92 Peterbilt 377 Gauge Problems

Those older trucks aren't multiplexed to the degree they are today. More than likely the 3rd temp sensor is for the engine ECM.

I'd look at fixing your gauge first. Your fan may be fine but you just aren't getting the engine hot enough for it to kick in. At least if the gauge works you can ballpark the coolant temperature to see if its actually hot enough to get the fan to kick in.

Did both gauges fail at the same time? If so I'd check things that are common to both gauges such as supply voltage to the gauge in the dash.

 

Easy to test on our old engines. Pull the sensor and put a 10 dollar heat gun on the sensor. They get up to almost 600 degrees. Unfortunately for the oil sensor you have to weight till next oil change. I bought a new oil temp sensor regardless and hooked the wires up to it before I changed the oil. Low and behold with the heat gun, the temp gauge in dash went up pretty quick. So there you have it. Coolant temp sensor you can probably pull and not lose coolant. Or just plug the hole and test. Test the sensor in the thermostat housing

 

Your third coolant sender will be for the high water temp light. Single wire normally open switch. Ground the wire should turn the light on to test the circuit to the light. Sender wont close til probably 230-240*.

 

Not stupid. Yeah you gonna have to pull sensor. Shouldn’t be any pressure on cold engine. Just plug hole with shop towel twined up to size of hole. A little may weep out but it’s only gonna take you less than one minute with the key on to see if your gauge registers the heat

 

What I use to test temp sensors on the whole truck when I have a gauge not registering. Must the time it winds up being a ground wire. Turned out to be the sensor in oil pan for my oil temp gauge not registering
1500 Watt Dual Temperature Heat Gun 572°/1112°

 

If you just have a single wire going to a temp sensor, which I think most do, take the wire off the sensor and with the wire disconnected your gauge should be all the way down. Then ground the wire and it should go all the way up. If not you have a wire or gauge problem. Someone correct me if I'm wrong