This 1994 Chevrolet caprice came in with the complaint that the engine runs hot. I ran the engine while monitoring the temperature with the dash gauge, a Tech 2 scan tool and an infrared heat gun. The coolant gauge was reading hot but with the infrared heat gun it was not.
Data stream also confirmed that it was not running hot.
The cooling fans came on at 219 “F” and there were no signs of a overheated engine except for the dash gauge.
The cooling fans were actually on when I took this picture but with the flash and the speed of my camera they look like they are not turning.
It was time to shift my testing to an inaccurate temperature gauge. The temperature sending unit on this caprice is located near the rear of the passenger cylinder head.
After the engine had cooled down somewhat, I reached under the exhaust manifold and unplugged the sending unit.
I got out my decade resistance box and set it at about 1400 ohms.
I then connected one end to ground and the other end to the sender harness. I did use a jumper wire with the proper size terminal on the end that was probing the harness connector.
The temperature gauge went to full cold as it should have.
I reset my decade resistance box to 55 ohms and connected the leads as I had earlier.
Now it reads full hot as it should. I now have a diagnosis of a faulty temperature sending unit.
I replaced the sending unit and ran the engine again to confirm the repair.
I was also again comparing it to scan data.
So in the end this was not a run hot condition. Instead it was an inaccurate gauge reading leading the driver to think the engine was running hot.
I realize that most readers will not have a decade resistance box but you can go to an electronics supply company and buy a 1400 ohm and a 55 ohm resistor to test with.