crazy temp gauge

[tdangle]

The sending unit died so the temp gauge read stone cold all the time. Verified it was the sending unit by grounding the wire and meter pegged. Replaced the sending unit with an AC Delco replacement. Was reading crazy, would peg and then drop to 1/2 way within seconds. Replaced the sending unit again, figured I got a bad one. First I verified the resistance vs temp of water and everything seemed OK. Installed the new one same thing. Replaced the thermostat thinking it was a problem. No change. Drilled 1/8" holes in thermostat thinking I had an air bubble in the system. Seemed to work OK, but I drove it today and gauge would read 1/3 way up and within minutes or quicker would peg the gauge, than as I watched it would go back to normal. Checking the engine with a thermal gun, it is running 185-195 or so which is normal. 180 thermostat, coolant reserve bottle, good rad cap 50/50 mixture of water and coolant.

[Stocker]

From a recent thread about sporadic temp gauge readings:

Originally Posted by LockDoc

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If any thread tape or sealer was ever used on the sender when it was installed in the head, that could also cause what you are seeing....

LockDoc

[tdangle]

I would think tread sealant would cause the gauge to only read lower as it could cause a higher resistance not a lower resistance. They come with sealant already installed

[cleszkie]

Low coolant level and/or steam pockets in your water galleys will cause temp gage to sporadically peg as the steam pockets pass by the temp sender.

[tdangle]

Originally Posted by cleszkie

Low coolant level and/or steam pockets in your water galleys will cause temp gage to sporadically peg as the steam pockets pass by the temp sender.

This is the only thing that makes any sense. Just have never run across this before. Coolant level is good, guess I just see if it works its way out.

[TBONE1964]

It sounds to me like your gauge is just dying. If you take it out of the cluster and shake it, the needle will probably flop back and forth like a wet fish out of water.

If you move the needle with your finger. it should have some resistance. If it flops around, the gauge is bad.

I can help you with any cluster diagnoses you need. I also specialize in 67-72 gauge cluster restoration, repair and parts. I do have new ac delco temp gauges in stock as well as very good used OE gauges.

It sounds to me like your cooling system is working fine. The coolant would be boiling out of it if the gauge pegged due to high temp of the engine.

Temp gauge is the most common of all of the cluster gauges to fail.

Tom

[ncpetersc]

Could it be a nick in insulation on the wire to the sending unit somewhere? Maybe intermittently contacting the frame(ground) from vibration and pegging the gauge?

[tdangle]

Originally Posted by ncpetersc

Could it be a nick in insulation on the wire to the sending unit somewhere? Maybe intermittently contacting the frame(ground) from vibration and pegging the gauge?

It's possible. I'm hoping to have some time this weekend to do some testing. I'm going to place a fixed resistance to ground to the wire after unhooking it from the sending unit and see if it floats around while driving. If so then either the wire is shorting to ground somewhere or the gauge itself is flaky. If so I'll contact Tom and get a new one.