Electrical Temp Sending Unit Wiring in 47 GMC

[Gippetto]

I bought a set of reconditioned factory gauges tha were successfully used in a 53 AD running a tuned port chev. My truck has a 55 235 with mechanical temp sensor port in head, not yet running. I got around that with a 261 thermostat housing which has an extra 3/8 npt port.
I blindly purcased a temp sending unit with no clue to resistance ranges or anything. First thing I did was wire it up on a bench. The temp gauge on the back of the cluster has three wiring lugs. One has a + which I hooked the positve battery to, another lug was clearly labled ground and the third I ran to the single terminal on the sending unit, then I also grounded the threaded body of the sender. When I powered up the gauge needlle started to climb . I switched it off before it pegged, it was rising rapidly but didnt look like a spike. The only thing that made sense was to reverse the 12 volt postive with the lead to the sensor. From here on nothing moved the gauge. I put the sensor in hot water and couldn't get the needle to move. Even when I wired it back to original config where it srarted to peg I get nothing. I measured the resitance through the temp gauge and I get readings so I suspect and hope I didnt blow the gauge. The sending unit itself read .500k ohms and decreased from there when I heated it. Apparentlly it works ok but is it the proper resistance range for my gauge? Also, I am not clear on the wiring of the three lugs on the back of the gauge or did I fry it when I reversed the leads.
Any suggestions? Maybe I'm better off just buying a replacement gauge and also get the proper sender for that gauge.

[VetteVet]

You need to keep in mind that the gauge cluster and the sending unit resistance of the OEM truck engine was designed for a 6 volt system and unless the units have been reconfigured for that or you are running a six volt system the units are not going to match.

If you connect the correct voltage to the positive terminal and the sending unit to the sending unit terminal, with the gauge body grounded to the ground terminal on the gauge, you should be able to touch the sending unit wire to the truck ground and with power on, the gauge should go to full hot.

You probably have the wrong sending unit for your gauge cluster and I would get the correct one for your gauge and whatever voltage you are running in the truck.

[Gippetto]

The truck the cluster came out of was 12 volts and the first time I powered it the gauge started to go full. Now I can't get the needle to move now.

Thanks for straightening me out on wiring. Why would it go full hot if it is sitting at room temp?

[VetteVet]

The sending unit regulates the resistance to ground of the voltage going to the gauge. If the unit is not calibrated to the gauge then the gauge needle will not move or it will move to hot if the sending unit resistance is too low or too high. When the coolant temperature rises, the resistance of the sending unit to ground is lowered and the gauge reads higher temperature. That''s why I suggested you ground the sending unit wire to the engine block or to another good ground.

The gauge has a resistor on it to protect the movement coils inside the gauge and I don't think reversing the + - would burn up the gauge. Have you measured the resistor on the gauge? If it reads zero then the gauge coils might be damaged. I seem to recall it can read anywhere from 40 to 70 ohms.

Your sending unit terminal to thread body should read nearer to 250 ohms cold I believe.
.500K would be 500 ohms. I would buy the correct sending unit for the cylinder head and engine temperature and then see if the gauge pegs when you ground the sending unit wire. If it doesn"t move then the gauge is likely bad. Make sure the gauge cluster is grounded,or the fuel and temperature gauges and the lights won't work either.

The sending units are different for gauge dashes and lighted dashes so be sure it is for a gauge dash.