If my truck has a temp gauge does the temp light function?

[MattyBs71]

Bringing this thread back just to ask....so factory full gauge equipped trucks have the Temp warning light in the fuel gauge but never actually lit up or were connected to anything? Doesn't this drive anyone else crazy? Why even have it there??? The Brake warn light illuminates upon cranking the starter but Temp one never does. It never has in any 67-72 truck I've driven. So that's just the way it is?

[ray_mcavoy]

Originally Posted by MattyBs71

Bringing this thread back just to ask....so factory full gauge equipped trucks have the Temp warning light in the fuel gauge but never actually lit up or were connected to anything? Doesn't this drive anyone else crazy? Why even have it there??? The Brake warn light illuminates upon cranking the starter but Temp one never does. It never has in any 67-72 truck I've driven. So that's just the way it is?

Yes, the temp warning light is present but not wired up to be functional in trucks with the factory gauge cluster.

The only reason it's even there is because both the warning light & gauge clusters make use of the same fuel gauge assembly (which includes the little red plastic lens / filter for the temp warning light).

On a related side note, the fuel gauge assemblies used in many of the larger medium duty trucks have "tandem lock" and "overspeed" or "low vacuum" warning lights instead of the "brake" and "temp" lights.

[MattyBs71]

Originally Posted by ray_mcavoy

Yes, the temp warning light is present but not wired up to be functional in trucks with the factory gauge cluster.

The only reason it's even there is because both the warning light & gauge clusters make use of the same fuel gauge assembly (which includes the little red plastic lens / filter for the temp warning light).

On a related side note, the fuel gauge assemblies used in many of the larger medium duty trucks have "tandem lock" and "overspeed" or "low vacuum" warning lights instead of the "brake" and "temp" lights.

Glad this is finally put to rest in my mind lol.

[70STOVEBOLT]

If the brake light dies not illuminate when the key is twisted, what is the fix?

[ray_mcavoy]

Originally Posted by 70STOVEBOLT

If the brake light dies not illuminate when the key is twisted, what is the fix?

One side of the brake warning light bulb is wired to +12V (uses the same source as the fuel gauge, etc.). The other side of the bulb is connected to the tan wire that goes to the pressure differential switch on the brake distribution valve or proportioning valve. That switch completes the circuit to ground (to light up the bulb) if there is a pressure failure in either the front or rear half of the braking system. Another branch of the tan wire goes to a set of grounding contacts inside the ignition switch. Those contacts temporarily complete the circuit to ground (also lighting up the bulb) when the key is turned to the start position.

If the brake light doesn't come on during the start-up bulb check, the first thing to check would be the bulb itself. It might simply be burned out.

If the bulb checks out okay, locate the tan wire on the back of the ignition switch and use a jumper wire to temporarily connect it to ground. If that causes the brake light to come on with the key on, you might have a bad set of grounding contacts in the ignition switch, or the metal body of the switch is not grounded (not making good electrical contact with the metal dash). But if the light still doesn't come on, you're probably looking at a connection problem somewhere ... possibly a bad connection where the bulb socket makes contact with the traces on the cluster printed circuit or maybe a bad connection in the cluster plug.

[70STOVEBOLT]

Thanks for the explanation, I will not be at the point of testing it for awhile. I will try to remember to post in the thread afterwards.

[ratman90]

Rich69shortfleet did you just run a wire from the sensors to the #5 pin on the plug and it all worked as needed?
Thanks
Larry