Temp Gauge Issues

[Awann99]

I’m trying to get the temp gauge working on my tach/vacuum cluster. It sits dead in the middle when the key is off or on. It will jump to the hot side and back to the middle when I start the truck. I pulled out the fuel wire from the fuse box and the temp gauge pinned at hot. I’ve tried grounding the green wire to the engine and the gauge doesn’t budge. The green wire doesn’t seem to have any breaks and it is connected firmly to the fuse Panel box from the engine bay to the cab. Is there a way to just bypass the gauge cluster and run the temp gauge separate?
Thanks.
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[franken]

I'll engage BS mode for a minute...

Since the sender has one wire, I'd guess it provides a ground to a variable resistance based on temperature. That suggests the gauge is fed by 12V on one side and the other goes to the sender.

12V, gauge, sender, ground.

I'd suggest looking at the wiring diagram and checking with a voltmeter before wiring the gauge separately...

[Awann99]

Originally Posted by franken

I'll engage BS mode for a minute...

Since the sender has one wire, I'd guess it provides a ground to a variable resistance based on temperature. That suggests the gauge is fed by 12V on one side and the other goes to the sender.

12V, gauge, sender, ground.

I'd suggest looking at the wiring diagram and checking with a voltmeter before wiring the gauge separately...

I checked with a volt meter and got around 3.45 volts from the end of the green wire that connects to the temp sender. I also looked at a wiring diagram and rechecked all connections
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[RustyPile]

Originally Posted by Awann99

I’m trying to get the temp gauge working on my tach/vacuum cluster. It sits dead in the middle when the key is off or on. It will jump to the hot side and back to the middle when I start the truck. I pulled out the fuel wire from the fuse box and the temp gauge pinned at hot. I’ve tried grounding the green wire to the engine and the gauge doesn’t budge. The green wire doesn’t seem to have any breaks and it is connected firmly to the fuse Panel box from the engine bay to the cab. Is there a way to just bypass the gauge cluster and run the temp gauge separate?
Thanks.
Posted via Mobile Device

You need to do some more trouble shooting before condemning anything..

First - Gauge readings with the key OFF are meaningless.. There's no power applied to the gauges when the ignition is OFF.. I'm not sure why removing the fuel wire would affect the temp gauge?????

When the ignition switch is in the start position, it temporarily grounds the green wire. The gauge moving during the start engine cycle indicates gauge circuit is intact and working. Two issues come to mind. Either the calibration resistor on the back of the gauge is bad, or the sending unit is bad or the wrong type..

With the key on, disconnect the green wire at the sending unit, the gauge needle should move to full cold.. Ground the wire and the gauge needle should move to full hot.. Remove the sending unit. Rig up some jumper wires and connect a digital volt meter to the case and connector on the sending unit.. Place the sending unit in a pan of water and heat the pan on a stove.. As the water heats up, the resistance of the sending unit should change.. If it doesn't change or only changes a small amount, the sending unit is bad.....

If the sending unit passes these tests, you have to check the gauge.. Remove the cluster so you can get at the resistor. Remove the resistor from the gauge and test it with a DVM.. It should read somewhere around 85 - 90 ohms.. You cannot properly test the resistor while it's connected to the gauge.. Don't shortcut this step --- remove the resistor.. If the resistor tests good, the gauge itself is bad...

[fishmunger]

A while back I did a bunch of research on here about the temp gauge issue and I found a couple guys that swore that by adding a little extra ground wire from the gauge itself to the dash it would fix your problems...?

[fishmunger]

Like I was saying, I read every thread I could on the temp issue and I copied and pasted all that seemed pertinent into a word doc so I could print it out and take it to the truck with me. I copied and pasted it below. Sorry if its a bit disjointed but might save you some time

TEMP
Order your temp sending unit for a 72 K5 with a 350. This will make sure the sending unit matches the gauge. Not sure what your new engine has but I would change just for piece of mind.

To test your temp gauge, pull the green wire from the sending unit and ground it with the ign key in the run position. If the gauge pegs, you should be OK with gauge and wiring. Have some one watch the gauge as you ground the wire. You only want to touch it to ground briefly. If the gauge does not move, then check for power at the number 3 terminal on the cluster connector. This is hot feed for temp and fuel gauge. It is a pink wire.

As stated above, check your dark green wire & gauge.

