The temperature gauge in my 79 vette is stuck in the red (hot) area and will not move. I've tried disconnecting the temp sending unit and starting the car to see if the gauge would return to zero, but no luck. I've also checked for loose connections on the around the temp gauge, there were none, and the printed circuit board on the back is less than one year old. Could this be the temp sending unit? Any suggestions on trouble shooting would be greatly appreciated.

 

I would start with the temp sending unit first. It's common for them to keep a solid reading like that when they go bad.

 

wait a minute. If you disconnected the sender and it had no effect, what makes you think its the sender? Ground the wire at the sending unit and check to see if the gauge moves. Check the resistance on the temp sensor with the engine cold and hot. If it is different, the sender is working.
Check the gauge with the ohmmeter to see if it is open, if its open its bad (gauge)

 

yea, especially when they're disconnected...

 

If the temp guage is pegged in the red, this indicates a dead short somewhere. When you disconnected the sending unit and the guage still was pegged, you have shown that either (a) the wire from the sending unit to guage is grounded out or (b) the guage has gone bad.

Best bet at this time is to remove the wire from the back of the guage and see what happens. If the guage then shows no temp then your guage is probably OK.

Next run a new wire from the guage to the sending unit. If the guage returns to working normal, then bingo...you've shown that the original wire is grounded out somewhere.

 

Thanks for the replies! I disconnected the temp sending unit and turned the key hoping that if the sending unit had failed the gauge would return to zero. My Vette has the gauge cluster with the printed circuit board on the back. All the other gauges (oil pressure, fuel, volt meter are working) and I believe there is a common ground for the cluster by way of that printed circuit board. I'll try checking the resistance and check to see if the gauge wire got grounded somewhere.

Thanks guys,
Bob

 

Right on the money!!!

 

Guess I wasn't thinking clearly on my original post. Since your's is a 79, you have a rather complicated plug in that connects several wires to your instrument cluster. When you unplug the wires, if you carefully look at the cluster then you can see which line of solder runs to the temp guage. Trace this line of solder back to the plug contact. Then you can determine which wire in the plug is the one for your temp guage. After making this determination, simply use a good volt meter to see if the wire is being grounded after you've disconnected it at the sender.

 

Here's the procedure to diagnose temp gage problems:

Tests
A. Disconnect the green wire at the sending unit and turn on the ignition switch. The gauge hand should stay against the left side stop pin.

B.Ground the green wire disconnected from the sending unit. With the ignition switch still "ON," the gauge hand should swing across the dial to the right stop pin.

Corrective Measures
If the gauge hand does not stay to the left as in test "A", either the wire is grounded between the dash unit and the engine unit or the dash unit is defective.

Test further by disconnecting the dark green sending unit wire at the gauge. Turn on the ignition switch. If the gauge hand stays on the left hand stop pin, replace the disconnected wire. But, if the gauge hand still moves, replace the dash gauge.

If the gauge hand does not swing across the dial as in test "B", there is an open circuit in the wire between the sending unit and gauge, the gauge is defective, or current is not reaching the dash gauge.

Test further by grounding the sending unit terminal (dark green wire) of the dash gauge and turn "ON" the ignition switch. If the gauge hand now moves, replace the disconnected wire. If the gauge hand still does not move, connect a test lamp to a ground and to the power (pink wire) terminal of dash unit. If the lamp does not light, test the wire between the ignition switch and the dash unit by connecting a test lamp to the "Accessory" terminal at the ignition switch and to a ground. The test lamp should burn.

If the gauge hand operates correctly as in tests "A" and "B" but the gauge does not indicate temperature correctly, either the sending unit is defective or the dash gauge is out of calibration. Replace sending unit one of known accuracy. If gauge reading is still incorrect, replace dash unit.

If the gauge hand is at maximum at all times and test "A" and "B" indicate that the wiring and the dash unit are in condition, the sending unit is bad and needs replacement.

If the gauge hand will not move, the dash unit is bad, or incorrectly installed. Correct the installation or replace the gauge unit.