My F550 has gone to the power reduction several times. The last time it was 70 degrees, trans temp 180 degrees, and was on a slight grade for 10 miles. The engine went to the reduced power mode. The computer showed no errors. What device(s) activate the overheat mode. Any way to test them?

 

No such thing on the PSD. Fail safe is only on triton(gas) motors.

 

If you have a manual tranny in an overheat situation the PCM uses the Coolant tempp sensor to adjust fuel mapping

DM: Engine Coolant Temperature Sensor (ECT) — Manual Transmission Only
Introduction

The engine coolant sensor is used as the primary input to the Electronic Control system to enable adaptive cooling. This provides a means of providing adequate cooling in severe engine temperature conditions. When ECT is greater than 107°C (225°F), the fueling rate of the engine is modified to provide cooling protection and prevent engine damage due to overheating. An ECT sensor is used only on manual transmission equipped vehicles.

 

Why do you think this feature isn't provided for an auto? My fan clutch kept locking up on my tow to Asheville, but my ECT is only about 195 F according to my IR gauge. After I turned the A/C off, the fan only came on once near the top of a long grade.

 

Because thats the way ford designed it. The autos do not have the adaptive cooling programming the ECT merely drives the gauge.

Fan Clutch Test

Spin the fan blade (8600) by hand. A light resistance should be felt. If there is no resistance or very high resistance, the minimum and maximum fan speeds must be checked as follows:
Fan Clutch Test—Minimum Speed Requirement

Use a suitable marker to mark the water pump pulley (8509), one of the fan blade retaining bolts and the crankshaft pulley (6312).
Connect a tachometer to the engine.
Install a throttle adjusting tool.
Connect the Digital Photoelectric Tachometer.
WARNING: To avoid the possibility of personal injury or damage to the vehicle, do not operate the engine until the fan blade has been first examined for possible cracks and separation.

Start the engine and run it at approximately 1,500 rpm until the normal operating temperature has been achieved.
Operate the strobe light at 4,320 rpm for 5.4L and 6.8L engines and 3600 rpm for 7.3L diesel engines, and aim it at the water pump pulley. Adjust the engine speed until the light flash and the water pump pulley mark are synchronized.
Aim the strobe light at the fan blade bolts. Adjust the strobe light until the light flash is synchronized with the marked fan blade bolt (the fan blade appears to stand still).
The fan blade speed must not be greater than 1,900 rpm on 5.4L engines and 2,000 rpm on 6.8L and 7.3L engines.
Turn the engine off.
If the fan blade speed was greater than 1,900 rpm (on 5.4L) (on diesel engines and 6.8L, 2,000 rpm), install a new fan clutch (8A616).
Fan Clutch Test—Maximum Speed Requirement

Perform Steps 1 through 5 of the Fan Clutch Test—Minimum Speed Requirement.
NOTE: The temperature of the air hitting the fan clutch should be above 96°C (205°F) for maximum fan speed.

Block off areas on each side of the radiator in the engine compartment and the front of the radiator grille (8200). This will raise the temperature of the air striking the fan clutch and should cause the fan blade to operate at maximum speed.
Place the climate control function selector switch in the MAX A/C position and the blower motor switch in the HI position.
Adjust the strobe to 4,320 rpm for 5.4L and 6.8L engines and 3,600 rpm for 7.3L diesel engines.
WARNING: To avoid the possibility of personal injury or damage to the vehicle, do not operate the engine until the fan blade has been first examined for possible cracks and separation.

Start the engine and adjust the engine speed until the strobe light flash and the water pump pulley mark are synchronized.
Aim the strobe light at the fan blade retaining bolts. Adjust the strobe light until the light flash is synchronized with the marked fan blade bolt (the fan blade appears to stand still).
If the fan blade speed is less than 3,600 rpm on 5.4L engines and 3,500 rpm on 6.8L engines, 2,850 rpm on 7.3L engines, install a new fan clutch.

 

The wife and I both have allergies so we always run in the MAX A/C position since it doesn't draw in outside air. Maybe doing the heater mod will improve A/C efficiency and help with the fan clutch locking up so often.

I think it would be cheaper to just replace the clutch than to pay a Ford dealer to do all the testing involved to see if it's bad. My temp measurements show that it's probably 205 F when it locks up, but I'm thinking if the viscous part of the clutch is weak and it's not spinning fast enough at lower temps, that might be why it's getting hot enough for it to lock up solid. Do you know if the lock up is a bimetal spring that's separate from the viscous part?