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**hatzie** · 01-13-2018, 11:09 PM

The idiot light sender wouldn't move the gauge needle off the lower post til you were in an overheat situation.

The IR thermometer readings confirm what I already suspected. You are not overheating... there's something wrong with the gauge sender or power circuits or the gauge itself. At 204°F the gauge should sit almost dead centre.

The TEMP sender wire in the engine compartment is green with a Packard 56 Female terminal at the sender and Packard 56 Male at the firewall plug. Pretty easy to reproduce and install with no splices. You can re-use both plastic housings.

The TEMP sender wire could be damaged inside the cab. It's worth a look.

The other thing that could cause odd behaviour, besides a bad gauge, is the cluster power and ground splices about 6" from the Instrument cluster plug. I'd lean toward damage to the sender wire somewhere because a dead short to ground will peg the TEMP gauge at HOT.

Is the E-Brake returning all the way to the top? Not quite returning will cause the E-Brake pedal switch contacts to intermittently close to ground.
Does the BRAKE lamp go out when you unplug the pin terminal on the Combination (proportioning) valve switch? This switch could be failed or the Combination valve plunger has moved because you have a leak in one of the brake lines.

Is the exposed copper that mates with the GM printed circuit terminals on the cluster plug lifting off the plastic and making contact where it shouldn't?

01-13-2018, 11:09 PM	#13
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,912	Re: Temp Gauge or Overheating?? The idiot light sender wouldn't move the gauge needle off the lower post til you were in an overheat situation. The IR thermometer readings confirm what I already suspected. You are not overheating... there's something wrong with the gauge sender or power circuits or the gauge itself. At 204°F the gauge should sit almost dead centre. The TEMP sender wire in the engine compartment is green with a Packard 56 Female terminal at the sender and Packard 56 Male at the firewall plug. Pretty easy to reproduce and install with no splices. You can re-use both plastic housings. The TEMP sender wire could be damaged inside the cab. It's worth a look. The other thing that could cause odd behaviour, besides a bad gauge, is the cluster power and ground splices about 6" from the Instrument cluster plug. I'd lean toward damage to the sender wire somewhere because a dead short to ground will peg the TEMP gauge at HOT. Is the E-Brake returning all the way to the top? Not quite returning will cause the E-Brake pedal switch contacts to intermittently close to ground. Does the BRAKE lamp go out when you unplug the pin terminal on the Combination (proportioning) valve switch? This switch could be failed or the Combination valve plunger has moved because you have a leak in one of the brake lines. Is the exposed copper that mates with the GM printed circuit terminals on the cluster plug lifting off the plastic and making contact where it shouldn't? __________________ 1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials... 1973-1991 GM Light Truck Service, Unit/Overhaul, Wiring Manuals, Parts Books and Some 1968-1999 GM Service Training textbooks in PDF 1988-1998 GMT400 CK service manual project 1988-1993 GMT400 CK parts books. 1967-1972 GM Light, Medium, & Heavy Truck Service Manuals in PDF Please include at least the year and model in your threads. It'll be easier to answer your questions. And please let us know if and how your repairs were successful. Last edited by hatzie; 01-13-2018 at 11:25 PM.