66 Temp Gauge Problem

[Bob H.]

I drove my 66 tonight and on the way home noticed the temp rising slowly over normal . I wasn't far from home but within a minute or two the temp went from 200 to the red zone so I puled over to find the car not overheating. By the time I dropped the hood and looked at the gauge again it was off the chart almost to the 2-3 oclock position. Did my gauge just die or could something have caused it? I had pulled the lead off the sending unit and even pulling battery power the gauge is now still buried. Is this what happens if the gauge goes bad?

[Jack H.]

Re: 66 Temp Gauge Problem

There are two opposing electromagnets in the gauge. One pulls the pointer needle to the 'cold' side of the dial. Its circuitry is fixed and it relies on the gauge to be properly grounded in the dash cluster.

The other electromagnet has a variable drive circuit that includes the temp sender in the engine. It pulls the pointer needle to the 'hot' side of the dial via the variable resistance of the sender.

So, when a temp gauge 'pegs' to the hot side, it does so for one of two reasons:

(1) The hot side went 'bezerk' (saw less than 40 ohms which is the temp sender's minimum resistance).

(2) The cold side 'lost it'... Either the electromagnet in the gauge open circuited or the gauge lost its ground reference at the dash.

I've seen both situations. So, it's a matter of electrical sleuthing to determine what happened in your case.

BTW, if you aren't electrically skilled, a decent auto electric shop ought to be able to diagnose this in about 30-minutes...

[Bob H.]

Re: 66 Temp Gauge Problem

Hi Jack. Thanks for the explanation. I now understand how it works. Since I have now removed power and if the gauge was ok shouldn't it pull back to "0" now? It hasn't after sitting overnight. I assume with no power the ground doesn't matter?

[John H.]

Re: 66 Temp Gauge Problem

Bob -

To verify gauge function, with the key "on", remove the connector from the temp sending unit - the gauge should peg to full "cold". Then ground the connector terminal to the engine, and the gauge should peg to full hot.

If the gauge doesn't respond that way, either the gauge has failed or there's a wiring issue between the gauge and the sending unit.