I have a very nice 1989 F-150 Lariat. This truck has been well taken care of and has all origanal finish and interior. The temp gauge started drifting down to zero, then coming back up to normal. I checked and found the temp sensor broken. I replaced this sensor and it is not reading at all. I replaced it with another new sensor and this sensor was tested before install. Still no reading. Sensor is reading 5.4 volts at the plug. and it shows proper ohom change when tested. I then removed the dash gauge, and tested the pin plug in back of gauge and read ground on one, 12 volts on one and lower volts on third pin. I tried another gauge from a junk yard still no reading. Any thoughts. I plan on keeping this truck stock so I am not going to install a manul gauge.
Oldbill

 

I have a similar problem with the temp guages on my 84 F-150 and 87 E-350. Also had same thing happen with dad's old 86 Bronco XLT. All seem to never stay true. I still haven't figured it out on the E-350 and I've had it for 5 years now. I know dad had fixed it on the Bronco a couple of times but he doesn't remember what he did or replaced since it's been about 8 years since we sold it.

 

This probably pre-dates you guys with the eighties vintage trucks. There used to be a voltage regulator attached to the cluster, problem is the fuel gauges should be affected also. Go back to basics and check for ground wires, from engine to firewall, grounds for the cluster itself. Connect a jumper from gauge out to engine just to verify.

 

All the other gauges work fine and the ground is good on gauge pin to frame. I am begining to think there is another sender, that I have not found. My book shows only the one I changed, located at the thermostat. and it is a two wire sender, and feeds info to the compuer.
Oldbill

 

maybe its an error in the computer?

 

Hooray I fixed my gauge problem. My brother found the answer, there is two sensors on my 89 f-150 one for the computer and another (Well hidden one for the temp gauge ) The temp gauge sender is located down on the cylinder head to the left and below the distrubiter. It is hard to see and small. Anyway thanks guys for the advice, and helping me look.
Oldbill

 

What I need to know is the location of BOTH sending units. I have changed the one right behind the thermostat housing (the one with 2 wires). The other one that has been mentioned, is the one that I'm not sure of. The two places that I think could be the other sender are between the #2 and #4 injectors (driver's side 2 front cylinders) on the intake manifold w/2 wires or the other place at the left front corner of the intake manifold (driver's side) w/1 wire. If someone could help me with this, I would really appreciate it. If you may have a picture or a diagram, that would probably help alot. Thanks!