Temp sending units

[dr09911]

I have a '77 C10, 250 6 cylinder with gauge option, 3 speed manual. I am having temp gauge problems and I am starting with the sending unit to narrow the problem. There is one at the the thermostat housing that has a "nail type" head on it. There is also another in the head that has a "spade type" head. The wire from the harness (dark green) goes to the spade connector, then over to the "nail" head type unit. I am being told the "nail" head unit is for the gauge option. The "spade" unit is for just an idiot light. I have both. Anybody else encountered this problem??

[sierra4x4]

I have never seen anything like what you have their but I believe that if you ground out the wire at the sending unit then your gauge will read to the high side of the gauge if the wiring and gauge are good.

[dr09911]

Thanks Sierra4X4, all those checks have been done. The only thing I can think of is this application is for either a light or gauge having 2 sending units in series. After work, I put the new sending units in and problem was still there. I removed the instrument cluster cover and pulled the temp gauge. I cleaned up the studs, the contacts and pushed the gauge back in. After letting the truck run a few minutes, the gauge was reading hot again, but I noticed the light behind the gauge was not burning. I reached around to twist the bulb and felt a bubble in the printed circuit...pushed the bubble and the temp needle jumped to about 170 and stayed. On a test drive, I would hit a bump and the needle would go to hot. I would push on the backside of the printed circuit and it would jump back normal. I know what my weekend project will be!!

[D13]

Same as my 77. In normal driving the spade is 'open' to ground and the 'nail' changes resistance to feed the green wire and thus the temp gage. When you overheat (BTW if you do this on an integral it's probably toasted the head) the spade turns on the light.

[dr09911]

Thanks for the explaination D13. No intergal head here. Luckily the truck was not running hot, just a false temp reading caused by a problem with the printed circuit board.