1984 F150 2WD 4.9L Symptoms:

When I turn the ignition switch on (not started yet) the temperature gauge goes past Hot until it hits the stop and stays there after starting.

Truck drives okay until it warms up then lacks top end power - begins to stumble when trying to accelerate (like no timing advance).

Throwing Engine Coolant Temperature error codes 21 (ECT out range) and 51 (ECT Input too high).

ECT sensor ohm resistance readings check out okay for both cold and hot conditions.

ECT harness continuity to Engine Control Computer (EEC) checks out okay.

That leaves either the temperature gauge or the instument cluster voltage regulator to be at fault do you agree? Am I missing something? Any idea how much either of these 2 items cost?

Thanks,
Karen & Paul

 

I would say that the wire to the temp sender is grounded. Fix that and the rest might clear up.

 

yeah... the temp meter should peg immediatly if youve got it grounded... For kicks and giggles, take the temp sensor wire off, and see what happens

tks alot
~Nate

 

Last night I removed the instrument cluster and bench tested the temp gauge - 12.9 ohms - right within spec. Opened the instrument voltage regulator but there's nothing but a little heater coil wrapped around a bi-metallic strip. Put everthing back together but the temp gauge still pegged the hot stop.

Disconnected ECT sensor - still pegged red hot. But, after more fiddling about, it suddenly showed cold. I started the truck and let her warm up. The gauge slowly went up to normal. But, when I drove the truck it acted more like a tach then a temp gauge! Every time I accelerated it pegged the hot stop. Weird! Now it's back to continually hot again.

I think it's the instrument voltage regulator ($71 + tax from Ford...!!! and a temp gauge is $72 + tax)

 

Instrument Voltage Regulator supplies Voltage to Fuel Gauge, Oil Pressure Gauge and Water Temperature Gauge and problems with the regulator affect all three the same. Sound like there is an intermittent short to ground in the
Temperature Gauge wire going to the Sending Unit or possibly a shorted Sending Unit. I doubt the Gauge circuits would have any effect on engine performance.

 

I'm measuring ~5v at the ECT sensor and ~14v at the temp gauge