I have a 49 3100 with a 55 235 engine. Temperature gauge has been working fine for several years until a week ago when it stopped reading anything. I put a new thermostat in and temperature gauge still does not move. I think my next step will be to replace the sending unit. Anyone able to give me any other ideas what it could be. Thanks for any help.
Do you have the mechanical type gauge or the electric type gauge? If it is the electric gauge and it stopped reading suddenly one of three things happened I think: 1---the gauge stopped working, 2---the sender unit stopped working (most likely, I'd bet) or 3---you've suffered a bad electrical connection or bad wire somewhere. To test your sender get a handheld meter and set it for resistance...maybe something like 0-200k ohms. Remove the wire at the top of the sending unit. Connect one probe from the tester to the top of the sending unit and the other to a good ground. Turn on the ignition with the engine cold and make a note of the resistance (may be something like 74K ohms or so). Then warm up the engine and take that same reading again. It should be much lower. Please let us know what that reading is and we can start chasing the demons.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
I'm not sure what you mean by "mechanical or electric". There is one wire (bare) from the sender to the temperature gauge. And the gauge just stopped reading. I warmed up the engine and the gauge does not move. So i'm thinking it is the sending unit.
Does the "sender" have a threaded stud and a nut or a slip on connector for the wire as shown in Guitplayer's picture below holding the wire on? If so, it is electric and the wire should not be bare.
If not, the gauge is mechanical and the wire is actually a tube containing a gas that increases in pressure as the engine gets hotter, moving the needle in the gauge. It the tube had been broken the gas escapes and the needle sits on cold forever.
Last edited by Bill Hanlon; 07/25/20234:35 PM.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
That "bare" wire isn't a wire- - - -it's a copper capillary tube attached to a bulb that's bolted into the cylinder head. The bulb and the tube are filled with a volatile liquid- - - -usually ether. Eventually, the tube gets damaged or springs a leak, and the needle stops moving. That type of gauge is almost impossible for a backyard tinkerer to fix. I have had some success buying a cheap mechanical gauge and swapping the liquid-filled bulb to an original stovebolt gauge head, but it requires an extremely cold liquid to condense the pressure medium like dry ice and alcohol, or maybe liquid nitrogen. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
The gauge is mechanical and must have a leak. So I will have to replace the whole thing tubing and gauge. So my question is where can I get the whole unit, bulb, tube and gauge Thanks Doug
If you like AC Delco , NOS is available on Epay.. Not sure where the classic part is made. I would only like to do that change ONE time. I put ALL AC N.O.S. in my cluster. Todd
Yes, that's why I asked if you had a mechanical gauge. If you happen to live where there's a gauge repair service, they can repair it. I had mine repaired 40+ years ago and it is still fine. Think the guy charged me $15 or something like that. He just installed a tube and filled it with either ether or pure alcohol. I can't recall which he used. Good luck!
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
