I have a '78 F150 4x4 351m auto that has a temp gauge that does not work. I pulled the dash gauge cluster and replaced the temp gauge itself....but it still does not work...it reads cold (far left) all the time. The sensor that screws into the block does not appear to have any moving parts. How does one go about determining what part of the gauge or sensor is at fault??????

 

buy a nice set of triple guages and dont look at the unreliable old guages anymore! really though, if you replaced the guage, then the sending unit(the thingy in the block) is most likely bad, if thats not it there is a short somewhere in between the two.......

 

Attach the plug-in to a ground spot on the engine (use alligator clips)
Get in the truck
Turn the key to RUN
Does the gauge PIN to the right ?
If so, the gauge works.

Ford's sending units works on a *grounding out* system.
The more oil pressure, temp of coolant,Amount of Fuel,the more the sending unti grounds without as much resistance.

That's why, the needles pin to the Right side when grounded.
Of course if the needle does move,it usually mean the circuit is open or the instrument cluster voltage regulator is bad

 

mine was reading cold all the time also. No matter what I did it would not move. Then thie fine folks here told me to replace the thermostat and now everything is fine, the gauge reads about halfway when warmed up.

 

run a ground wire from the panel voltage regulator it is the metal box on the back of the gauges and run the wire to a good ground source that should fix the problem

 

Thanks for the info everyone. I tested the gauge as outlined above and found that both the original and replacement gauges were no good. What are the chances that out of the five dash clusters I have for parts, I picked two defective gauges to install? I also took the opportunity to replace the amp gauge, too. Its nice to see all that movement on the dash.
Thanks again.
John