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Discussion Starter #1
It's a 94 Chevy truck, I don't think the gauge has ever registered while driving.

There's a new GM sending unit.

The needle on the gauge goes all the way to the right with ignition ON when the gauge wire is removed from the sending unit and the wire is grounded. It's my understanding that that means the gauge is ok.

The needle never moves when the engine is running, just hangs out on the left side...

But, the needle will register after the engine has run a while and been turned off.

Any ideas as to what the problem is?
 

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Originally posted by Glenn1018:
It's a 94 Chevy truck, I don't think the gauge has ever registered while driving.

There's a new GM sending unit.

The needle on the gauge goes all the way to the right with ignition ON when the gauge wire is removed from the sending unit and the wire is grounded. It's my understanding that that means the gauge is ok.

The needle never moves when the engine is running, just hangs out on the left side...

But, the needle will register after the engine has run a while and been turned off.

Any ideas as to what the problem is?
Did you use teflon tape on the threads of the new sending unit? The sending unit must be grounded to the block and if you used tape it isn't grounded so the gauge would act like its not even hooked up. My guess.
 

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Discussion Starter #3
As much as I don't feel like draining that thing again, I just might and try your idea without a sealant. Or, I think I'll just run an Ohmmeter across the sending unit and the block and see what it says.

It never worked since new - factory installation.

Removed and replaced the original sending unit Saturday (new aftermarket sending unit had the wrong terminals) and used Loctite PST when replacing.

Got a new Chevy sending unit yesterday. It came with red stuff in the threads, but I wire brushed it out and applied Loctite PST before installation.

Couldn't take the suspense - just ran an Ohmmeter across the sending unit and the exhaust manifold - no resistance, so that rules out the sealant as the culprit.

Thanks for the suggestion.
 

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Originally posted by Glenn1018:
Couldn't take the suspense - just ran an Ohmmeter across the sending unit and the exhaust manifold - no resistance, so that rules out the sealant as the culprit.

Thanks for the suggestion.
Lost me but that's not hard to do. You measured between the outer body of the sender and the manifold and read nothing or open? Should be a dead short (0 ohms) but the exhaust manifold is not the best place to read from. Usually rusted and dirty.
 

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Discussion Starter #5
One probe was on the exhaust manifold, the other was on the brass part of the sending unit - zero Ohms, needle on the VOM all the way to the right.

Just checked again to intake and the needle jumped to zero Ohms.

It uses some sort of bisexual sending unit which can be used in one wire or two wire applications. This is a one wire deal. Tried two aftermarket, Elgin (NAPA)and Wells (AutoZone), sending units Saturday, but they had two round prongs where the electrical connection is made. Got one from a dealer yesterday which is like the original - one little round prong and one little spade terminal. The gauge acts as it has for 10 years - 100* (all the way to the left) while driving/running.
 

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I Have a 93 Z71 that we landed pretty hard a while back while out messing around in the mud. The big plug with the 2 clips the goes into the back of the gauge cluster came unclipped on one side. You might check to see that it’s secure and obviously while your back there check the wires on the gauge and all the grounds. Sounds like you covered everything else…
 

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The temp sending unit will have around 1350 to 1400 ohms at 80 deg F. and will be around 300 to 400 at 200 deg F. Measure the resistance when cold and when hot if you get something close to these values the sender is good. The gauge may be the problem.
 

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Bisexual units..I like that one. Have to remember to use it. Yep, relays swing both ways.
 

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Discussion Starter #10
Thanks for the replies.

The thermostat is original. I plan on changing it and the radiator cap pretty soon - that'll take care of the cooling system, flushed, new hoses, etc. The truck has nearly 177,000 miles on it and runs fine, and I figure that may be because I don't work on it.

Will check hook-ups/wiring behind the gauge during lunch today - I haven't poked around back there. I have a feeling that's where the problem is.

Here are some Ohm readings on the original sending unit at room temperature:
Threads to round terminal - about 2.2 K Ohms
Threads to spade terminal - about 2.2 K Ohms
Round terminal to spade terminal - 0 Ohms
That's with a cheap analog VOM, so I'm guessing at the readings.

Thanks again.
 

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Discussion Starter #11
I looked at a wiring diagram as well as the wiring. Not much to tell about the wiring - nothing obviously loose.

The book shows the dark green wire from the sending unit going to the firewall as one wire, but on the interior side the book shows two dark green wires coming out. One goes to the ignition switch (found that one, I think) and the other goes to the instrument panel cluster/connector (didn't find that one).

Given the following conditions:
1) The gauges tests ok when removing and grounding the sending unit wire with ignition ON.
2) Gauge stays to the left (around 100 degrees) while driving, but will creep up and give a temp reading of around 170 after engine is turned off.
3) When starting, just before the engine fires, the needle goes all the way to the right, but returns to 100 (far left)after started.

Would you suspect the ignition switch, the instrument panel connection, or something else?

It's been like this since new.
 

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I’m going to go with an open thermostat. It sits around 100 degrees while running because the fan and water pump are keeping it cooler. After you shut it off the circulation stops and the heat rises. Sorry didn’t read the whole post if you’ve checked that, my bad
 

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Discussion Starter #13
Thanks, I'll replace the thermostat this evening and see what happens - maybe it's been stuck open for 10 years.
 

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”3) When starting, just before the engine fires, the needle goes all the way to the right, but returns to 100 (far left)after started.”

This is normal. The dark green wire that connects to the temp gauge is routed to the temp sender and the ignition switch. At the ignition switch the wire is grounded when cranking/starting. This tests the indicating light or gauge each time the engine is started. Here’s a pic form a diagram.
 

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Discussion Starter #15
Thanks Elree, I thought that was normal, but wasn't sure. So I guess that rules out the dark green wire to the ignition switch as being at fault, and leaves the dark green wire to the instrument panel cluster as a possible cause for the problem? Could the ignition switch be faulty and lose or make a connection when the pressure is taken off the key after starting?

Changing the thermostat in a few hours - we'll see...
 

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I'd like to, if I may, bring this back up, because I am having the exact same problems, but want to double check. I'm having the same hanging out at about ~100 or 120ish, then rising to normal operating temp when the car is turned off. When I start my gauge goes to the right all the way as well then back to the left.

My question though is if I turn the car off, and the needle rises, which is presumably OK, then turn the power/ign back on, but do not start the car, should the needle go back down to 100, or stay at the operation temperature until the car is actually started again?

That's the only difference I can see in mine, unless that was what was happening to Glenn too. I'm just wondering if since it's routed through IGN if turning the key back on would 'reset' the gauge so to speak.

Thanks.
 
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