Temp Gauge Not Working

[mattyd7]

My '79 Spider has a problem with the temperature gauge. It seems like no matter how warm the engine gets, the gauge stays all the way to the left. After it gets really warm, if I flick the gauge with my finger, it will jump up a tiny bit to the first line (120F) and stay there.

If I ground the wire to the temp sender, the gauge jumps right up to max temp. If I ground the wire to the overheat sensor, the gauge jumps to max temp. So it seems like the wires between both sensors and the gauge are OK. Also, it proves the gauge can move and reads max temp with zero resistance/full voltage.

I checked the resistance of the temperature sender to ground. When cold, I believe the resistance was about 3000-ohm. After it got warmed up, I checked and it was down to about 300-ohm. So it seems like the temperature sender is working, and reducing resistance as the coolant heats up. Does anyone know if these resistances are in the right range? Also, I think this verifies that warm coolant is getting to the block, correct?

I pulled the dash off and all the wires to the gauge look good. I also just did a total refurb on the grounds on the whole car so they are all freshly cleaned and with new spade connectors. This didn't change anything for the temp gauge.

So...what could it be? It seems like the sender is working, and the wire between the sender and gauge is OK. Could it be a bad gauge, and it only works when grounded because it's getting so much voltage at that point it gets forced to work? The temperature gauges are very expensive, so I'd hate to buy one if mine is fine. Are there other tests I can do to pinpoint the problem or diagnose the gauge? Check resistance or voltage across pins on the gauge perhaps?

I've ordered more shop manuals, the ones I already have do not get into any details like this, just the basics about checking for resistance change at sender, and grounding the wires to check the gauge. So in the meantime I am hoping you guys can help.

Any help appreciated!

[bradartigue]

The gauges are less than $30.

Sometimes the senders mess with the gauge. Disconnect the overtemp sender and run it with only the temperature sweep sender. If it works correctly then the problem is the overheat sender. If not then remove the gauge from the cluster and run a new, short wire to both gauges and test. If it works then the wire probably has a bunch of crud in it/corrosion/and you can replace the wire.