1995 f-250 4x4 standard cab 460 auto. My temp gauge is pegged. I replaced the sending unit (NAPA) and have the same issue. If I unplug the sending unit it the needle drops a little, but not much. Attaching the sending unit only pegs the gauge again. After running for 15-20 minutes it'll drop to the O in normal, bounce rapidly a couple needle widths, then peg again. It'll cycle itself like this over time. All other gauges read normally, engine temps are normal, and no CEL.
Any ideas ? Anyone experience this problem before ? Thanks for any help !!!
Your temp sensor is a resistor that changes with the temp. Resistance should be lower as the temp goes up.
This makes me think that you have a partial short somewhere in the connection from your gauge to the sensor.
When you hook your sensor, it adds resistance in parallel, which lowers the overall resistance further. (This is just part of my reasoning. Not important if you know what I mean ). This causes the gauge to read hotter.
Anyway, check your wiring and panel. I would get an ohmeter and check the resistance of the sensor wire to ground. Then I'd see if there were any connections along the way to help isolate the problem. (If you can disconnect the wire at any point, you could see which side has the partial short).
Oh, and pull the positive batt wire off while you're checking things. There are reasons why it could cause confusing readings.
Replace the Temp sender.
Another poster just had the same problem when replacing the sender with one from NAPA.
They must have a bad run of senders.
Also make sure you are changing the right item. The one you want to change only has one wire to it and it pushes onto the sender. It sets behind the distributor and screws into the intake manifold itself.