good.

Gauges are rock solid.