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On my 1996 F-150 with the 4.9 L (300 CID) inline 6, for some time now, the engine barely reaches the normal coolant temperature as read on the gauge. Historically, it would climb until it was in a nice healthy spot in the middle of the gauge indicated normal range. About a year ago, I tried to fix this by replacing the thermostat, but the problem persists.

Assuming I correctly installed the new thermostat and it works correctly, I think the problem is either that the signal going to the gauge is bad or the gauge itself is the problem. The gauge is not cheap and probably tedious to replace so I'll assume the former case.

Given that, I have the following questions:

  • am I correct in thinking that I should try replacing the coolant temperature sender?
  • is there a good way to check if the thermostat is actually working correctly?
  • am I correct in thinking that the temperature sender is the device that measures temperature and sends it to the gauge and that the temperature sensor is the device that measures temperature and sends it to the engine control unit?
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1 Answer 1

up vote 2 down vote accepted

If you replaced the thermostat about a year ago, it does seem less likely that the problem is the thermostat. If your heater is working properly ( good amount of heat once the engine is good and warm ) and the hose coming out of the top of the radiator doesn't get hothot until your engine reaches about 180-190 degrees, then you can almost certainly rule out the thermostat problem.

Now on to the temperature gauge. You can test the gauge by running a small wire from the negative battery terminal and tap it to the wire going from the sensor to the temperature gauge. If this causes the gauge to jump up each time you tap it, then it seems like your gauge is good too, which would leave the temperature sensor or the wire leading from it. If you can't get the gauge to jump up when you ground it, then you know pretty well that the gauge is the problem.

As for the difference between temperature sender and temperature sender? Those two terms seem to be interchanged too often. You'll have to physically determine which one is wired up to the gauge cluster and go from there.

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Thanks. I think I remember replacing the temperature SENSOR when I had an OBD II code saying something about coolant temperature being way off, and that fixed it. That being the case, I'm pretty sure that means that, at least for my truck, the sender provides a temperature signal to the gauge and the sensor provides a temperature signal to the ECU. –  Chad Apr 3 '13 at 18:51

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