I'm planning to change the sending unit first... any other common problems that could cause the temp guage not to work on these? Still a noob to C3's as of this past weekend so, I may have some dumb questions over the next couple weeks.

 

take the wire off the sender, poke a paper clip in the end, and ground the paper clip does it go full hot? if so, it's the sender. if not, it could be the power source, or the gauge itself.

 

The temp sender is a varistor that changes the voltage to ground. Unplug the sender, touch it to ground, and turn on the key. If it pegs on hot, it's the sender. If it does not move, it's the gauge/fuse.
This trick works on most all gauges.

 

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.

 

Very good post. It saved this to a word file for latter!

 

This theard was SUPER helpfull! Thank you all!

 

A couple questions....
I don't have a left side stop pin but, it rests a little below 100 with the key on and green wire disconnected. Is this the left side stop?

What's normal operating temp for an 80 350? Mine is doing better with a new sending unit but, doesn't get above 150* or so.

I didn't get to finish the testing above but, test A and B worked so I started with replacing the sending unit.

 

Sounds like it's now working, and my guess is you have a 160 degree thermostat, and cool weather.

 

I just went through the same dang thing!!!!! Read this thread before you do anything!!! Learned from the School of Hard Knocks!!


http://forums.corvetteforum.com/show...ght=temp+gauge

 

If you dont mind spending A $1 go to Radio Shack and buy a 86 ohm resistor and put it in place of the sender your gauge should read 220*