I have a 1989 F150 4.9L. The coolant temp gauge is resting at L even when the engine has been hot. The coolant system works, but need info on solving this problem. The needle doesn't flutter or move just stays at L. Is it the temp sensor or the gauge itself.

 

I check the wires from the sensor to the gauge and if not id test the sensor, remove the sensor from the block and put it in a glass of water about 140* and u should get a reading of about 667 OHMS, then put it in a glass of water at about 77* and you should get 2796 OHMS then try 32* and you should get 9420 OHMS. as the water temp gets colder as your testing it the OHMS reading goes up. If you dont have digital OHM meter then id jus replace the sensor.

 

Is there any possible way the gauge itself is broken? All other gauges are working good.

 

Sure! Did you hit it w/ a hammer? That would do it! The coolant Temp. sender (not to be confused w/ the ECT sensor) is usually the problem, If you disconnect the sender, the gauge should show lowest range. If you ground the wire to the block, you should show the high range. This simple test should rule out the gauge. If you disconnect the ECT sensor, no effect on the gauge, but the engine will run rich til you plug it back in.

 

I know that the 300 L6 sender is very tough to reach, I had to lay in the right side of the engine compartment and reach my left hand around the intake to unscrew the sender from the block with a 5/8" wrench.

 

The location makes it hard to confuse it w/ the ECT sensor doesn't it! It almost seems there was some malice involved in deciding where it should be located.