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Erratic temperature gauge

316 Views 4 Replies 3 Participants Last post by  Arthur Dent
Hi all!

The other day I noticed my temperature gauge was being erratic. After having driven for a little while (engine was at operating temp according to temperature gauge), I parked waiting for some family and let the truck idle. I noticed the temperature gauge dropped steadily towards cold, and as it got closer to "full cold", the needle started jumping erratically. As soon as I drove off, the temperature needle came back to where it usually sits and stopped jumping around.

Is this the thermostat failing?
Where does the temperature gauge get its input from? Is it the thermostat, or is there another temperature sensor somewhere that is going crazy?

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Don't know your 3.1 engine, but most temp sensors operate much the same way. There are much like bridge circuits where one of the resisters is placed somewhere in the cooling circuit where it's subjected to changes in the temperature of coolant and it responds with a change in it's resistance which then unbalances the bridge and this results in current flow thru the meter. A change like your reporting is probably not thermostat related but sensor related. There may be a bad connection at the sensor. I would be inclined to start at the sensor, even maybe changing it. Check for corrosion at the connection and assure all grounds are clean and connected.
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Thanks for the quick reply. Makes sense. I was able to identify the sensor that feeds the gauge and it’s..well..a little dirty..I’m sure the connection can use a clean. This post was helpful ([3.1D ] 4JG2 Temperature sensors)

Will report back once I get a chance to look at it. Cold and snow in the forecast for the next days, so may not be anytime soon 🥶
This is a symptom of a bad wire connection or a broken wire.
Perform a shake test on the harness and connectors. When you see the gauge jump you are really close to pin pointing the fault.

Itsmehb is correct that it is a bridge.

More specifically the coolant temp gauge circuit is called a Wheatstone Bridge. This 150+ year old circuit is the basis of all comparative analog and digital measuring devices.
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Apologies for the tardiness in getting back to this thread. A quick jiggle test revealed the fault lies in the connection at the sensor, at the thermostat housing. Also, very dirty there.
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