Where does your temp gauge read now? If it's pegged already, that frayed wire may be grounding out.

Otherwise, with key on, ground the sender end of the wire to the block and the gauge should peg past hot.
Then at least you know the wiring and gauge are working.


First things first.
That yellow wire is connected to the key switch and is the old ignition wire for the points. It is shown as the pink/black wire in the diagram. It goes through the firewall block by the master cylinder into the cab and to the ignition switch.

You must have a different wire to power the HEI distributor.

You have to check the yellow wire for power with the key on and it if is hot then you have to tape it off or insulate it somehow or it may short against the block or starter and blow a fuse or start a fire in the harness.

You need to locate the green wire with the slide on clip and connect it to the sender. There are two senders for our trucks, one for the idiot light in the dash and one for the temperature gauge in the gauge dash. What do you have in your dash? The sender will work in either head because the heads are interchangeable. They will fit on either side of the block but they will be reversed.
The sender just grounds the wire as the engine heats up but at a different rate for each sender. If you find the dark green wire just hook it up and see if the gauge or light works. If it doesn't work or it works but not correctly then start a new thread and we'll help you with it.

When you connect your green temperature gauge wire to the sender and the gauge pegs, it means the sender is shorted to ground and has no resistance.
the sender is a variable resistor to ground and should read high ohms when the engine is cold and low ohms as the engine heats. You can verify this by doing a continuity check on the sender. Grounding the temp wire will do the same thing and is a good test for the gauge . If the gauge is good the needle will go to full hot when you ground the wire providing that the wire is not open or grounded somewhere in the harness.


There are a few pink wires in the harness can you see where it goes. There is one to the ignition and one to the fuse panel as well as one to the cluster which feeds the gauges in the cluster.
With the wire unhooked from the sender the gauge should read full cold. With the wire grounded, it should read full hot. If the gauge does not move when you ground the wire, the wire is open somewhere between the sending unit and the gauge or the gauge is bad.
the correct connector is a female 56 style terminal (Napa part# NW 725147)
ran into this last summer of the gauge reading full hot. It turned out to be as noted in the picture above that the bottom lug on the gauge needs grounded to the mounting tin and the tin must be grounded to the back of the cluster.
I have had to run a separate ground wire for the temp gauge on almost every cluster I have owned.
so you are saying run a ground wire from the bottom prong of temp gauge to body of truck somewhere? Never seen this in my life and owned 6 67-72 trucks. Willing to try anything at this point.
Picked up a tach cluster in the spring, and when I put it in, everything seemed to work fine except for the temp gauge, which went to max with the key on. The old one was functioning well, so figured the sending must still be good. Tied in a ground on the bottom post to the dash, and voila! Gauge read cold, and went up as it warmed. Didn't have it running long enough to see how high it climbed, but should be fine.
Thanks guys!

just ground the sending wire on the engine and see if the gauge moves....great place to start
Gauge is working!!!!!! Got into the harness because I realized I had no continuity through the wire. Someone had gotten into the harness and cut the wire. Idiots!!!!
Spliced wires back together, check for continuity and sure as heck, gauge works. Sending unit is perfect, as the gauge gets to the first hash mark and stays put. The harness is taped and looks sweet going up the firewall. Thanks for all the advice.

Just for reference-the gauge is not grounded to the cluster(see above) and I Teflon taped the sending unit. Just my two cents...

When not grounded, the gauge goes full hot.
AC Delco part # G1852 new temp sensor comes with plastic sleeve to attach wire sold by rockauto.com $14.35 +shipping

Alternative part #'s

AC DELCO 1234869

AC DELCO 1513321

[Already Gone]

I'm having issues with my temp gauge too. About a year and a half ago while driving it just went to cold and stayed there. During Highway driving it would move a little towards hot. Last summer in 80 degree weather with the A/C on and driving 75 MPH it went to half way.. The gauge is about 3 years old and the sender a year old with hardly any miles. I bought a new sender and installed it and no change and have good solid connection with original connector on the sender. All that is left is the gauge which I plan to change out in the sprig with an NOS one I bought. If anyone has any idea what my problem could be please post as I really don't want to pull my dash out. Oh yea my dash is a Speed Warning, Tach and Vacuum Gauge cluster redone by board member, PTs70CST, Paul Timmerman and only been in the truck for 3 summers.