I have a 1987 D150 with the 5.2L 318. I’ve been having problems with the temperature gauge. The gauge is always running low. Sometimes when I start the truck, the gauge will jump the mid-range (normal temp) then back down to almost nothing. I believe the gauge works, so I think the problem is with the coolant sensor.

I’m having problems selecting the correct sensor. When I select my truck on auto websites it shows me several different sensors. I attached a photo of my current sensor (at least what I think is the sensor! This is my first older vehicle I’m working on). If anyone could please help select the correct one I would appreciate it.

 

Coolant sensor will have one wire, and I believe it's on the other side of the upper radiator hose.

 

There is nothing on the other side of the radiator hose. This is the only sensor I could locate. I replaced the thermostat a few months ago and did not see anything on the other side when I removed it.

 

They look like vacuum lines. How long have you owned the truck? I wonder if the PO didn't pull the sensor and plug the hole.

This is the sensor with the wire attached.

This is what the plug in looks like, and the sensor not plugged in.

 

That helps a lot. Then what did I take a picture of? I will check when I get back to my truck. My Haynes manual had the sensor in a different spot completely so I’m all confused.

 

So I identified the problem.... I couldn’t see the sensor because there is no sensor. There’s just a hole. I located two loose wires that look like they were cut near where the sensor should be.

 

I wouldn't think it would have 2 wire sensor but maybe. I'd try to see what the wires go to. The coolant sensor wire should move the needle if it's grounded.
Is the sensor broke off?? It should be leaking coolant if it's just an open hole.
Your truck doesn't happen to have the lean burn system does it?? Big black box with wires on the side of the air cleaner.

 

that is smog related stuff that you took a picture of. the coolant temp sensor should be right need the heater hose on the intake. 1 wire. the gauge one is always there unless you have an aftermarket intake. the computers one is usually on the drivers side by the upper rad hose.

 

Upon further inspection I do not believe the sensor is missing. I checked earlier quickly at work, but I believe I was wrong. Here are pictures of what I found under the hood. I still believe my original photo is of the sensor.

 

That's a vacuum switch, not a sensor.