Temp sender issue

[tdangle]

My sending unit went out and the gauge was not registering. I replaced the sending unit with a AC Delco replacement. So far as the engine is warming up it just keeps reading higher and higher until it pegs the gauge. I checked and the water passage at the top of the manifold is only about 180-185 ish. So I figured bad sending unit. Got another one and checked it prior to installing, comparing the resistance and water temp to a chart posted. Looked fine. Replaced it and it does the same thing, but as you let the engine run it drops to where it should be. I replaced the thermostat (180) with a new one thinking this was the problem. No change. Every thing checks out, but very unnerving seeing the gauge peg on warm up. Never had a problem with an air pocket before, wondering if this is the problem.

[dmjlambert]

That is interesting. Sorry I have no answer but just more questions. In what part of the engine do you have the sender installed? How much antifreeze do you use, or is it straight water? Do you have an overflow tank for your radiator? How much confidence do you have in the radiator cap keeping 13 to 15 lbs pressure in the system?

[garyd1961]

Do you have a by-pass hose from your intake to the water pump? If you have the heater coil hooked up it will serve as a by-pass. The by-pass keeps water flowing through your motor until the thermostat opens.

[tdangle]

running a 50/50 mixture of coolant, have an overflow bottle and fairly new radiator cap. Heater is hooked up, no bypass. Totally stock 350. Sending unit is stock unit in drivers side head. I'm stumped as prior to changing out the sending unit, never had this issue. Truck does not overheat even with AC running in 100 degree temp. I'm kinda afraid of driving much as I would have no way of knowing if I did overheat until too late.

[dmjlambert]

That is a crazy thing and would also have me stumped, too. Idea: Disconnect the sending unit wire and attach your ohm meter directly to the sending unit while installed, and see what resistance curve you get as the engine is warming up. If the sending unit has good output then it would have to be the wire going to the gauge or the gauge.

[dmjlambert]

Also, you could clamp a jumper cable from the neg battery terminal to somewhere on the body of the truck, and see how it behaves, to test if you have a good ground between engine and the rest of the truck. The ground for the whole truck normally goes through the engine and to the rest of the truck through ground straps between the engine and the body.

[Shinysideup]

So it does this on the FIRST warmup after install, does it do it the subsequent times as well?
Thinking if an air bubble it got burped out when thermostat opened. I use the practice of drilling an 1/8" hole in thermostat to purge bubbles.

[Already Gone]

Timely post for me. My temp gauge just stopped working a month or so ago. Its new as well as the sender and they've only been in the truck 2 years. It does work but only just comes off the cold mark when engine is at full opertaing temp. Any suggestions about what could be wrong would be appreciated. Was heading to a car show a 100 miles away a month ago and it stopped working half way there.

[LockDoc]

-
Remember there are two different senders for our trucks, one for the three hole light warning clusters and one for the 7 hole gauge clusters.It's not unheard of for a parts store to sell you the wrong one for your application.

The first picture is for gauges and the second is for lights.

LockDoc

[dmjlambert]

Stopped working, hmm.. Well that is different from what tdangle reports, so you may need to make a separate thread about it if the conversation needs to get involved. But the same troubleshooting I suggest in post 5 and 6 above is what I would start with, chart your resistance curve and check grounds. Here is that thread with the chart from davepl that tdangle was probably talking about. http://67-72chevytrucks.com/vboard/s...d.php?t=714839

Things that apply to any problem with stock gauge and sender:
1. Only the stock sender with 1/2" NPT will work with a stock gauge and it's possible to get the wrong sender like LockDoc mentions. Some folks have reported that the part number is TU5, and the part number I used is GM part number 12334869, AC Delco number G1852. Since I have a 3/8" NPT hole in my engine, I bought the 1/2" sensor and had forum member A1971Blazer turn and thread it. Other folks have had forum member Brian Mac turn and thread theirs.
2. There's a finite number of things involved that you need to check, so that is good. The sender, the wire going from the sender to the instrument cluster, the firewall connector, the cluster printed circuit, the gauge, the ground connection from instrument cluster to truck, the ground connection from engine to truck. You can check all that with an ohm meter and some wire with alligator clips.

Here are a couple more threads about the subject.
http://67-72chevytrucks.com/vboard/s...d.php?t=703388
http://67-72chevytrucks.com/vboard/s...d.php?t=627880

[tdangle]

I’ll get back to it tomorrow. Correct sender unit have 2 due to Amazon screwup. I’ll double check grounds but shouldn’t have changed. Resistance checks out. Used a resistance box to verify gauge. It’s OK

[Already Gone]

Originally Posted by dmjlambert

Stopped working, hmm.. Well that is different from what tdangle reports, so you may need to make a separate thread about it if the conversation needs to get involved. But the same troubleshooting I suggest in post 5 and 6 above is what I would start with, chart your resistance curve and check grounds. Here is that thread with the chart from davepl that tdangle was probably talking about. http://67-72chevytrucks.com/vboard/s...d.php?t=714839

Things that apply to any problem with stock gauge and sender:
1. Only the stock sender with 1/2" NPT will work with a stock gauge and it's possible to get the wrong sender like LockDoc mentions. Some folks have reported that the part number is TU5, and the part number I used is GM part number 12334869, AC Delco number G1852. Since I have a 3/8" NPT hole in my engine, I bought the 1/2" sensor and had forum member A1971Blazer turn and thread it. Other folks have had forum member Brian Mac turn and thread theirs.
2. There's a finite number of things involved that you need to check, so that is good. The sender, the wire going from the sender to the instrument cluster, the firewall connector, the cluster printed circuit, the gauge, the ground connection from instrument cluster to truck, the ground connection from engine to truck. You can check all that with an ohm meter and some wire with alligator clips.

Here are a couple more threads about the subject.
http://67-72chevytrucks.com/vboard/s...d.php?t=703388
http://67-72chevytrucks.com/vboard/s...d.php?t=627880

Thanks for the info. In 2016 a AC Delco G1852 was installed along with new dash and gauges and i've only put 2000 mikes on truck since.

[Sparky dave]

I replaced mine and new one does the same thing ( goes off the scale) then returns back to normal when engine cools. I think mine is wrong sender unit and has the wrong resistance reading to the gauge. Was going to wire in a resistor in-line (at operating temperature) until the needle registered 3/4 the way up the gauge. Or you could just get the core t sender unit. I can’t get parts so cheaply being in England

[tdangle]

Update:

yes i have the correct unit ACDelco G1852. I'm starting to think the old one before it went out completely was not reading correctly. Started it up from sitting overnight. Radiator full. Pretty much pegged as it was warming up. I kept checking the hoses and the upper radiator hose was cool to touch as it warmed up. both heater hoses and lower radiator hose were warm to touch. Then as the thermostat opened and the upper radiator hose got warm the gauge went down to mid scale. Drove it a few miles and it stayed right at mid scale Low 80's out side kept the AC off. then after about 3 miles noticed the gauge was at the lower end of the normal range. This is where I'm use to it being. Stopped and went in a store came back out and the gauge started reading mid scale again.

Not really sure what is going on. The is the original engine, I believe it was rebuilt once so it's fairly high mileage. Could I have a problem with the engine???

[Shinysideup]

Glad to hear you fixed your air pocket via my advise, glad I could help!