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Faulty Temperatue Reading

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Old 07-30-1999, 07:41 AM
Charla Fiske
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Faulty Temperatue Reading


I purchased a 1995 F150 Lightning in overall fine shape with an easy 38,000 miles a few weeks back, but I have a condition I believe to be caused by a faulty temperature sensor. After reading this message board it seems temperature problems both real and faulty indications are not uncommon in the F150.

About 45 seconds after starting the engine the temperature guage starts rising rapidly (cold to hot in a couple of seconds)until it pegs high. The engine has not even warmed up at this point. A minute or so later the guage drops to around the 1/4 point (below center) and remains very stable at that point as long as the truck is driven -- even in the 100 degree weather we are having.

If the engine is stopped briefly after fully warming up, the cycle repeats. In this case there is still no indication that the engine is really running hot. By the way, at first I missed the fact that this occured when the engine was warmed-up and replaced the thermostate thinking it may be sticking.

I suspect the sensor is bad, but I am amazed that it is so consistent -- I would suspect a failing sensor to be more erratic????

Anyway, has anyone seen this?

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Old 08-13-1999, 02:35 PM
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Faulty Temperatue Reading

In case anyone is interested I finally fixed this one.

Received a couple of responses on another Internet Truck chat from experienced Ford mechanics. One agreed the sensor should be replaced first -- I did, but the problem continued. The second one said it was the guage -- internal shorts were pretty common and could cause indications like this. Waited until I received my Service Manuals from Helm and ripped into it. The problem was caused by a bad connection in the plug between the engine compartment and the instrument panel. The contacts in this plug we in really bad shape -- turning green with copper oxide. Very strange since the truck is low milage and the weather is not all that bad here.

Found a couple of interesting things out about the 1995 Ford in debugging this one:

1. The 1995 Ford does not use a voltage regulator in the instrument panel at all. Several instrument related problems in this section refer to the voltage regulator as being a potential problem -- don't worry about it on the 1995, you don't have one.
2. Ford took a really big short-cut on the Oil Pressure Guage -- it is simply connected to a pressure switch on the engine block. If the pressure is below the switch point the guage shows no oil pressure, if it is above the switch point the guage is feed through a 20 Ohm resistor to move the needle somewhere on the normal range. If you look at your oil pressure guage and expect it to actually indicate some relationship to pressure, forget it on the 1995 Ford -- a red idiot light would tell you just as much.
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Old 10-01-1999, 07:59 PM
john john is offline
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Faulty Temperatue Reading

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