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the "Check Gauge" light goes on quite frequently when driving my Ford Ranger 95 SC STX 4.0 because of the Temperature gauge, when the air conditioning is on. The temperature here is over 35C and the light only lights on when slowing down on the highway. The pointer got only to two thirds of the ranger and never got close to the red H, but is it normal to the light to keep getting on?

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Your vehicle has a thermostatic fan clutch. This is a disc with fins mounted between the fan pulley and the fan blades. When the air coming through the radiator is not very hot, the clutch does not engage, so the fan blade spins only weakly. When the radiator is hot and requires more cooling, especially when the vehicle is not moving, the thermostatic clutch engages the pulley hub to the fan hub so the fan blades spin with greater force to move more air and provide more cooling.

If the thermostatic clutch is worn or defective, the engine can overheat when the vehicle is moving at low speed or when it is stopped. This is due to lack of sufficient cooling air flow.

With the engine cold and not running, spin the radiator fan by hand. If it spins freely and makes many turns before it stops, the fan clutch should be replaced.

With the engine hot and on the verge of overheating, turn off the engine and again spin the radiator fan. You should feel resistance and it should stop quickly when you spin it. If it spins freely and goes around many times, the fan clutch should be replaced.

For professional use only: I have seen a mechanic test a thermostatic fan clutch on a hot, running engine by taking a rolled-up newspaper and inserting it into the spinning fan blade from the side. With a bad clutch, the blade stopped easily and the mechanic was able to spin the blade backwards with the engine still running, proving that the thermostatic clutch was bad. This is scary and dangerous, so I'm not suggesting that you do this, but if you have a mechanic to do repairs like this, you might mention this test as proof of a bad thermostatic clutch.

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  • Technique for manipulating a fan as described in the last paragraph - a disposable rolled-up newspaper is good because it is strong enough to push a fan blade, but won't damage the fan if it spins up all of a sudden. Your fingers, on the other hand will take damage and aren't disposable.
    – Criggie
    Oct 7, 2023 at 20:09
  • I did all these tests and it the the fan clutch is in fine working order, activating correctly and turning fast. Any other ideas?
    – Ivan
    Nov 15, 2023 at 16:51
  • @Ivan You never mentioned the condition of your coolant. If it is clear and clean, and if you have already replaced your engine thermostat, I have no other ideas. If the coolant is rusty on a '95 engine, you may have internal corrosion that has released sediment and flakes of rust that have blocked some of your radiator tubes, greatly reducing the ability of the radiator to remove heat. If possible, flush engine and do a commercial reverse power-flush of the radiator. If not available in your area, replacing the radiator is the only other possibility that comes to mind.
    – MTA
    Nov 15, 2023 at 20:59
  • Thanks again. The radiator fluid is new and is clean, no rust or other debris floating. I haven't flushed the radiator, though, and I think this will not be easy to do. I will try to replace the thermostat and see if it solves it, before considering replacing the radiator.
    – Ivan
    Nov 22, 2023 at 18:44

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