View Full Version : Temperature gauge/sending unit question

06-26-2008, 08:55 AM
I've looked at the recent discussion on temperature sending units and from reading through that thread, I understand that if I ground the temperature gauge, it should go to hot.

On our '54 Commander (Grace), when I start the engine, the gauge is at cold. Within a couple of minutes, it gradually rises to hot and pegs there. This is long before the coolant is hot. Then, when I shut the engine down and open the battery cut-off switch, the gauge remains pegged at hot. It eventually returns to cold, but that might take an hour, half a day, or overnight; I don't really know how long it takes.

I'll tell you all that when I was last driving the car 5 years ago, the gauge operation was all right. I haven't changed the sending unit or the gauge, though I did rewire the whole car. The gauge was never out of the instrument bezel, and all of the other electrical gauges (oil pressure excluded, of course) seem to work all right. The coolant level is what it should be, though currently I'm running water only since I want to flush the system before adding antifreeze. Previously, I removed the freeze plugs and cleaned out the block (what a messy job).

The question is what might be the problem.

Kindest regards,

Alan Mende
Hummelstown, PA

06-26-2008, 09:31 AM
I am having some of the same problems. You have probably seen my thread. Sometimes I can use a test meter and check the resistance in the sending unit wire and it reads about 200 ohms. When hot it should go down. (Look at my thread). When I start the car and the ohmeter will peg at 0. Not what it should be . I have de-volted the guage with a resister to 6 volts so that should be ok. I have another guage and going to test that soon and will let you know. When it did work some I got the similar result with the guage pegging as it shorted to test the guage. Still scratching my head.


06-26-2008, 09:40 AM
Did you change the voltage when you re-wired? You would still need to end up with 6V at the guage. (I think I am right)
You may notice from the thread on my question that a respondent indicated the resistance on the ohm meter for the sending unit should be greater than 130, about 51 ohms at operating temp and 21 at hot. These were numbers he got when testing.


06-26-2008, 11:24 AM
Our electrical system is 6 volts, though up until we started the restoration, there was an 8-volt battery in the car.


06-26-2008, 06:57 PM
Knowing something about electronics, I would ask you if you changed the gauge of the wire when you rewired the car? If you changed the factory wire, you may have changed the resistance in the wire between the sending unit and the gage. I do NOT know what the factory wire is. For example, if it was single strand 16 ga. wire, and you put in single strand 18 ga., the resistance would be lower and the gage would see ground inappropriately. Same if you changed single strand to multi-strand. If the ohms between the sending unit and the input of the gage are lower than factory specs, the winding on the gage needle will react as if ground potential were reached sooner than in actuality.

'50 Champion, 1 family owner

06-27-2008, 06:19 AM
Excellent reply. I found an earlier thread where someon had indicated that the resistance, I think at the guage was greater that 130 ohms cold, 51 at operating temp and 21 at hot but not sure where to take the reading. I could take a reading direct from the lug of the sending unit cold, and operating but not sure what it is supposed to be at the guaage. If those were the numbers, I would not be sure what resistor to put in the wire to reach those numbers at the guage. Makes one admire the guys who do all this originally and then we spend years screwing it up.
thanks for the tip about the wire resistance altering the output