Temp Gauge Help!!

[JoePolo]

So I am 80% sure I have a bad gauge. Fired up a truck that hasn’t run in a very long time. I had the cluster out at one point to repair blank/vacuum gauge hole and add a clock there since someone had killed the intermediate tin.
Temp gauge is pegged hot, I can see the needle. The sensor was not even hooked up at any time testing. I have not ohm gauged the sensor wire yet. I hooked up a ground to the gauge with alligator clips. All no change. And the gauge is pegged with the key off, so new gauge??? I have other clusters to steal a gauge out of, hoping I can do it from underneath, my seat it out and I would rather not pull the cluster again.

[pjmoreland]

Did you observe the needle move to the pegged position when you first started your truck? The winding that pushes the needle to the right has its circuit completed by the sender which you don't even have hooked up. This indicates the sender wire or the gauge post on the passenger side of the gauge that the sender wire leads to is shorted. I have seen it happen where the gauge is mounted crooked in its mounting holes which can cause one or more of the mounting posts to short out on the metal mounting plate.

If you're flexible, you should be able to get back behind the dash to remove the temperature and battery gauges without removing the cluster. Not sure it's easier that way though.

[JoePolo]

Originally Posted by pjmoreland

Did you observe the needle move to the pegged position when you first started your truck? The winding that pushes the needle to the right has its circuit completed by the sender which you don't even have hooked up. This indicates the sender wire or the gauge post on the passenger side of the gauge that the sender wire leads to is shorted. I have seen it happen where the gauge is mounted crooked in its mounting holes which can cause one or more of the mounting posts to short out on the metal mounting plate.

If you're flexible, you should be able to get back behind the dash to remove the temperature and battery gauges without removing the cluster. Not sure it's easier that way though.

I will test the sender wire to make sure it’s not grounded. But my mind says why would it not return once the key is off even if it was the sender wire. I will have to look at some pictures of the cluster when I was working on it to see if it was always pegged. I did not watch it go to the pegged position.

[Rich69shortfleet]

The needle could be stuck. You might try very gently pushing it slightly to the left.

[pjmoreland]

Originally Posted by JoePolo

I will test the sender wire to make sure it’s not grounded. But my mind says why would it not return once the key is off even if it was the sender wire. I will have to look at some pictures of the cluster when I was working on it to see if it was always pegged. I did not watch it go to the pegged position.

That's a good point. Temp gauges do generally return back to over toward the C side in a lethargic manner.

[JoePolo]

So when I finished the cluster it was definitely not pegged hmm, still wondering why it will not go back down even with battery disconnected and key off.

[pjmoreland]

Nice looking cluster

[JoePolo]

Thanks I just hope I can fix this water temp gauge without pulling it again. It’s so weird that the gauge will not return without power. Almost like it’s stuck.

[JoePolo]

Originally Posted by Rich69shortfleet

The needle could be stuck. You might try very gently pushing it slightly to the left.

Thanks yah I confirmed it was over to the left when I built the cluster. So maybe having the sensor disconnected when I started the truck freaked it out.
I’m going to attempt to do some more diagnosis tonight. I was so happy to just get this truck running finally.

[pjmoreland]

Did your temp gauge work correctly in your truck before you pulled the cluster to add the clock?

[JoePolo]

Originally Posted by pjmoreland

Did your temp gauge work correctly in your truck before you pulled the cluster to add the clock?

No idea, I bought the truck taken apart since the 1990s. The clock is an isolated circuit, does not run through the cluster except using the tin as a ground. Has it's own dedicated power wire. I could disconnect it to be sure, but highly doubt it's back feeding some how.

[pjmoreland]

I'm wondering if the dash wiring harness is for a gauge cluster with warning lights instead of gauges. You can identify it by looking at the connector that plugs into the back of gauge cluster. If the harness is for a warning light cluster, then the wires in terminals 1 and 12 (both at one end of the connector) will be dark blue and gray. If the harness is correct for your full gauge cluster, then those wires will be black with a white stripe and black.

A warning light harness places a ground wire on the terminal that would then be running to your temp gauge. The power wire, though, would be running to the cluster metal housing/ground, so the fuse would blow. I wouldn't expect the gauge to move at all in this scenario.

[JoePolo]

Originally Posted by pjmoreland

I'm wondering if the dash wiring harness is for a gauge cluster with warning lights instead of gauges. You can identify it by looking at the connector that plugs into the back of gauge cluster. If the harness is for a warning light cluster, then the wires in terminals 1 and 12 (both at one end of the connector) will be dark blue and gray. If the harness is correct for your full gauge cluster, then those wires will be black with a white stripe and black.

A warning light harness places a ground wire on the terminal that would then be running to your temp gauge. The power wire, though, would be running to the cluster metal housing/ground, so the fuse would blow. I wouldn't expect the gauge to move at all in this scenario.

I will double check that tonight, since today I was searching sending units, and the one in the truck, appears to be the one for a light. This cluster was in the truck when I got it and has a factory tach with factory tach wiring, not an add on. And it was hooked up in this truck because I had to disconnect the oil pressure tube. I will double check those colors though this evening when I am under the dash. Also I would assume if the fuse blew I would lose the other gauges, such as the fuel gauge, which is definitely still working but I will check the fuses as well. Thanks for that info on the wiring.

