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65 Temp Gage Maxs Out when Ign On

My 1965 L79 Corvette temperature gage will occasionally move quickly all the way to the right (max) when energizing the ignition circuit before a cold start. After the engine starts, it stays at max for a few miles and slowly moves back left to normal. I've also have tried not starting the engine when I see this happening. In this case the temp gage needle still stays pinned when removing the key but slowly moves back down over night even with the ignition off. I've checked the connector at the sending unit and it seems okay.

Could this be my temperature sensor going bad? Or maybe the temp gage (it's an original 65 gage with no yellow or red colors on the scale ..... those were 66 gages)? Thanks.

Reply to
John Smith
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Most likely the wire between the temp gauge and the sending unit is touching a ground someplace. After 38 years it's possible that a piece of now brittle insulation has broken off exposing the wire, and the wire is rubbing against a piece of sheet metal or the chassis somewhere. If not, it would have to be the sending unit going bad. The fact that the gauge itself goes up and down means that it's fine.

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-|>|- Diode -| My 1965 L79 Corvette temperature gage will occasionally move quickly all the

Reply to
Diode

Reply to
Steve Garner

Thanks folks ... I tried your suggestion of grounding it and disconnecting it and the gage moved as expected late this afternoon. I did not use thread compound when I installed a new sending unit five years ago after a mechanical/electrical restoration. I replaced all the wiring harnesses and other miscellaneous wires 5 years ago and also had the temp gage replaced. Therefore, all things seem to be pointing to an intermittent problem with the sending unit (somehow grounding out while sitting cold over-night every now and then).

Incidentally, this morning the gage was still pinned to max (from yesterday's incident), even though I had disconnected the sending unit connector before going to bed last night. This morning, before starting the car, I tried reconnecting it, both with and without the ignition energized, and it still would not move back from max. I then left it disconnected and started and ran the car until it was fully warmed up and while idling, I reconnected the wire to the sending unit and presto ..... the gage snapped back immediately to normal temp.

I guess it's off to my local NAPA parts store tomorrow for a new sending unit .... Thanks again, JS.

Reply to
John Smith

Most of the older cars with bi-metalic meters had an "instrument voltage regulator" which provided a steady 7 volts for the meters to operate on. The reason is if voltage were to rise, the the meters would rise also. Not good for accuracy.

Reply to
Philip ®

Okay .... Uhm, does that mean my thinking that the Sending Unit is going bad is incorrect?

Reply to
John Smith

If the gauges voltage regulator has gone bad, the gauge can either pin out from too many volts or not work at all.

My 86 Jeep CJ7 has a mechanical gauges voltage regulator and if it sticks closed, my temp and fuel gauges will pin out. Give the dash a thump and the points start working again and the gauges work.

I have to open mine up because the points are going bad, I am only getting 3.5 volts to the gauges most of the time instead of the 5-7 they are looking for. This means my gauges read low most of the time. Full tank of gas only shows 2/3, normal running temp is below the green, not in the middle.

Maybe you should put a multimeter on the power lines for the gauges to verify what is coming into it before spending money?

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

John Smith wrote:

is incorrect?

Reply to
Mike Romain

Reply to
Steve Garner

Your's might not. I was specific about the type of meter that would (bi-metalic).

Reply to
Philip ®

You are correct .... my 1965 Corvette does not have an instrument voltage regulator as confirmed by my review of the shop manual wiring diagrams.

Anyway, a new sending unit today did the trick. Thanks to all!

JW

Reply to
John Smith

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