so with the scan gauge, im trying to figure something out.
FDC is what the PCM is asking the fan to do % wise
and FSS is what the fan is actually doing? (RPM)
or is FSS RPM also a desired number?

 

I'm pretty sure FSS is actual.

Not sure about FDC.

 

FSS is actual

 

cool. thank you guys.
will be monitoring this tomorrow while helping my mom move to her new house pulling 7x14 trailer loaded down.

 

I've never been able to get FDC to work. My fan speeds vary with temp and load but you can diagnose a bad fan clutch that has no DTC by watching it at idle and at engine speeds over 2000 rpm. If the viscous fluid has leaked out, FSS will read 400 rpm at idle, at 2000 rpm, FSS will not exceed 650 rpm.

I have replaced mine twice in the last two months. The original had a bad heating circuit and it I replaced with a Ford unit. It lasted a day and was found to have leaked out all of the viscous fluid due to being stored improperly. It did not give any indication other than overheating while towing and when not towing, temps would hover around 200*. Ford would not warranty because I misplaced the receipt, so I opted for a AutoZone sourced unit with a lifetime warranty. It's been on for about 3 months now and has been working properly. When the temps climb to 195*-197*, the fan speed increases from roughly 800 to 1100 and blows the temp back down to 190*.

I have another 6.0 truck that has a bad heater circuit, yet will actuate the fan to 100% when temps reach 215*. Outside of that, it just coasts at like 700 rpm, off-idle and 400 rpm at idle.