1988 7.3 IDI Temperature Gauge Problem

[rempfer]

All of a sudden the red 'ENGINE' check light has come on and the temperature gauge is pegged at extreme 'H' (hot) end of the scale. My F250 is running normally (not overheating) so I'm pretty sure either a sensor has failed or some other problem has occurred causing these indicators to misbehave.

I have a schematic which shows a separate wire each for the light and for the gauge coming from the engine sending units. it seems to me extremely unlikely that both sending units on the engine would have failed at the exact same moment, so what would you experts say is really going on here? Could a failed temperature sending unit alone cause BOTH the pegged gauge AND check engine light? The wiring under the hood from these two sensors looks good and the connections to them are clean (corrosion free). Could a blown fuse cause this situation?

Your help in how to proceed to diagnose the problem would be appreciated. The cooling system is working properly so at this point I know for sure that the engine overheating is NOT the problem.

Thank you,

Richard Rempfer
Elma Washington

 

[Austin86250]

try unplugging the lower temp gauge my friends truck had the same issue turned out the lower temp sensor was bad we just unplugged it
your temp gauge works as should with the lower one unplugged

 

[Cubey]

Go buy a real temperature gauge. If you're on a budget, I recommend Equus 8242.

 

[rempfer]

Go buy a real temperature gauge. If you're on a budget, I recommend Equus 8242.

The temperature gauge is not the problem.

 

[rempfer]

try unplugging the lower temp gauge my friends truck had the same issue turned out the lower temp sensor was bad we just unplugged it
your temp gauge works as should with the lower one unplugged

Sure enough worked, so is a redundant system and sending unit has failed. Thanks, Austin86250. Will probably replace that sensor anyway, just because.

Would anyone have the correct part number for this lower simple off/on switch that closes on overheat condition only?

 

[Austin86250]

Sure enough worked, so is a redundant system and sending unit has failed. Thanks, Austin86250. Will probably replace that sensor anyway, just because.

Would anyone have the correct part number for this lower simple off/on switch that closes on overheat condition only?

the bottom coolant sensor doesn't really add anything especially on the brick noses as the only time it does anything its already to late/ you'd know somethings up before it turns on
that sensor has been unplugged on my bull nose for many years and on my friends bricknose for a couple weeks no problems
also i couldn't find that sensor but i didn't put in that much effort to find it either just 30-50$ i dont need to spend

 

[Joezam]

The lower sensor is head temp, it is there to prevent cracking the head or blowing head gasket. All it does is turn on the hot light when the head overheats