79 250 2nd temp sensor

[79Square]

I am a newbie, and just got a 79 C10, I6 4.1 250. I'm having issues with the temperature gauge readings, and the gauge stays at the low 'C' level. I am replacing the thermostat and the temp sending unit(s), and noticed that there are two sensors. One at the thermostat housing (see pic 1) and one in the cylinder head (pic 2) I have removed both (pic 3). Can someone tell me what the small one is? and how to replace it. I have been to a couple of parts stores locally, as well as googled, and can not find it anywhere. The thermostat sensor is a 3/8 npt, and the smaller one appears to be a 1/8 npt. Also, they both are connected to green wires, and when I ground them, they both peg the temp gauge to full hot 'H'.

Thanks for you help.

[Palf70Step]

The thermostat housing is the one for the factory temp gauge. I had same issues with mine and it turned out to be the actual gauge. Make sure you get the sender for the gauge and not the idiot-light model.

The second one kind of looks like something for an aftermarket temp gauge?

My 81 250 had a block off plug in that head hole

[ray_mcavoy]

The 1979 wiring diagrams show both a metal temperature switch and coolant temperature switch (or sender for gauges) on the 250 & 292 I6 engines. And it shows that they are both wired to the same circuit (#35, dark green wire) that goes to the temperature light (or gauge).

The coolant temperature sending unit (the larger one in the thermostat housing) provides the variable resistance that operates the temp gauge. While the metal temperature switch (smaller one in the head) will connect the circuit to ground, pegging the gauge full hot, if the cylinder head gets hot enough to trip the switch.

[79Square]

Thank you for the quick replies. You all confirmed my thinking, they both work in tandem. One solution is to just plug it, but I'm sure it would help if I could use both.

The problem is I can't find it. The parts stores and web searches don't even list as a part for the 79 I6 250.

Does anyone know where I can find one?

Thanks!

[ray_mcavoy]

I think the metal temp sensor is GM #14040900. It's a discontinued part but there are a few NOS ones currently listed on eBay.

Have you tested the existing one to see if it is defective? I believe the only time it will affect the temperature gauge reading is if the cylinder head metal gets hotter than the trip point of that switch. Unfortunately, I don't have any specifications on that switch to know where that trip point is but I'd think it would be under 350°F. So you could test it by hooking it up to a multimeter (set to measure continuity) and heat the end up with a heat gun or propane torch. The meter should show an open circuit with the switch below the trip point temperature and show a closed circuit (continuity) above that temperature.

[79Square]

Problem solved. I was able to find the smaller sensor at O'Reilly here in town (had to be ordered). It is a Standard TS135. It shows that it was for 1979-1981 models.

Here is an online link to one https://www.carid.com/standard/coola...specifications

Thanks for everyone's help.