C3 1970 Temp Gauge Jumping

[Mark P.]

On my way back home yesterday from a long trip, my 70 350/350 overheated and the temp gauge was jumping around like crazy. Turned out it was a stuck closed thermostat, but even after I removed it, the temp gauge still jumps around (it didn't do it before) even though it's running within the right temp range. Did I do something to the sending unit when it overheated or is it a connection problem?

Thanks,

Mark

[Jack H.]

Re: C3 1970 Temp Gauge Jumping

During overheat episodes, the temp gauge can 'twitch' as a result of low coolant and pockets of steam running past the sender's probe. If the sender didn't 'fry' in the overheat event, it should recover once the system has 'burbed' any trapped air bubbles.

Also, the lead wire to the temp sender might have skinned insulation with the wire intermittently grounding. Check the temp sender lead wire for intact insulation....

Next, you can test things. Disconnect the lead wire from the temp sender and see if your temp guage reads consistently 'cold'. It it's jumping/twitching, there's a problem at the gauge end (bad ground to the instrument cluster?).

With the gauge lead wire disconnected, you can put an ohm meter on the temp gauge (being CAREFUL not to burn probe wires on the exhaust manifold) and you should see steady resistance readings in the 800-70 ohm range (resistance drops as engine temp increases). If resistance readings are erratic, it's time to replace the temp sender ONCE you're convinced there's no more air trapped in the cooling system.

[Les Jacobs]

Re: C3 1970 Temp Gauge Jumping

Jack,
Above you say "With the gauge lead wire disconnected, you can put an ohm meter on the temp gauge (being CAREFUL not to burn probe wires on the exhaust manifold) and you should see steady resistance readings in the 800-70 ohm range (resistance drops as engine temp increases). If resistance readings are erratic, it's time to replace the temp sender ONCE you're convinced there's no more air trapped in the cooling system. "
I think you meant to say, put the ohm meter on the temp sender, not the temp gauge. Les

[Mark P.]

Re: C3 1970 Temp Gauge Jumping

Thanks so much for the responses. I'll get right on it and figure out what the real problem is. I appreciate the help so I don't end up chasing the wrong issue and throwing money where it doesn't need to go.

Thanks again,
Mark

[Les Jacobs]

Re: C3 1970 Temp Gauge Jumping

Hey Mark There is actually one more thing you can do if you want to kill the horse again. Go to Radio Shack (or electronics supply) , and pick up two 50 ohm (or 51 ohm) 1 Watt resistors, and two small alligator cables. Disconnect the temp sender wire and connect the two 50 resistors (wired in series) between the sender wire connector and ground. (Turn key on, and the temp gauge should read about 3/4 full scale. (actual value isn't important). But you now have a stable sender substitute, so if your gauge is still jumping all around, you have ruled out the sender. Cost for the test..less than 10$. Les

[Mark P.]

Re: C3 1970 Temp Gauge Jumping

Thanks, Les. Sounds like a great way to check it. Appreciate the help.

Mark