Testing ccluster coolant temp guage

[coldweatherblue]

My temp gauge has been acting funny for a while, fluctuating, running >3/4 for 10 seconds then fluctuating back to 1/2 way. I've replaced/bled nearly every hose/sensor/radiator/fan etc in the cooling system and from what I can tell it is functioning properly (good pressure, no leak, cool lower hoses, warm upper hoses, fan works, etc).

I've taken a temp of the thermostat housing when the car was above the 3/4 mark several times and the external engine temp is always around 168-170 degrees, which should be within internal normal limits.

This morning it's about 15 degrees outside and as I was pulling in my driveway my temp gauge went all the way over to the red, pegged to the right. The valve cover was cool enough for me to keep my hand there without being burned, there was no coolant overflow, and the temp at the thermostat (via infrared thermometer) was 170.

Does this sound like the dash gauge? I've bled the car many times and have never gotten air out of the system. The coolant reservoir never overflows even though the gauge sometimes gets close to red for ~15 seconds.

I know my cluster was replaced by PO and I have to hit the dash for the temp gauge to work anyway, so I'm kind of assuming it's bad.

Just wondering how I can verify this before buying another cluster or taking it apart, etc. Anyone know a way to test the coolant sensor resistance and the temp it should correlate to? What are normal thermometer temps at the thermostat btw, does 170 sound about right?

thanks

[jlevie]

What you describe sounds like a short in the sensor or wiring between the gauge and the engine.

[dnguyen1963]

When a temp gauge behaving badly, I always check the SI board/batteries and the ground nut. If everything looks good then proceed with what Jim posted.

[coldweatherblue]

Are you aware of any method to test the temp gauge by measuring resistance across the engine coolant temp sensor? it would make sense that if I knew the correct calibration of the dash gauge I could measure the temp sensor and correlate that with expected position of the dash gauge.

From what I've read 1/2 on the gauge is about 180-190 internal engine temp, do you know to what resistance across the coolant temp sensor this would correlate?

[jlevie]

The calculated sensor resistance around that point is:
178.75F - 68.39 ohms
186.88 - 59.68
195.00 - 52.26

[Gregs///M]

The temp gauge sensor should read around 1000 ohms at room temperature.

The gauge pegging to the red would indicate that the brown/violet wire is shorting to ground.

I would start by making sure both washers are installed on the temp gauge, the 7mm nut is tightened down, and the cluster main board has enough solder to make contact.

This is bad...


This is good (I added solder)

[Gregs///M]

Also, if you have never replaced the SI board batteries, it will give you inaccurate temp gauge.

[E30NJ]

Didn't they also have a bad ground issue?