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Discussion Starter #1
I'm really not sure if my factory temp gauge is working correctly. Right now I have a mechanical guage connected to the port in the head, and the factory gauge is connected to a sending unit on the intake manifold. I'm using a 160 degree stat.

After 20-30 minutes, the mechanical guage reads ~190, which is what I would expect. However, the factory gauge consistently reads 210, which might be okay if you consider 10-20% varability.

The funny thing is that if I turn of the engine and then just turn the ignition to the "on" position, then the temp gauge reads ~160-180 (which would be pretty accurate) If I start the engine, the guage climbs back to 210.

Shouldnt the gauge have the same reading regardless if the engine is running? Can anyone help me out?
 

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Since there's no way for the water to change temperature that fast, the difference is caused by the voltage change.
When you turn the car off, the gage is running on battery voltage. Somewhere around 12 volts. When you start the the car back up, the gage is now getting a higher voltage produced by the alternator. Somewhere around 14.5 volts. The gage responses to the higher input voltage.
 

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If you are using a generic aftermarket sending unit for the factory gauge I would get a NOS original or a reproduction one that is calibrated to the old gauges before suspecting the dash gauge.
For example I have tried a couple Napa sending units in my 90 truck and they both read 210 while the Delco unit read 10-15 degrees lower as the original did.
I have however noticed on several web sites that even the new replacement Delco sending units may not register correctly on some old original gauges.

Rene
66_Malibu
 

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While I agree with John. "the difference is caused by the voltage change."

It shouldn't make a difference. The gauge is designed to give consistent indication with varied supplied voltage. Check the resistor on the back of the gauge. The top post of the gauge should have a good connection to the resistor, the bottom post should also have a good connection to the resistor and ground, cluster metal back. The cluster metal back also needs to have a good ground. If you have an ohm meter check the resistor. W/the 12 pin cluster connector disconnected, the gauge top post to ground should be around 85 ohms. The bottom post should have 0 ohms to ground.
 

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Discussion Starter #5
Thanks for the input guys! I’m going to take a close look at the resistor, ground, and sending unit. One question I still cant figure out is why the temp gauge seems to be accurate when the key is in the “on” position, but when I start it up, the gauge climbs up to 210. (this is after reaching normal operating temps). It still seems to me that the gauge should read the same regardless it the engine is running or not. Is there something else I should be look at in order to remedy this, or is this normal as John’s eluded to? Thanks! Scot
 

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Discussion Starter #6
okay here's what I found out tonight.

1)There is a yellow "sticker" on the resistor.
2) With the 12 pin connector disconnected, I get a resistance reading of 67 ohms (assuming I'm using/reading the meter correctly)
3) The bottom post on the resistor is properly grounded.

So where does that leave me? I thought the resistor was supposed to be orange? What about the ohm reading of 67? Could that be the problem?
 

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Usually the resistor is orange. Do you know if this gauge/cluster has been changed from the original factory configuration? Either the factory put that resistor on for calibration or it has been changed for some reason.

If this were my gauge I would check the calibration. Compare indicated temp to a standard. Suspend the sender in a pan of boiling water, the temp will be 212 deg at sea level lower as the elevation increases. Pull the gauge pointer and replace it so that the gauge reads mid scale, 210 deg. Plus or minus a 2 or 3 degrees is within the accuracy of the gauge. Now the hard part is a 100 deg standard. I don’t have a suggestion for an easy 100 deg standard. My only recommendation is a good thermometer and a lot of patients. Put the sender in a 100 deg standard and see what it indicates. If it’s close to 100 then the resistor in the back is correct if it’s way off then the resistor needs to be changed. You will not get an instantaneous response the sender takes time to stabilize at the new temp.

Note to anyone wants to calibrate their temp gauge. If there is no reason to doubt the correctness of the gauge resistor, a one point calibration in boiling water is all that is needed.

To answer you question about John’s comment on the applied voltage. In your case I think it could be why it indicates a different temp when running. However if the gauge is working as per design the applied voltage will not effect the indication.
 

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Discussion Starter #8
Elree,

Thanks for all the help. Do you think it would make any difference if I used the orange resistor? If so, is there somewhere I could find one without searching the world?

I think that the cluster came out of a 70 Monte Carlo since I had to replace the SB tach with my BB tach.

The guage goes to the far left when I disconnect the sending unit wire, and it pegs at 250 when I ground the wire. I'm using a sending unit that I picked up at the local parts store (Orielly), but I've heard I should try a GM part.

I checked the resistance again tonight and it was 67 ohms with with 12 pin connector disconnected. I dont know it this matters, but with the 12 pin connected, the resistance read ~45 ohms. The sending unit (cold) read ~515 ohms.

I dont know if this information helps, but I tried a few more things tonight, and now I'm starting to think I have an electrical problem.
The temp gauge is still reading 210 when warmed up. If I turn the engine off with the lights on, the gauge will climb up to 240-250 degrees. If I turn the engine off with the lights off, the guage remains at 210 until I turn the key to the on posistion (it drops to 160-170, then very slowly climbs to 210).

(Also, I used Jungle Jimmys instructions on how to convert an idiot light harnes to a gauge harness)

What about the printed circuit board? Could it be time to replace it? It is a little worn out in a few places including where it attaches to the temp gauge.

Sorry for the long post, but I want to get this figured out before I put the windshield back in.
 

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If the gauge came from the factory with the yellow resistor on it then changing to the orange will not help.

Also, I used Jungle Jimmys instructions on how to convert an idiot light harnes to a gauge harness” I’m not familiar with Jungle Jimmys instructions, cant say if they are right or wrong.


The fact that the gauge indicates to the left when the sender is disconnected and full scale when grounded leads me to believe the circuit card is not the problem with the temp gauge.

Could be a cluster ground problem. Make sure the cluster back is grounded. The dash harness has black wires with ring terminals, one connects to the cluster back one connects to the parking brake assembly.
 

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Discussion Starter #10
Elree,

You'll have to excuse my ignorance, but can you elaborate on calibration of the temp gauge? Do I need to leave it in the car, or should I remove it? What wiring/grounds/power source needs to be hooked up to the back of the gauge?

I actually took the gauge out last night to take another resistor reading. It ended up being 90 ohms.

I thought I'd try your calibration suggestion before I end up sending it off to have checked/fixed.
 

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This pic shows how to connect the gauge. The gauge shown is not from a Chevelle but is electrically the same. For the sender (-) connection strip about three inches of the wire and wrap/twist it around the threaded area. I use a alligator clip on the sense connection and use it to suspend the sender in the water, keep the top 1/2" of the sender out of the water. The gauge pointer can be removed from the stem for repositioning if needed.
 

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Discussion Starter #12
Elree,

Thanks for all of your help. The diagram was exactly what I needed! I performed the calibration as you instructed and found out the guage read nearly 240-250 when the water was boiling. I pulled the needle off (which was held on by 4 pronges) and adjusted it so that it was at 210. Then I just waited as the water cooled down and periodically checked to see if the temp gauge read the same (or close) to the water temp.

The only question I have now is what increments are the "hash" marks on the gauge in (70 SS)? Based on my calibration, 100 degress is actually somewhere close to the 9:00 position, and the first mark represents 170 degrees. (I think the marks are setup to be in 10 degree increments, and are not linear)

Anyway, after running the engine for awhile, the mechanical gauge read ~180, and the dash gauge that I just calibrated read ~170. Hopefully that did the trick. Thanks again for your efforts.
 
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