P0538 Ford F350 Diagnose and fix
A P0538 Ford F350 refers to a Fan Speed Sensor Circuit No Signal issue. This engine uses an electrically controlled viscous clutch fan to move air across the radiator. The powertrain control module (PCM) uses the fan speed sensor (FSS) input to monitor the cooling fan clutch speed. If the indicated fan speed is lower than the calibrated value during the key ON engine running (KOER) self-test, the diagnostic trouble code (DTC) is set.
Ford trouble code diagnostics shows these possible causes:
FSS VPWR circuit open in the harness
FSS PWRGND circuit open in the harness
FSS circuit open in the harness
FSS circuit short to voltage or ground in the harness
Damaged FSS sensor
Ford TSB 07-3-10 fix for P0528 F350
Ford has issued a service bulletin 07-3-10 to address a P0528 F350 issue. Ford states that in some cases, the rotating fan can cut the wiring harness to the viscous clutch. The issue can be caused by a broken fan shroud retaining clip that allows the harness to move and be hit by the fan blade. The fix requiring installing a new pigtail and a new securing clip to prevent harness contact with the fan blade.
If the Ford TSB doesn’t apply
Check the red wire at pin #5 of the fan clutch connector. This wire supplies battery voltage to the fan clutch from the PCM power relay. Next, check the brown/light green wire for battery voltage coming from the fan speed sensor from the PCM. You should see battery voltage on both wires.
If so, proceed to momentarily ground the dark blue wire on pin 4 while the engine is running. This will cause the viscous clutch to lock up and you’ll notice an increase in fan speed. If you don’t notice a change in fan speed, the problem is in the viscous clutch itself.
If the fan responds by increasing speed, you’ll have to monitor the fan speed signal on the red/orange wire. This will be a digital 12-volt square wave the rises and falls with fan speed. If you don’t see a change in the digital signal while the fan in changing speed, check the brown/pink wire for good ground. If the ground is good with low voltage drop, you may have a bad fan speed sensor in the fan clutch assembly.
Fan runs constantly
If the viscous clutch run the fan full time, check all of the above plus the inputs from the engine coolant temp sensor and the cylinder head temp sensor to determine why the PCM is asking for full cooling
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