Unknown sound while towing

[wolfman]

Just pulled my 4900 lb camper with my 99 f250 SD, 7.3 PSD, 4r100 transmission. Had the transmission locked out of overdrive. About 20 miles into my trip, a sound started coming from the engine compartment that I can only describe as a fan coming on and off constantly. Turning the A/C off would stop it, and going into overdrive would stop it. Even with the A/C left on, it would eventually stop for a while, but would soon return. Towing in SC, so it is pretty flat but mighty hot. Talking to a friend with the same truck with a manual transmission, and he said his did it all the way from SC to Texas and back. Any ideas ? Also, shouldn't this truck be able to pull 4900 lbs in overdrive without trans damage. Thanks.

 

[93turbo_animal]

sounds like you were just hearing your fan kicking in and out. Most of the time you probably don't work it hard enough to get it to come on strong enough to hear. With the AC on you were creating more heat to get rid of and in overdrive the rpms were lower which wouldn't let the fan make as much noise so you probably just didn't here it. My trans actually runs cooler in OD then in D pulling 10000lbs the key is to not let the trans get hot so if you can run od with out the trans wanting to downshift then upshift and back and forth then run it but if it wants to keep shifting back and forth lock it in D

 

[wolfman]

Thanks for the info. Makes a lot of sense and relieves some stress.

 

[Agnem]

I would agree 100% that's what it is. I've only ever heard the fan come on in my Excursion one time, and that was towing the Night Moose home from Rhode Island. The first time I heard it, it reminded me of the IH 4600 I rented. About 20 miles outside of town the fan came on and I thought there was a C-130 landing on my roof! When it started cycling, that's when I realized what it was.