Temp & fuel gauges - what am I overlooking?

It's annoying when you're doing everything logically and still getting illogical results. We've all been there.

These guys have you on the right track but I'll just let you know what my order of operations would be if your gauge doesn't work as soon as your test box is connected.

I'd disconnect the wire to the temp sender and place the test box in the same general spot so the wire doesn't get disturbed too much.

Ground the test box to the intake, check your test box to intake ohms and then your intake to battery ohms.

I'd set the test box to 23 ohms (middle) and then check the resistance at the sending unit wire as it goes into the bulkhead.

Check the resistance under the dash as it comes out of the bulkhead.

Check the resistance into the gauge connector. Should still be in the ballpark of 23 ohms.

You could still have an intermittent open in the wire somewhere but at least you have a baseline that your resistance feed is good as the wire currently lays.

Then check the gauge cluster to dash frame ohms and the dash frame to battery ohms.

Last check would be ensuring that the IVR is showing the same voltage behaviors as it sits in the car that you got with it out of the car. Use the same ground points here as you used when checking ohms in the previous step.

I can't see how it wouldn't work if everything checks out fine. Only other possible issue would be an intermittent open in the wiring under the dash that you are disturbing when doing your checks. At that point I'd check the resistance at the gauge again and start wiggling the harness around to see if you get a fluctuation on your gauge. Might be smart to do the same shake test as you test the IVR voltage as well. Since the IVR is pulsing, you'll probably have to go slower and hold the harness one way for a few seconds and another way for a few seconds to see if you can find a spot where the voltage drops out.