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Discussion Starter #1
I'm having a (never ending) problem wit the temp gauge on my '68 vert. It's an orignal 327. Last year, I installed OER gauges, factory fit wiring harness, and a Lectric Limited sending unit. The temp gauge works good for the first 10 minutes or so after I first start the car (temp climbs normally) but after the car wamrs up, the needle continues to climb until it's pegged to the right. I checked the temp on the motor (radiator, hoses, water pump, intake etc etc) with an infrared thermometer and everything reads normal.

At times, when I see the needle beginning to climb past half way, I can press on the gauge housing (so the guage itself kind of moves) and sometimes the needle starts back down but inevitably, it starts creeping back up again until it's pegged. If I let the car cool down for awhile and then start it, the gauge is back down to zero - and we start the whole cycle over again!

On the forums here, I've read Marc C has suggested pulling the needle off and putting it the center of the gauge after the car reaches normal operating temp and I was thinking of doing that. But, if the gauge itself (not just the needle) is pegged, wouldn't putting the needle in the middle of the gauge put the needle as high as it could go on the gauge?? In other words, wouldn't the guage read as high as it could at normal operating temp?

Anyone have this issue or have any suggestions?? This has been driving me nuts...
 

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If the guage keeps climbing upscale it's because the gauge, or the whole pod isn't grounded properly. When you push on the guage, you are probably temporarily grounding it.

I beleive (its been a while since i've been in my cluster) the temperature gauge has an insulator (white phenolic bushing) on its ground leg (bottom threaded pin) to insulate it from the metal mounting plate. You may have to connect a ground wire directly to that leg of the guage to get it to work right.

Also the OER guages have the external shunt resistor built into them, so if you are using the old shunt that was on the back of the original guage, with a new gauge, that may cause the problem you are seeing.
 

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Discussion Starter #4
Mark
I'm not using the restistor from an original gauge. I just installed the gauge as it was purchased.

I have another gauge from OER that I purchased that I am looking at. The gauge has four pins at 12, 3, 6 and 9 o'clock. The 12 and 6 o'clock studs have the resistor the 3 and 6 o'clock studs have the white plastic bushings at the very bottom of the stud. They only come through the gauge housing about three theads up the stud. Should I stil try grounding the bottom threaded pin??

I think the cluster itself is grounded fine because my fuel guage works and all of the lights in the cluster work as well...

Could I simply have a faulty gauge that works to a certain point or has some loose windings in it?

I'm confused as to why it works to a certain point and then pegs to the right. It stays at the bottom of the gauge when the car is cool, slowly climbs when the car warms up and then just keeps climbing and pegs to the right. If it wasn't grounded, it wouldn't work at all correct?
 

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A temperature gauge (or gas gauge) consists of two sets of coil windings, one wounde clockwise, one wound counter clockwise. The coil connected to the temperature sender drives the metter upscale towards hot. As the resistance of the sender decreases as the car heats up, more current passes thru the upscale coil and the needle moves up towards hot. The other coil is a balancing coil since it has a fixed resistance associated with it and its current flow never changes and it tries to push the needle down towards cold. In a correctly working system with a stable temperature the current in the two coils balance each other out and the needle stops moving, somewhere around 1 mark above 1/4 scale.

A couple of things could make your gauge do what it is doing. Insufficient current in the downscale coil, caused by the wrong shunt resistor (or a missing resistor), or a bad ground. I doubt that coil could be completely open circuited because the gauge would peg high pretty quickly, probably less than 20 seconds. Check to make sure the OER gauge does indeed have the internal resistor in it. Another thing could be that the sender is mismatched to the gauge either in range of resistance or rate of change over its temperature band, and simply is allowing to much current thru it as it reaches operating temperature.

My sender measures 571 ohms at about 75 degrees F and drops to around 300 ohms at 180 degrees.
 

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Mark
What's your thoughts on why the needle sometimes starts going back down if I press on the gauge housing? It's almost as if the gauge moves and something makes a "connection" and the gauge begins heading back down and then it just goes back up again?
 

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What does the gauge do when you ground the sendeing wire? And what does it do with the sending wire disconnected?
 

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Just wanted to "conclude" this thread in case anyone has a similar problem in the future. Mark C was right on - as always. I added a ground jumper wire to the bottom stud of the temp gauage and connected the other end to the plate it's mounted in and what do you know...a working temp gauage. It was at 1/4 for my entire cruise around town tonight.

Thanks!
 

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I am having the same problem as dsccribs with my gage and found this thread. I am using a re-pop temp gage with the internal resister. I have done some leg work with a volt meter as I was going to ground the bottom or 6 o'clock pin but when I checked it with a volt meter I found line voltage on it so I did not think it was a good idea. I have pulled the gage and am stumped as to how it is grounded other then at the sending unit there is no other ground that I can find. Is this right?

Another thing with the gage in my hand if I push or pull the pins I can get the gage to move across from cold to hot. I am beginning to think it is poched.

Any other ideas would be welcomed!

Thanks
 

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Line voltage on the bottom leg is correct when the guage isn't grounded. Theres no current flow until you ground that pin, so theres no voltage drop thru the windings. The counter coil (the one that moves the needle towards the "C" side grounds thru the bottom pin to the metal plate that the gauge bolts into. That metal plate is connected to the front plate with a short black jumper wire as there is only one guage ground wire coming from the console wiring harness and it attaches to the front metal plate. Make sure the other three pins have the little plastic collars on them so that they don't get grounded to the metal plate as they pass thru it.

Theres not much clearance between the coil windings and the metal case, if you bend the gauge you a probably grounding one of the coils to the case, or grounding one coil to the other. Don't do that.
 

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Thanks Mark I grounded the bottom pin and everything is working fine. I was not putting any more pressure on the pins then you would by picking them up. I was just concerned that I would be shorting the circuit out by grounding the bottom pin as I was reading full voltage on it.

Thanks!
 
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