I get no reading from temp gauge on 2004 Bonneville.

When turning on the ignition it does not swing to hot and drop back to cold. I replaced the temp sensor and it had no effect. Disconnect two wire connector, turn on ignition and same result.

Open circuit somewhere?

Bad ground?

My wife said temp gauge went off the peg a few months ago.... you would think that she would mention this....

FWIW, I put new gasket and upper manifold on it earlier this year.

I'm trying to get a schematic from local library. They have a lot of GM service manuals.

very common for the clusters to be bad in alot of GM vehicles. if the gauge isnt sweeping its defective.

I took the gauge cluster out. It'* pretty easy. The ring around the ignition key snaps out as well as the triangular panel for the DIC buttons. After that there are two plastic push rivets at the top underside of the large bezel that covers the cluster, radio and climate control. I broke one of the rivets removing it. Once the rivets are out, you can just pull carefully on the bezel and it un-snaps. There are no screws holding it in.

The cluster also snaps in. There are about 6 push to release clips. Most are on top of the cluster and two at the bottom sides. Pull out on one end while pushing clip levers to the side one at a time and the cluster comes out in one big piece. The electrical connections are on the back and plug in so there is no harness to disconnect. I checked for corrosion on the electrical connections and found nothing.

Next I pulled off the drivers side door sill cover. It just snaps in place. Near the parking brake there is a plug with about 10 black wires going into it. This is the ground for the temp gauge, fuel gauge, speedometer and a lot of other things. I could not find anything unusual here either.

I called the dealer and was quoted $600 - $700 for a new rebuilt cluster with odometer set to my correct mileage. This seemed a bit much as the labor to remove the cluster is about 10 minutes and installation would be even less.

A search of the internet turned up at least two companies that specialize in instrument cluster repair (probably what the dealer was going to use). I was quoted $125 by one guy to fix the temp, fuel and speedometer gauges. UPS shipping was going to be a few $ more depending on if I wanted overnight service.

It seems that the stepper motors used to drive the needles are a week spot for certain years.

Good information. Always a big savings when you can fix it yourself