Water Temperature - Something New and Notable

[bradartigue]

When diagnosing an annoying overheating (as indicated by the water temperature gauge):

Something I've noticed now twice in the last few months, on two different cars, is something I've never seen before: the water temperature overheat switch causing the needle to partially sweep, creating a false overheat reading (and not pegging the needle).

Perhaps a better way to describe it is to describe how it works and how to diagnose it before you get thoroughly annoyed. Let's assume your car is reading higher than normal, and your fan isn't coming on. Also let's assume your car is not visibly overheating. If this is the case, unplug the overheat switch from the cylinder head - and don't accidentally ground it. If the needle returns to normal than your overheat switch is bad.

How it is supposed to work: the overheat switch simply pegs the water temp needle when the temperature gets too high. It is not supposed to do anything prior to this.

Failure state: As described above, the switch will partially engage at lower temperatures, adding a few degrees to the actual reading and causing a false reading.

Diagnosis: As described, test the water temperature with the switch disengaged (e.g. remove and isolate the connection because if it grounds you will peg the needle). Alternate diagnosis: If when grounded the wire does not cause the needle to peg then the gauge is faulty.

Remedy: Replace the faulty switch.

I've never seen this happen before and now twice in a year - I can only attribute it to these things getting old, and that sender being one few people ever bother to replace.

Hope it helps.

[hayesbd]

Brad,

Thanks for the tip. I have a '73 1592 with an in-head thermostat and my temp gauge always goes a bit right of center and then settles in to the normal position a bit left of center after a few miles. It does not deviate much after that. I am going to disconnect the overheat switch and see if this behavior changes.

Brian

[RRoller123]

I wonder if anyone has rewired the overheat switch so it lights a bright red dash warning light instead of pegging the gauge?

[So Cal Mark]

I've seen about a dozen overheat switches cause this

[SoFlaFiat]

I wonder if anyone has rewired the overheat switch so it lights a bright red dash warning light instead of pegging the gauge?

That's exactly what I just rigged, using a small LED in my temp gauge.





Hopefully I have side stepped the issue.

[RRoller123]

Makes sense!

Pete

[81SPIDERMATT]

what a great thread .... the amount of threads that are started about "car running hot" seem to pop up quite often .... and while everyone wants to help the original poster get to the issues and help ... it is very seldom that fingers get pointed to the switch or switches Brad is talking about ... and such an easy way to test that switch has been offered up .... kudos to brad ... truly thankful that he is on our side

[70spider]

Ok so after reading this post I decided to check on my temp gauges. And as per normal the PO had the sensor and switch in the wrong positions, thus the wires were crossed. So after switching the sensors to their correct positions as well as the wires (repair job was needed) I warmed up the car. The temp gauge needle went to its usual position, the 0 of the 190, I then unpluggged the over-heat switch (the black one in the front, right?) and the needle dropped to the 1 on 190. So in conclusion does this mean I have a bad over-heat switch? If so can I run the car without the overheat switch connected until I get my new one?

[seabeelt]

It doesn't hurt to change it, they are less than $10. If it is original, it will probably be a bugger to get out.... Won't hurt anything to run with it disconnected.
R/

[spider2081]

Thanks All
I checked the wire diagram for my 81 and found the green wire should go to the temperature sensor in the head. The temperature sensor should be located close to the #3 spark plug. Not sure if this is the case for other years. My temperature sensor is colored pink. The gray/black wire goes to the over temp switch. As previously mentioned this sensor is black. It is located close to the #1 Spark plug. I am mentioning this because I have read other posts concerning where the sensor and switch are located. I don't think the position in the head is as important as having the correct wire to the device.

[bradartigue]

Thanks All
I checked the wire diagram for my 81 and found the green wire should go to the temperature sensor in the head. The temperature sensor should be located close to the #3 spark plug. Not sure if this is the case for other years. My temperature sensor is colored pink. The gray/black wire goes to the over temp switch. As previously mentioned this sensor is black. It is located close to the #1 Spark plug. I am mentioning this because I have read other posts concerning where the sensor and switch are located. I don't think the position in the head is as important as having the correct wire to the device.

Agree...and they seem to be wired up all kinds of ways. I've seem some FI cars with them in the very front of the head. All kinds of variations.