Temp Gauge Is Not Working

[Powerhouse Ranch]

Temp gauge is not working, pegged past hot. Never moves.
-New gauge
-New wiring
-New temp sensor
-New cluster circuit board

Any thoughts/tests I could perform? Thanks guys.

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[bucket]

Damn, that's really pegged. The sender signal must be contacting ground? Maybe put power and ground to the gauge (out of truck, using truck's power), with no signal wire, to verify it's not a faulty gauge. Then if it seems ok, jumper the sender wire to the gauge directly, still uninstalled in truck.

Is it pegged with the ignition off too?

 

[Powerhouse Ranch]

This is what I just put in it, looked identical to the original that I took out. I have proper grounding of the block and cab.

CHEVROLET OER 748691 OER Coolant Temperature Sending Units | Summit Racing

Free Shipping - OER Coolant Temperature Sending Units with qualifying orders of $109. Shop Coolant Temperature Sensors at Summit Racing.

www.summitracing.com

 

[Powerhouse Ranch]

Damn, that's really pegged. The sender signal must be contacting ground? Maybe put power and ground to the gauge (out of truck, using truck's power), with no signal wire, to verify it's not a faulty gauge. Then if it seems ok, jumper the sender wire to the gauge directly, still uninstalled in truck.

Is it pegged with the ignition off too?

yes, pegged with the ignition off too. This is the second gauge now. First one was borrowed from a spare cluster we had laying around. That pegged and we bought an LMC one - the one pictured.

 

[bucket]

Is the first one still stuck pegged? Maybe verify the operation of that one since it's out already.

 

[Powerhouse Ranch]

Is the first one still stuck pegged? Maybe verify the operation of that one since it's out already.

That's good thinkin. I'll try that out.

 

[bucket]

With power and ground on the gauge, no sender wire, should be cold. Grounding out the sender terminal should put the gauge to hot. But I don't think it should show THAT hot.

 

[edgephoto]

Remove the connector and measure the resistance of the sender to ground. Then connect the sender and check the resistance at the gauge. They should read the same.

Have you verified you have a sender and not a switch? They look the same but the sender varies resistance with temp and the switch changes state at a preset temp.

 

[bucket]

Remove the connector and measure the resistance of the sender to ground. Then connect the sender and check the resistance at the gauge. They should read the same.

Have you verified you have a sender and not a switch? They look the same but the sender varies resistance with temp and the switch changes state at a preset temp.

I thought if a switch was installed, the gauge would not read, unless the engine overheated. At which time, the guage would peg on hot.

I've seen a sender installed on an idiot light before. As the engine got up to temp, the idiot light would gradually illuminate and stay dimly lit during normal operating temperature.

 

[Powerhouse Ranch]

Remove the connector and measure the resistance of the sender to ground. Then connect the sender and check the resistance at the gauge. They should read the same.

Have you verified you have a sender and not a switch? They look the same but the sender varies resistance with temp and the switch changes state at a preset temp.

tis a sender, yes

 

[zrtwooo]

I'd love to know how you resolve this as i have the same situation with my 82...

 

[Sad Sack]

Are the threads supposed to be bare on the installed sender for a ground and not to be taped or such? Looks like some goo was used.

 

[edgephoto]

I thought if a switch was installed, the gauge would not read, unless the engine overheated. At which time, the guage would peg on hot.

I've seen a sender installed on an idiot light before. As the engine got up to temp, the idiot light would gradually illuminate and stay dimly lit during normal operating temperature.

You are correct.

 

[edgephoto]

Are the threads supposed to be bare on the installed sender for a ground and not to be taped or such? Looks like some goo was used.

Easy to rule out. Connect a jumper wire from the metal hex part to ground. Then check operation.

 

[75gmck25]

The temp gauge circuit is very simple.
- 12 volts applied to one gauge terminal when ignition is on
- other gauge terminal connects through cluster wiring to the green sending unit wire in the engine compartment, which is clipped on to the sending unit terminal

- sending unit is a resistor that varies resistance based on temperature of the engine
- low temp/ high resistance in sending unit - gauge needle moves a little, but reads low
- high temp/ low resistance in sending unit - gauge needle moves higher
- sending unit body/threads must be grounded to the engine to complete the circuit, so do not wrap threads with sealer tape

For testing
- if the sending unit wire is grounded (zero resistance), gauge needle goes to max
- you can use small resistors (like from the old Radio Shack), connected between the sending unit wire and an engine ground, to simulate the 0-max ohm resistance and see what the gauge reads. Max resistance is quite high (maybe as high as 3000-4000 ohms, or more) not the 90 ohms max for a fuel sending unit.

The resistance “profile” (what resistance matches each gauge reading) of the sending units has changed over the years of the squarebody, and some of the early Delco sending units have been discontinued. The connector on the end of the sending unit has also changed ( button vs. pin, etc.). It may take some time to find a sending unit that is the correct match to your gauge.

I found only a fairly close match to my ‘75 SBC sending unit, since the original Delco part has been discontinued.