Thread: over heating....or is it?

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First gen Chevy small block....As soon as I start it up or shortl after the the needle on the temperature gauge jumps as high as it can and sits there. Why would it do this ?

Perhaps a bad sending unit.

Why don't you try a temperature gauge with numbers on it so that you will know what the real temperature is??? Just hot and cold on a scale is not very accurate.

Originally posted by Dave Severson
Why don't you try a temperature gauge with numbers on it so that you will know what the real temperature is??? Just hot and cold on a scale is not very accurate.

It does have numbers on it. It gos over 300.

Originally posted by nystreetscene
First gen Chevy small block....As soon as I start it up or shortl after the the needle on the temperature gauge jumps as high as it can and sits there. Why would it do this ?

Sending unit as in the temp sensor?

SOUNDS LIKE A BAD GUAGE TO ME

Yup. Is it a mechanical or electrical gauge??? If it's electrical it could be either the sending unit or the gauge itself.

Could just be the wire shorting to ground.

You can use an ohm meter to check your sender. If it is a GM
sender, the following two senders are typical:

Standard Ignition # TS-71 ... 1/2" male pipe thread.
Replaces GM8993106, GM8993107, GM25037345.
Resistance at 100 degrees F. ... 1425 OHMS
Resistance at 220 degrees F. ... 147 OHMS
Used with 240 degrees F dash gauge.

Standard Ignition # TS-76 ... 3/8" male pipe thread.
Replaces GM8993146, GM8993164, GM25037346.
Resistance at 100 degrees F. ...1425 OHMS
Resistance at 250 degrees F. ... 147 OHMS
Used with 260 degrees F. dash gauge.

Notice that both have the same upper and lower resistance BUT the corresponding resistance is at different temperatures. This still gives you a quick check of the sender.

Since the gauge is going full scale at low temperature, the sensor is suspect. A broken wire can also take the resistance low. Since a temp gauge is really a volt meter, the lowered resistance is makes the gauge go full scale (less voltage drop).

If the sender reads OK, you can use a bypass wire to check the wiring.

mike in tucson

I found the problem today. I had the temp sensnor wire sandwhiched between the valve cover and gasket. That was causing it to short out. Thanks everone for your input.

Glad it was the easy thing!