1989 F150 supercab 4x4 w/5.0 EFI and automatic trans. I just bought this truck about a month ago and there are several problems with it but this one has me stumped. With the engine warmed up, I can turn the heater/ac fan to high and the temp guage goes slowly to hot.The red engine light also comes on. When I set the fan all the way to low, the guage goes back to normal in just a few seconds and the engine light goes out. There is no sign that the engine is actually getting hot and I'm sure that it isn't. One of the other problems that I have that maybe I should mention is that the oil pressure gauge (needle) is always pegged. When the key is off the needle is its normal position to the left. Key on engine off it is still in the normal left position. Engine running...pegged. I have done the obvious and replaced the sending unit but that didn't change anything. I don't kbow if this is related to the temp guage thing but I thought it worth mentioning. Thanks for any help in advance. Ivan

 

The Oil pressure gauge is not the most accurate of units in those trucks. If you're worried about your true oil pressure, install an aftermarket setup.

As for the temperature guage, have you tried replacing the TCU (temperature control unit)? Mine was reading wacko a couple of years ago...lots of random fluctuation. The unit is right by the thermostat and comes off nicely with a 5/8" deep socket. I replaced mine and that issue went away. I believe the unit was under $20.

Good luck!

M

 

you have a ground problem . you need to check all of the ground connections around your dash and instrument cluster.

or it could be a power supply problem where you have a bad connection some were between the battery and the blower motor that is causing it to starve the instruments,

 

Mr M and Bruce.........Thanks for your input. I will check out both of these solutions and see what I come up with. It will be a few days before I can post again. Thanks

 

Sorry for the late reply but I have been slammed for the past several months in work. Thanks for the help. I belive it was the ground problem that was causing the problems. My oil light is still high but at least it isn't pegged any more and my temp guage reads close to the "N" in normal all the time now (I had replaced both sending unit and temp switch before my first post. Thanks for that tip.)

There are several problems with the truck still and I'm trying to solve them one by one. the next one is that the truck is seemingly cutting out. One person has advised me that it could be my map sensor. Another says it's a vacuum leak. Both guessing by listening tp engine run. I get and error 41 (always indicating lean). When it's idling it seems to be running rich. Going down the road it acts like I'm shutting the key off for a 1/10th of a second and then back on repeatedly. I have replaced the egr, the 02 sensor, and the egr valve position sensor. Thanks for your reply. I'm going to do a new post on this also so it shows up as a new problem.