78 f250 crew 429 with gauge not light. Gauge is pegged out on hot. Thermometer says 206 degrees at thermostat housing. Is there a way to test the instrument to make sure it is properly reading the sender? I replaced the sender and the problem is unchanged
Tia todd

 

Is the fuel gauge reading okay or is it pegged also? If it is, I would suspect the instrument cluster voltage regulator. If the fuel is okay, the first thing I would suspect is a short somewhere in the wire that goes from the sending unit to the gauge. To check this, disconnect the sending unit and see if the gauge still pegs. If it does, then there's probably a short somewhere that is grounding the circuit.
If the gauge doesn't move when you disconnect the sender, then I would suspect the gauge itself.

 

Agreed, sounds like the gauge is shorted downstream. Most likely the wire running out to the sending unit is pinched against sheet metal somewhere. Does the fuel gauge exhibit the same behavior?

 

All the other instruments work fine. It's not pegged all the time it just reads way hotter than it should. I will check for a short. That is simple. Sounds like a new gauge is in order

 

I don't understand what you're saying. First you said it's pegged on hot. Not you're saying it intermittently reads hotter than it should? In your mind, what "should" it read? That's subjective. Why do you think you need a new gauge? They hardly ever go bad and you can easily test it first.

 

the way I test temp gauges is to turn the key to the "run" position, while the temp sender wire is disconnected from the sender, run a jumper wire to a known good ground and to the temp sender wire, what does your gauge do ? if it immediately pegs hot, your gauge is fine. if it doesn't move, then you may have a bad gauge or something in the IPVR ( instrument panel voltage regulator) as stated earlier. if you're gonna replace the gauge, I would recommend a new temp sender, available at Ford part # F1SZ-10884-A.

 

As stated in op when the gauge is pegged (all the way to the right hand limit of its sweep) my infrared thermometer reads 206 degrees at the thermostat housing. That was immediately after slow speed low rpm trailer shuffling in 90 degree ambient. So I think that the gauge is reading hotter than it should. During 60 mph trailer dragging the gauge is at the mark past the word " normal". Same ambient. Hopefully this explanation is more clear. During all driving conditions it reads too hot. I have replaced the sender

 

it sounds like to me your gauge is faulty/not reading correctly. you can either replace the gauge or install a 5k ohm resistor inline to reduce the gauge readings. the easiest would be to replace the gauge. have you put in a mechanical temp gauge just for the heck of it to see what it reads ?

 

I have considered it. But have not. I may try a aftermarket under the dash as a back up. I have way too much invested in the engine to overheat it