First to fill you in on what I've done to my 79 F150 - 351M truck -- I've known for a while my temperature gauge usually only went just barely into the normal range on the dash gauge, so thought my thermostat might be sticking open because I had a 192 degree thermostat . I have taken the dash apart and installed all led lights in the dash, also put in a new cluster voltage regulator and headlight switch, all the dash lights are working and looking great! Also installed a Retro Sound radio. After doing all that I had not started the truck, but today I also installed a new 192 degree thermostat and took the truck for a ride. Now everything is working except my temperature gauge is not moving at all. I'm looking for suggestions on the problem. Could it be the printed circuit on the back of the dash was damaged, my sending unit on the engine just went bad, or the gauge went bad?? Any help would be GREATLY appreciated!!! Not looking forward to having to take the dash all apart again if that is needed!!

 

Well don't do that just yet.
As a first and easy test, I would (with the key OFF at first) remove the temperature sender wire at the engine, and ground it to a known good grounded bit of metal.
If you still have the original 90° push-on connectors, you'll likely want to put a bit of wire inside and ground that. Or a screw, or other appendage thingy.

Go into the cab and turn the key to ACC and watch the gauge.
If it goes up immediately, how far does it go? Just watch for a minute or less and then turn the key back OFF. Not sure what all could be harmed by leaving it on, but I just always got in the habit of leaving the key in ACC (or especially in the ON position!) as little as needed when testing the full sweep of a gauge.
If you get full sweep, then it would seem that everything in the system except the sender is working. Replace (or test) the sender. But make sure it's getting good contact with the engine and is not too rusty or has too much sealant on it.
If only a partial sweep, or none at all, then you have more digging to do. Unfortunately...

The senders on our engines are not always a happy thing to access either. It's on the front, facing forward, down in an area that's often hard to get to with all the factory stuff still installed.

Good luck.

paul

 

Thanks for the idea, I'll try it sometime this coming week. Do you know if the printed circuit on the back of the dash has anything to do with the temperature gauge or is it mainly just for the lights?

 

The PC does provide power & a path to the sender for the temp gauge.

I'd start as Paul recommended above before doing anything else.

I've had the connector for the sender rust & fall apart, the connector for the sender get knocked off the sender while tinkering under the hood, & the sender go bad. The gauges themselves are very reliable in my experience.

 

And has been reported many times, the original PC boards deteriorate and turn to dust on a regular basis. So it could be just about anything having it's way with your gauge reading. But I like to start with the simple stuff and hope for the best.
Works out as often as it does not.

Paul

 

Thanks for the reply! Yep I'll try testing the sending unit first, from what I've learned it's in kind of a tight place to get to, I just hate to have to take the dash apart again if the problem is with the dash or gauge !

 

I want to thank everyone that replied to my original post!! This Forum is great and has a lot of information!
I did what was first mentioned to me, which was ground the wire for the temperature sender wire to see if my gauge needle moved ( it went all the way over to the right). So I thought I needed a new sender but when looking at the plug that fastens to the sender I noticed the inside metal part looked rusty. I took a very small sanding tool from my Dremel tool and with my fingers kept rotating it on the inside metal part to clean it up. The stud on the sender looked ok, I did spray it down with some WD 40, I put some dielectric grease on the inside of the plug after cleaning it and put it back on the sender. Started the truck up and pulled it out of the garage and let it warm up, the temperature gauge is working perfect now!!
Before the gauge would just barely go into the normal range so I thought my thermostat was stuck open so put a new one in. Now I wonder if the original thermostat was working but just not getting the right reading from the temperature sender, I left the new thermostat in the engine and now when warm the needle is right between the E and M on the temperature gauge working perfect!
Thanks for the info, as I wasn't looking forward to having to take the dash apart again!!

 

A great success! No more taking the dash out. At least for now! :-)
Even when these things were less than 10 years old, those 90° plugs would often get a little corrosion (It is a pretty rough environment after all), but also come loose.
The seam in there was pretty flexible, and they did seem to just come loose quite often. I probably had to take a pair of needle those pliers and gently squeeze the contact points inside several times over the years.
There’s probably a great tool we could come up with just for that so we don’t crush it if we get a little ham-handed with it.

 

I was going to say the same thing, they come loose.
Guy in the 80 truck area had the same thing happen, reading low or not at all and was a loose 90*

I keep forgetting to check mine, 300 six so also had to get to.
As my gauge also reads low and have replaced the stat 2 times, forgot I did it the first time
I had also replaced the sender as the gauge did not work at all when I first got the truck.

Good to hear it was somewhat an easy fix.
Dave ----