Factory temperature gauge issue

[68GMCCustom]

I'm having an issue with the temperature gauge on my '72. The gauge has always read cold and wouldn't move. The guy I bought the truck from said he thought that meant the gauge and not the sender was bad and provided a factory gauge replacement when I bought the truck. I finally switched out the old temp gauge and when I went to test it as soon as I hit the key the new gauge swung to the hot side and when I started the truck the gauge gauge pegged to the hot end. I have put a meter on the wire after taken off the sender and it has some resistance and is not shorted to ground. Is my new temp gauge bad, the sender bad, both or something else?

Thanks in advance for any assistance!

[davepl]

Depends on where the gauge is getting the erroneous ground signal from. If you're certain its not from the sender, then it's in the cluster or the gauge.

I don't know what the resistance is on the idiot light senders, but it should be high enough I think that it shouldn't cause this even if you had the wrong sender.

What I did was boil mine in a pot of water with a thermometer and an ohmmeter, wrote down the readings of resistance v temp, and mapped it out. So I know mine's right, and you can't go wrong doing it that way...

[ray_mcavoy]

Seeing as this problem started after you replaced the gauge, I'd recommend double checking it's ground connection to the cluster.

The temp gauge has 2 coils inside. A "hot" coil that pulls the needle to the hot side and a "cold" coil that pulls it to the cold side. Both coils share a common power feed connection.

The "cold" coil is grounded to the cluster housing through a resistor located on the back of the gauge. So a bad connection between the gauge and/or resistor and/or cluster housing (or an open circuited resistor) will prevent any current from flowing through the "cold" coil and pulling the needle to that side.

However, the "hot" coil is grounded through the variable resistance sending unit so current can still flow in that coil. And without any influence from the "cold" coil, that will result in the needle pegging to the hot side.

[68GMCCustom]

Thanks guys. The problem was there when I bought the truck. I replaced the gauge since the PO had given me one with the truck, and its reading swapped ends on me with the gauge change. I'll check the grounds.