OK so dad has a 05 f250 with the 3v 5.4. It has a brand new jasper engine with less than 3,000 miles. We went Missouri from Florida to repossess a little Toyota truck that the guy didn't pay for. Anyways , we are towing the little truck with a uhaul trailer , combined weight of approximately 6,000 lbs. give or take. We drove all the way from Missouri to McDonough, GA. with the tow/haul turned off. Figured it best for towing thru the mountains and hills. Well before we stopped the tow haul light starts flashing. We pulled over for the night thinking maybe a cool off period was needed. Today we check the fluid and normal color , no burnt smell nothing to cause worry. Jump on interstate and it starts flashing again. We pull over and dad wiggled the rear speed sensor on top of the rear end and the light went out. Couple miles with overdrive on and it comes back. So I installed a new rear end speed sensor hoping to fix the problem and same thing flashes continously. Temp gauge stays perfect , speedo and odometer are working perfectly. But you can feel sometimes a studder but not often. It keeps going in and out of overdrive but not racing the rims, i.e.- 1700 then goes to about 2100. So we pulled over shut the truck down and restarted it. No light. I had him turn tow haul on and take off again just now and a few miles later it starts flashing again. This is really the first this truck has ever towed anything and there is approximately 120,000 on this trans. 278,500 on the rest and he's the original owner. Need help , suggestions , opinions, fixes etc. Like now , he just dropped over 7k in a new motor , don't want to have to put a new tranny at 5k in as well.

 

You need to get the codes pulled, the flashing tow haul light indicates a transmission issue.

Ford or most trans shops can pull the codes for you.

Until you have that info we can't go any further.