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Our Caprice 5.7L temp gauge is not working. At start-up, the needle quickly passes from cold to hot and back to cold. The cabin has sufficient heat. It has 189k miles and runs well with no other issues. In the past, I have changed both the temperature sending on the water pump unit and the temperature gauge sensor on the passenger side head. And I also have had the electrical connector by the head/exhaust wrapped in a heat shield. That connector looks brand new still. So this is all I've had to do to get the gauge working again, but not now. I know that the temp gauge connector has only one wire. I thank you all for letting me know where there are common wire breaks or something else that I am missing.
 

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If it was mine I would start at the RR head sender ( water pump sender has nothing to do with gauge)
Ign on ,
Unplug it look at gauge
Ground the single wire , look at gauge.

If it full scales in both directions , then it looks like sender.

If not , you are likely looking at wiring as the gauge full scales on the gauge check
( start position grounds the wire )

Next place would be the passenger side underdash connector .
I dont have the terminal - plug number handy at this time.
 

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The wire and pin may not be making good contact. You can disassemble the connector, and tighten the crimp on the wire.
 

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I am just going to rephrase some of what 95Wagon posted.

At start-up, the needle quickly passes from cold to hot and back to cold.
This is the gauge test. It shows the gauge works. It also shows that some of the under dash wiring is working.

Ign on , Unplug it (sensor connector) look at gauge
The gauge should show below low as there is no resistance in the circuit.
Ign on , Ground the single wire (sensor connector) , look at gauge.
Grounding the sender wire (connector) does the same thing as the ignition switch does when you start the car. IE: the gauge should show above the high on the gauge scale.

The circuit diagram:

What it does not show is the only connector between the instrument wiring and the engine wiring.
C205 is located behind the right hand(passenger) kick panel.

From this connector you can check the wiring at the half way point.
Short the dark green instrument wire (connect to a ground) and the gauge should move to above high.
With a multimeter set for Ohms check if the sender resistance is available between the sender and the dark green engine wire in the engine harness connector. At 100 degF 1365 Ohms at 260degF (overheated) it would be 53 Ohms. One test lead on the connector pin and one test lead on ground.

Most of us would bet the wire is damaged where the sender wire is over the engine. Look for damaged wire coverings. or where someone has added tape.

The best tool is a Fox and Hound system. Electricians and data wire installers use them. With no power to the circuit connect the transmitter at the sender then use the receiver to listen to the transmitted signal on the wire. With luck the signal ends at the wire break.
 

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I am following this post because my temp guage hasn't worked in years. My problem is the guage set about alow as it can go and never moves. So is my indash guage toast?
 

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I am following this post because my temp guage hasn't worked in years. My problem is the guage set about alow as it can go and never moves. So is my indash guage toast?
The wiring between the ignition "bulb test" switch and the cluster may be broken. Or the gauge may be bad.
 

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the stock gauge temp sender had a heat sleeve over it and its connector. The wire to it also runs down the back of the engine into the wire harness. It also had grey stripe split loom over that wire. Both those heat shield components should be used and replaced if needed.

I bought a new pigtail for the temp sender and soldered it in. I replaced the split loom with heat wrap split loom (silver foil wrapped fiberglass split loom tubing) made by Vibrant Performance and sold in 4' lengths

That would be the first wire and protection I would check
 

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Thanks guys
 

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Update. I was always under the assumption that you get a more acurate temp reading in the head. So I switched the locations of eash sensor. My aftermarket is in the head in place of the factory one. I put the factory one out of the head in the water pump. I did that after my gauge quit working. I am pretty sure the solid green wire is broken in the engine compartment. I am going to run a new wire from the sensor in the water pump to the green wire under the dash The guage tested good , so I should be good
 

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In water pump , as in the 3 wire mod that does both pcm and gauge ?
I have the stuff do that but, for now I have a tee in the water pump that I use for the PCM connection and now the gauge. My aftermarket is in the head on the passenger side. I tried to put it on the drivers side, but the plug would not come out. I used plenty of heat on it but still rounded off the plug
 

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Got to say , not "feeling" the " T " as it takes the tip of the probe out of the direct flow of the coolant.
How much of a difference in temp. do you think that makes?
 

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FWIW I did the "T" thing on my 327 using 1 port for dash light and other for my aftermarket gauge....I found the gauge as a result was reading about 10 degrees low so if you "factor" than math in you are good but as Gerry notes any "spikes" may not be revealed until later than they start. I have since abandoned that setup as my new motor has heads with a port for temp sender, preferred location.

My measurement of 10 degrees was with an IR temp gun at the base of T going into intake manifold vs the base of the sender going to gauge. YMMV
 

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Very interesting guys. I may have to do some rethinking on my setup. But I do have an aftermarket guage that is in the head. Don't you think that would cover me, over the idiot light in the dash?
 

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... if you have a pump failure I question if you will get a useful warning.
Speaking of OE arrangements ...
The senDER in the head (driver side in 2 & 3 door LT1s, passenger side in 4 & 5 door LT1s, not sure why ...) never runs colder than the senSOR in the H2Opump, and nearly always runs hotter than the H2Opump.
For this reason, the senDER in the head will always warn of excessive coolant temps or overheats before the sensor in the H2Opump, should coolant flow ever in any way fail.

I've personally witnessed several LT1 engines on the cusp of overheating, as per warnings provided by dash temp gauges.
At those moments, the hottest reading I've ever read from any of the corresponding H2Opump temp senders was not more than 220F / 104C.

Can no longer find Scott Mueller's LT1 article titled "LT1/LT4 temperature gauge anomalous readings" on the web. The HERD's tech articles have followed ours down the toilet. (HELP!)

Regardless, I still remember Scott Mueller mentioning that the head temp senDER can report up to 40F hotter than the H2Opump temp senSOR.

If I still had an LT1 car, I'd be crazy enough to try to move the H2Opump temp sensor to the 'other' head.
I'd NEVER be crazy enough to move either of the head temp senDERs to the H2Opump.
 

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I get what you guys are saying. and Thank you for your info. Thats why I like this place, always something to learn. Since I have the stuff for the 3 wire--------thats what I will do.
 
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