Temp gauge problem

[brackie1]

I am really frustrated with my temp gauge. It is 20 degrees hotter than actual engine temp. I've re-built the cooling system, new low temp fan switch, new low temp thermostat, and new temp gauge sensor. Any ideas on what is going on.

[Odoyle]

Check the temperature sending unit which threads into the top of the cylinder head.

[brackie1]

I just put a brand new one in and it is doing the same thing.

[Odoyle]

Have you tried installing the old temp gauge with the new sending unit.

[DanD]

Where exactly does it sit on the gauge ? They usually sit just to the right of center.

Mine reads higher when the lights are on.

[Odoyle]

Well if the operation of your headlights is changing the reading on the gauge, it sounds like some electrical issue, exterior to the gauge itself, is interfering with the reading. Do you have a wiring diagram?

[courtenay]

While my car is an '80 rather than a '78, it has two temp gauge sensors. Did you change them both?

[gschwab]

If your car has two sensors in the cylinder head, maybe they are connected wrong, or installed in the wrong place. One of them is an actual temperature sensor, the other one is a switch that sets the gauge to "hot" once a certain temperature is reached. Maker sure they are hooked up right, there is a resistor somewhere in that circuit that might mess up the temperature reading. Check out the wiring diagram for your car!

[brackie1]

Ok here's the thing. I did not replace the gauge it's the gauge I have always had. This is somewhat of a new problem that started a few months back. The two sensors in the head are wired correctly according to my shop manual. I have tested that. I may just be making a mountain out of a mole hill. The old senor which I installed back in June did fine for a while then the difference started to creep up until it was almost 40 degrees different than head temp. This new one has started out with a 20 degree difference.

[brackie1]

This an add on post. To answer one of the questions, the temp gauge reads 190 while the head temp at the sensor when measured with a thermal gun is 170 or so. Which is about right because my fan switch is a 170 and it comes on when the head temp is 170. I have tested the gauge by dead shorting the sensor and it goes immediately to 260 which is the way I have been told to test the gauge.

[gschwab]

This seems to be a really odd problem, if it were a bad connection (higher than usual resistance in the signal path), the gauge would read lower that it should. Have you checked the system voltage to make sure it is correct? If the voltage regulator on the alternator was bad, that could lead to increased voltage and a higher than normal temp reading.

[spider2081]

There have been some instances where the over temp switch ( the other sensor in the head) has caused your symptoms. When the engine is at temp and the gauge is reading higher than it should have you unplugged the wire on the other sensor to see if the correct reading is then displayed.

[azruss]

Does the gage reading change when other electrical is on, like the lights. If so, then you have a weak ground. This is usually in the ground connection at the gage itself, but can be internal as well. would also be worth cleaning up the ground to the instrument cluster. As with most fiat electrical issues, start with the grounds first.

[brackie1]

I drove it today and it stayed slightly above 190. I stopped to checked head temp and it was rea ding around 170. My fan had not come on and it is a 170 degree switch. There is no change with other electrical items on. I guess I will just live with it since I know what the differential in temp is. But it is puzzling.

[DieselSpider]

What is the reading where the sensor is bolted on? It is possible that there is a 20 degree variation between the head and the fitting where the sensors are mounted.

Any chance that the system could need to be burped a bit better or that it needs a good white vinegar flush to get rid of some hard water scale built up over the years?

I know I am guilty of skipped a flush and stretching things longer than the standard 2 years but re-mediate with a good complete flush using a 50/50 mix of distilled water and white vinegar before refilling with new coolant and since I live in Semi-Tropical Florida I use a Water Wetter type agent to enhance thermal transfer. I have been using the Lucas product lately and though I was skeptical at first it does make a noticeable difference in reducing extreme temperature swings.

I have a SUN Capillary Tube temperature gauge which eliminates electrical gremlins from the equation. You could put one on temporarily using the same location for the probe and see if the readings are different.

Mine is similar to this one from Dorman: