Continuing Temperature Gauge Issues

[tdangle]

I have been having strange temp gauge problems. Seems to really float around going from barely warm to almost hot and back within a few seconds. I have verified the temp of the engine with a thermal gun and it is fine. I have a new 180 thermostat, new sending unit (actually have 3 new units). I know as the sending unit goes to ground the gauge reads hotter. Couple of days ago the gauge dropped to cold and I figured the wire came off of the sending unit. I turned the ignition off and for some reason I turned it right back on with out even starting the engine. Gauge went from cold to normal. cycled the ignition switch a few times and gauge reading varied. First I was thinking the power to the instrument panel has an issue, but the fuel gauge reads correctly all the time. Is it possible I have a flaky temp gauge? I really don't want to just throw new parts at it with out somehow verifying the problem.

[Steeveedee]

My temp gauge did the same sort of thing, and my fuel gauge works fine, too. I put in a mechanical gauge that I bought for a car I ended up selling. I'll get to making it "right" eventually.

[garyd1961]

Do you have the heater core hooked up? If not do you have a bypass hose in its place?

[tdangle]

Originally Posted by garyd1961

Do you have the heater core hooked up? If not do you have a bypass hose in its place?

Yes all stock AC/Heater. Gen1 SBC had an internal bypass in the head. I've also turned the heater on to see if it made a difference. As far as I can tell the engine temp is not really changing much per the thermal gun. As soon as I started seeing a change in the gauge reading by just cycling the ignition switch I realize it's not a cooling problem its a gauge problem.

[tdangle]

Originally Posted by Steeveedee

My temp gauge did the same sort of thing, and my fuel gauge works fine, too. I put in a mechanical gauge that I bought for a car I ended up selling. I'll get to making it "right" eventually.

At least now I know I'm not going crazy.

[Steeveedee]

Originally Posted by tdangle

At least now I know I'm not going crazy.

Usually, when the gauge goes nuts, especially if it indicates low, the system is low on water. I ran the gauntlet with mine during trouble shooting. Like you, I could tell it wasn't over heating.

[tdangle]

Originally Posted by Steeveedee

Usually, when the gauge goes nuts, especially if it indicates low, the system is low on water. I ran the gauntlet with mine during trouble shooting. Like you, I could tell it wasn't over heating.

I wish it were that simple. I have a coolant overflow bottle and vented cap, the level in the radiator is to the top. Have a 50/50 mixture, fairly new water pump all new hoses and belts, new 7 blade clutch fan, and shroud. Been thru a couple of thermostats, even pulled the head on the passenger side to make sure there was no blockage.

It worked great until one day the sending unit went out and from that point on it has been driving me nuts. When its reading correctly (which is actually most of the time) it maintains the same temp whether on the highway or around town, AC on or off.

[Already Gone]

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[Ironangel]

Check your grounds, ALL of them. A greasy ground connection will fluctuate resistance...

[GMCPaul]

Originally Posted by Ironangel

Check your grounds, ALL of them. A greasy ground connection will fluctuate resistance...

^^X2^^ Definitely sounds like a ground issue, check your motor to frame & cab grounds and cab to frame grounds.
Paul Sr @ GMCPauls

[rpmerf]

Another thing I've heard. Use pipe dope instead of teflon tape, as the tape will not allow it to conduct as well.

Manifolds or headers? Heat shields?

Does the connector fit firmly onto the sensor?

[tdangle]

Originally Posted by rpmerf

Another thing I've heard. Use pipe dope instead of teflon tape, as the tape will not allow it to conduct as well.

Manifolds or headers? Heat shields?

Does the connector fit firmly onto the sensor?

They come from AC Delco with teflon tape on the threads. If the sensor was isolated from the block you would have no reading not floating higher than it should normally read. It works as a resistance to ground. Stock exhaust manifolds, heat shields long gone, but it worked previously. No change in ground straps. Connector is firmly connected. Again if loose would read low or not at all.

Going to work on it this weekend and hopefully figure it out. Will keep you all updated.

[TBONE1964]

Originally Posted by tdangle

I wish it were that simple. I have a coolant overflow bottle and vented cap, the level in the radiator is to the top. Have a 50/50 mixture, fairly new water pump all new hoses and belts, new 7 blade clutch fan, and shroud. Been thru a couple of thermostats, even pulled the head on the passenger side to make sure there was no blockage.

It worked great until one day the sending unit went out and from that point on it has been driving me nuts. When its reading correctly (which is actually most of the time) it maintains the same temp whether on the highway or around town, AC on or off.

"It worked great until one day the sending unit went out"

What type of sending unit did you replace with? I would not be surprised if you got a bad one right out of the box.

You may consider trying a different one to see if the condition changes.

I have a hard time trusting new parts these days.

[Sheepdip]

Just had a buddy who went thru the same issue, his was reading warmer than normal but the heat gun said all was good. 2 senders later the gauge was back to reading normal again.

[tdangle]

Think I’ve got it solved. The copper pad connector to the plug on the instrument panel pcb was crumpled up and shorting to the next pad which went to the hot temp light where I had put a bulb in for some reason. Also cleaned all the connections at the gauge. Took a short drive and it seems to be working much smoother

[tdangle]

Originally Posted by TBONE1964

"It worked great until one day the sending unit went out"

What type of sending unit did you replace with? I would not be surprised if you got a bad one right out of the box.

You may consider trying a different one to see if the condition changes.

I have a hard time trusting new parts these days.

I tried 3 new ones. 2 ac delco and one other brand