1979 chevy k10 heating issue

[bogolek10]

On my 79 k10 I think i'm having a temp gauge problem but no sure. After i start the truck the temp gauge to seems to work normally, but it climbs right into the red not superfast but fast enough that its not believable, feeling around the engine its not overheating, anyway I shut the engine off and turn the key on the gauge is in the normal mark, start it again and it climbs right back in to the red. I replaced the temp sensor on the side of the head, tried another gauge, no changes. on another not there is no coolant leaks, i replaced the theromstat just to roll it out, i'm stumped. any help would be great thanks

 

[chengny]

Just as general information:

Gauge failures are often caused by defective wiring or grounds. The first step in locating trouble should be a thorough inspection of all wiring, terminals and printed circuits. If wiring is secured by clamps, check to see whether the insulation has been severed, thereby grounding the wire. In the case of a fuel gauge installation, rust may cause failure by corrosion at the ground connection of the tank unit.

VARIABLE VOLTAGE TYPE
The variable voltage type dash gauge consists of two magnetic coils to which battery voltage is applied. The coils act on the gauge pointer and pull in opposite directions. One coil is grounded directly to the chassis, while the other coil is grounded through a variable resistor within the sending unit. Resistance through the sending unit determines current flow through its coil, and therefore pointer position.
When resistance is high in the sending unit, less current is allowed to flow through its coil, causing the gauge pointer to move toward the directly grounded coil. When resistance in the sending unit decreases, more current is allowed to pass through its coil, increasing the magnetic field. The gauge pointer is then attracted toward the coil which is grounded through the sending unit.
A special tester is required to diagnose this type gauge. Follow instructions included with the tester.


That's from the GM service manual. Check the green wire that connects to the temp sender, it runs along the valve cover and takes a beating. A ground fault in that lead will cause low resistance/increased current flow through the hot-side coil and ultimately, false high readings.

Temp vs. resistance chart:

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Here is a dwg that I made as an attempt to explain why- see if you can make any sense out of it:

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[bogolek10]

Ok that would make sense, I'll give everything a look, thanks for the help