[pjmoreland]

If the fuel gauge is working, then the wiring is probably correct. If a previous owner repinned the gauge cluster connector to work with the full gauges, then the dark green temp sender wire might still be routed to the ignition switch for checking the warning light when cranking the engine. This connection would cause the gauge to peg to the right when cranking. My '68 had been converted to full gauges by a previous owner. It still had that ignition switch connection, and the gauge would peg like that. I pulled the terminal with the two dark green wires out of the ignition switch connector and taped them up.

This still wouldn't explain why your gauge is pegged permanently. It very well might just be stuck mechanically like Rich69shortfleet suggested.

[JoePolo]

Originally Posted by pjmoreland

If the fuel gauge is working, then the wiring is probably correct. If a previous owner repinned the gauge cluster connector to work with the full gauges, then the dark green temp sender wire might still be routed to the ignition switch for checking the warning light when cranking the engine. This connection would cause the gauge to peg to the right when cranking. My '68 had been converted to full gauges by a previous owner. It still had that ignition switch connection, and the gauge would peg like that. I pulled the terminal with the two dark green wires out of the ignition switch connector and taped them up.

This still wouldn't explain why your gauge is pegged permanently. It very well might just be stuck mechanically like Rich69shortfleet suggested.

Yah that is what I am leaning towards something is stuck. I just don't seen a wiring explaination to it not returning once there is no power.

[PbFut]

Things are too far apart to check for you, and I may be remembering wrong. But I sort of recall my temp gauge not returning to zero upon turning off truck. When I would return after a cool down sit, gauge was still in a warm position. Turn key to on position or start the truck and the temp needle would move to cold position. I am about 50/50 I am recalling this correctly. Could be the years of cannabis eroding things.

[JoePolo]

Originally Posted by PbFut

Things are too far apart to check for you, and I may be remembering wrong. But I sort of recall my temp gauge not returning to zero upon turning off truck. When I would return after a cool down sit, gauge was still in a warm position. Turn key to on position or start the truck and the temp needle would move to cold position. I am about 50/50 I am recalling this correctly. Could be the years of cannabis eroding things.

Ahhh I have another truck I can test it on and see if it returns once you turn the truck off.

[pjmoreland]

Here's a video I made a while ago that shows how the needle behaves when you move it around by hand.

https://youtu.be/pK8lQ0DFHOo?feature=shared

[pjmoreland]

There are two sets of windings in the temp gauge that push the needle in opposite directions. The sender is connected to the winding that pushes the needle toward Hot. The resistor on the back of the gauge is connected to the winding that pushes the needle toward Cold. Maybe you've got a bad resistor or oxidation on the resistor pads so the needle is never getting pushed to Cold.

[JoePolo]

Originally Posted by pjmoreland

There are two sets of windings in the temp gauge that push the needle in opposite directions. The sender is connected to the winding that pushes the needle toward Hot. The resistor on the back of the gauge is connected to the winding that pushes the needle toward Cold. Maybe you've got a bad resistor or oxidation on the resistor pads so the needle is never getting pushed to Cold.

Great info and great video, it showed basically that it doesn't return once you disconnect the power as I thought it should. It stays where it was when you shut it off. This is awesome for my tests. I have a spare cluster I will steal the gauge out of and bench test it.

I also need to see what's up with the sender in the truck, it has a spade terminal which from my research is for the warning light not the gauge.

[dmjlambert]

Yes, that's the incorrect sender if it has a spade terminal. There's only one sender type for the stock gauge, and it is for a 1/2 NPT hole. This is the one I use: https://www.amazon.com/gp/product/B000C9SM22/
If you need the sender to fit a 3/8 NPT hole, you can buy the 1/2 NPT sender and have a machinist turn and re-thread it. There are a couple forum members here offering that service. I had forum member A1971Blazer turn and thread mine. Other people had it done by brian mac.

[JoePolo]

Originally Posted by dmjlambert

Yes, that's the incorrect sender if it has a spade terminal. There's only one sender type for the stock gauge, and it is for a 1/2 NPT hole. This is the one I use: https://www.amazon.com/gp/product/B000C9SM22/
If you need the sender to fit a 3/8 NPT hole, you can buy the 1/2 NPT sender and have a machinist turn and re-thread it. There are a couple forum members here offering that service. I had forum member A1971Blazer turn and thread mine. Other people had it done by brian mac.

It's in the stock location of the head so I should be okay, I just ordered one from NAPA. Of course I just filled the cooling system yesterday. Maybe if I do it really quick hahaha

I have eliminated that from the equation with it unplugged but still need to change it.

[dmjlambert]

If the radiator cap is off and engine cool not much coolant will come out. I just unscrew the old and plug the hole with my thumb while I'm picking up the new and start threading it. You may lose 1/4 cup of coolant at most. I use about 2 wraps of teflon tape around the threads, it will not prevent the sender from grounding.

[JoePolo]

Originally Posted by dmjlambert

If the radiator cap is off and engine cool not much coolant will come out. I just unscrew the old and plug the hole with my thumb while I'm picking up the new and start threading it. You may lose 1/4 cup of coolant at most. I use about 2 wraps of teflon tape around the threads, it will not prevent the sender from grounding.

Thanks.

[JoePolo]

Doing some tests, the sender wire is not shorted, it has resistance does not ohm at 0. The gauge on the back both studs have 12V at them and the ground lug is grounded (ohms to 0.00). Not to sure now other than stuck gauge