Temperature gauge won't move from cold

[Brysonsproject]

Hello, I've set up this experiment with a new temperature sending unit I bought and my original gauge, so that I didn't install it then realize the sending unit wasn't the problem. I figured out where these wires should be run based on checking for continuity in all of the wiring of the temp sending unit and gauge in the truck (I tore it down to the printed circuit board to check it all. everything else is shown and explained in this short video. Thanks!

Edit: found a guy on Facebook parting out 2 trucks so I took both of his temp gauges and hooked them up to my test and the exact same result on both. I must be doing something wrong because not all 3 would have the same problem right?

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[75gmck25]

I will have to get my wiring diagram out to verify, but I think there are only two wire connections to the gauge. That third and fourth post is just a locator pin, and it should not have power applied.

try this setup
- Connect a ground wire on the sending unit body only to the battery. This simulates grounding the sending unit to the engine
- connect a power wire from the battery positive to the gauge + terminal
Gauge now has power, but is not grounded
- connect a wire from the gauge negative over to the center post of the sending unit
Now the gauge is grounding, but the ground is going though the resistance in the body of the sending unit

 

[WP29P4A]

Try a simple test. Set your volt ohm meter to Ohms, test the sender and make a note of the meter reading, check the rest of your senders, compare the readings, then warm up the senders and test the ohms again. The sender is just a variable resistor that changes with temp, gauge displays needle position based on resistance.

You also shouldn't connect any wiring to the mounting studs. Yes I am aware that you are trying to test the gauge, but testing the gauge without the proper sender is like testing a light bulb without electricity. Typically gauges get 2 connections not including illumination. 12 volt power and the sender as the variable ground.

 

[haromaster87]

Surprised no one has mentioned the ceramic resistor on the back of the gauge. There should be a white rectangular piece that is mounted between two of the posts on the back of the gauge, this is a resistor that calibrates the way the gauge reads based on the input from the sender. The resistors very often go out of spec/fail by now, or the contact points on them get so dirty and oxidized that they are no conductive and therefore no longer adding their resistance value.

A common test for the gauge itself is to just ground the cable going to the sending unit and see if the gauge maxes out. The trouble is, the gauge will still "pass" this test even with a defunct ceramic resistor, but it will not ever read above the C mark when connected normally to the sending unit.

To test the resistor, remove the two nuts securing it down and slide it off the posts, then measure its ohm value it with a volt meter. I think the one for the temp gauge is supposed to read around 85-90 ohms, but that's for an 87+ truck, not sure if the earlier gauges used the same value.

If you get no value or a strange value on the resistance test, use the pointy test probes to scrape the contact points on the resistor until you see clear metal come through, then test the resistor ohm value again while making sure to contact the points you scraped clean. If you get a good reading then, scrape the whole contact pad until it's a shiny metal color again. If you still get a bad reading, the resistor has died. Out of all the temp gauge issues I've messed with on these trucks, it's rarely been a sender/wiring issue and usually came down to something with the resistor.

 

[GTX63]

I believe most manufacturers have eliminated the resistor on the back and now sell an updated gauge with the circuitry inside.

 

[haromaster87]

Well after confidently declaring this issue to likely be related to the resistor on the gauge, I have now encountered my first temp gauge issue where the sender, signal, and capacitor tests okay, but the gauge refuses to work correctly.

Brysonsproject - can you try to test the resistance values at the gauge cluster? On mine I plugged the gauge cluster back into the truck but left the lens and black plastic shroud piece off, so I can unplug/replug the gauges as needed. With the truck running, pull the temp gauge out of the housing and you can put the COM/Ground probe on the top most contact clip for the gauge and then the other probe on the lower left contact clip to measure the resistance value being received at the gauge cluster.

I think for your year truck it goes from like ~350 ohms(cold) to ~50 ohms(full hot), so you might have to switch between 3-2 digit testing for resistance. But otherwise, you should be able to see the resistance value start high and slowly come down as the truck warms up. This will at least tell you if your sensor and wiring are getting the signal all the way to the cluster.