Temperture gauge is pegged on high

[gmc684x4]

Is the sender for your model of truck ?
Example 96 99 vortec 350 engines use a different range temp sender then the 67 72 trucks

To make one work you would need to install a 67 72 temp sender in intake or thermostat housing

To use in stock location one needs to have the threads cut down as a vortec head has smaller port for temp sender

Your guage is proubly fine i think you have the wrong range of sender?

Sender range and guage have to match or they will not display correctly

What motor transplant did you install?

[dmjlambert]

My approach would be to remove the instrument cluster, take the bezel and lens off, and nudge the pointer gently to see if it dislodges from the pegged position. If it does not operate after that, replace the gauge.

I think it is likely you have the incorrect sender. At operating temperature it should be more like 120 ohms (roughly). 350 ohms will not take the gauge above reading cold. Does the sender look like this?


Check out this thread, jaros44sr recently resolved problem with temp gauge not reading correctly because of incorrect sender.
http://67-72chevytrucks.com/vboard/s...d.php?t=807068

[gmc684x4]

Here is an example guage has not been powered up in 15 years still reading the temp it was running before it was parked and the engine swaped to a different truck

[Seventy Two]

After it cools down I going to check the ohms on the sender again and see what comes up.

I think I will also remove the gauge and see if I can bump it off of pegged hot. Then use some jumper wires to directly connect from the battery to the gauge. Maybe it is a pinched (grounded) lead between the sending unit and the gauge.

crossing my fingers

[dmjlambert]

Make sure the sender looks just like the one pictured above. Other senders will not work. The resistance when warmed up is the important part. 350 ohms from the sender will show on the gauge in the cold range.

[Seventy Two]

Mistake in my last post, I measured 350 ohms and thought it would be less not more.

GMC68. It sure looks like the right gauge. The engine swap was a vintage 350 for a 350 crate engine and the dealer new it was going into a 72.

I agree the gauges just stop at the last reading but when it is powered back up it should adjust to the current situation right.

I think Im going to pull the gauge and see if I can bump it off pegged hot and then use jumper wires directly between the battery and gauge to test full cold and full not and see what comes of it. Maybe it is grounded lead.

Hears hoping.

[Seventy Two]

BTW, my needle is pegged on the right side of the H.

[Seventy Two]

I am going to try your suggestions and agree 350 ohms seems to high after being warmed up. Yes the sender looks exactly what you have shown.

Thanks for your input.

[gmc684x4]

New crate 350 i suspect the sending unit is wrong
Not all salesmen know the difference the temp sending unit is proubly wrong

Go to nappa or similar parts store with your meter ask gor the gauge sending unit for 67 72 check its cold reading and compare to what you have in the truck

The sending units for lights and guages and different between years
Quick look at rock auto show a 72 and different units for a 84 both c10 with 350

Yes the guage is correct vintage 1970 k20

I will look at the old 283 my dad had last in the 72 see what it reads

Most of the 70s smog heads i recycled in my collection

[gmc684x4]

User davepl posted this on a 70 shortbox senfing unit reading

http://67-72chevytrucks.com/vboard/s...d.php?t=725468

[gmc684x4]

Never thought of looking at Nappa they list ohm range values at 100 f 328 ohms at 220f 80 ohms that would put it at about 100 to 120 ohms at normal operating temperature

Makes sense ohm value drops low for guage reading as a clean ground will pin meter action

[Seventy Two]

Nice find GMC68!
I have a little progress myself. I pulled the gauge out and using two jumper wires from the gauge to the battery, I was able to peg the needle hot and cold by simply reversing the jumpers at the battery.
This suggests to me that the gauge is good and that I am dealing with a grounded lead or a bad sending unit.

Thoughts?

[Seventy Two]

Well I bought and installed a new sending unit, turned the key to on and immediately pegged the needle to hot.
So I am going hunting for an electrical gremlin between the sending unit and the gauge.
ugg!

[LockDoc]

Originally Posted by Seventy Two

Well I bought and installed a new sending unit, turned the key to on and immediately pegged the needle to hot.
So I am going hunting for an electrical gremlin between the sending unit and the gauge.
ugg!

Any chance you could post a picture of the back of the gauge?

LockDoc

[Andy4639]

Take said gauge out and look at it. I had one I pegged out. Took it out and it was just stuck. Tapped it with my finger and it's working fine.

[gmc684x4]

Do you have the schematics from the electrical section of this forum they did a fantastic job look there for a full version with the different options hei an s1 alt hei with cs30 alt etc

i cut and downsized it so i can include the basic schematic here

You can remove the cluster connector and the ignition switch plug temp sender then check with your meter that wire 20DG is grounded

Then separate the bulk head connector and re check for grounded wire on both sides

Following 20DG sender wire to bulk head inner bulkhead fuse panel to ign sw then to cluster

[tdangle]

The gauge works on resistance to ground. Wire off the sending unit with key on it will read cold all the way to the left. Ground the wire and tilt will read Hot pegged to the right. if it checks out then its the sending unit. Refer to the chart posted. When cold the sending unlit has high resistance and as it heats up lowers resistance to ground. Tape or bad ground will NOT cause gauge to read hot. It will cause a cold reading. Actually really simple to diagnose. After running these checks if the gauge works correctly, you have a bad or wrong temp sending unit.

Good Luck and it you want you cam PM me.

[Seventy Two]

I pulled the gauge and with jumper wires to the battery, i was able to peg the needle hot and cold by reversing the leads at thr battery. This suugests to me that the gauge is working fine. So after re installing the gauge I went and bought another sending unit to try. After installation was complete I turned the key on and pegged the needle to hot once again. So i am thinking it is a wiring issue and will start that hunt.
BTW, while I had the gauge out, I noticed that I can peg the needle hot or cold with my ohm meter on the continuity ( beeping) setting.
At this point I am thinking an electrical ground is happening that is pegging the needle to hot when the key is turned on.

[Seventy Two]

Hey LockDoc, I have a photo but not sure how to load it. I think I need a photo bucket or something. Is there tutorial on this forum that will explain how I do this?

[Seventy Two]

Andy, I did have the gage out and with jumpers to the battery I was able to peg the needle hot and cold by switching the leads at the battery (or the gage).

[Seventy Two]

GMC68, I will take a run at this and get back to you. Thanks for electrical references.

[gmc684x4]

Photos can be loaded easily at lower resolution
Full resolution pictures the site does not like uploading

[Seventy Two]

tdangle, With the sending wire off and key on it still reads hot and this is with a new sending unit from Napa.

[Andy4639]

The gauge is feed 12 volts through the harness and the wire going to the motor is a ground wire. If it has been cut are wire exposed to metal it will peg. I would start by checking the wire from the motor back to the cluster.

[gmc684x4]

Removing the wire from the sender will not make the guage start reading cold as the circuit is incomplete and will need to read 300 plus ohms to drop its reading
That it re pined reading after you zeroed out tells me that 20Dg wire is shorted to ground somewhere