79 250 2nd temp sensor

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 your help.

The one that threads into the cylinder head is the sender that drives the gauge. The other one - that goes into the t-stat housing - is a metal temperature switch (don't ask me what it does - I have no idea).

The gauge sender shows as P/N 8993164. But that style of terminal is getting harder to find. Here is one on e-bay for $30:

Chengny, thanks for the reply. The sensor you show is the larger one (3/8"), the smaller one is the the metal sensor out of the cylinder head and is (1/8" X 27) and the one I can't find. The terminal can be different. The smaller one is very hard to find. The parts stores and websites don't even list the part for the 79 I6 250.

How about a Standard TS135? It is on rockauto, Amazon, and Summit, along with other sites.

Metal temperature switches are primarily (maybe only) an inline engine thing with GM. All they do is close when the designed temp is reached or exceeded. This will either turn on the idiot light or peg the gauge at HOT.
Our '86 Cavalier 4 cylinder used one of the 1/8 thread metal temp switches, but used the collar button style connection instead of the spade connector. I suggest one of these, as the parts listing for the '79 250 is lousy now.



My '78 292L6 uses a giant, 1/2" metal temp switch along with the sending unit for the gauge, like your 250. My metal temp switch has been broken for years. I always planned to fix it when I had everything apart for my stalled, but nearly complete, fuel injection project.



The 292L6 was never an integral head engine, like your 250, so they are radically different.

Problem solved. I was able to find the smaller sensor at O'Reilly here in town (had to be ordered). And yes, Nonstop, it is a Standard TS135. It shows that it was for 1979-1981 models. It is a 'binary' switch that works in tandem with the gauge switch in the thermostat housing.

Thanks for everyone's help.

Did it solve you gauge problem. I have the same issue right now with my 79 c10, 250 6 cylinder. New gauge and large sensor, but gauge barely gets past C. I have not changed the TS 135