'84 F150 w/ 3 speed C5 transmission. I understand this has lock-up TC. When cold, the truck barely moves and reving the engine doesn't make it to the wheels. The tranny slips in all three gears and doesn't want to shift automatically. I manually shift to finally get it up to speed. After about 10 minutes of driving, I can feel a small lurch like the TC locks up and from then on it shifts and drives just fine without any noticeable slipping. I can drive all day without any problems unless it sits for over an hour or two and back to slipping again but then it only takes a few minutes to fix itself. I dropped the pan, replaced the filter and fluid (what was in the pan) and prior to that I added Lucas Transmission Fix. Nothing helped. Any suggestions? Vehicle has less than 80K miles.
-Norm

 

The seals inside the transmission (especially the one in the forward drum) can harden over time and cant seal to apply the clutches. Once warm they seal just enough to allow the clutch to engage. This is common on this trans and the C6. When this seal is removed it usually just crumbles and breaks rather than being flexible.

 

Thanks for the information and reply. Could I expect a transmission shop to have the parts to fix this or would it sit in the shop for days waiting for parts? What should I budget for the the repairs using a reputable shop?

 

Parts for this transmission are available, so there should be no delay.

 

Well it's been nearly two months and a couple thousand miles since I tried a "trick" from my local transmission shop. May have cost them a rebuild job; for awhile anyway. A real pro at the shop claims he's rebuilt many c4's, c5's and c6's. He new exactly what my described problem was; said he calls it "morning sickness". He said to put a couple ounces of brake fluid in the transmission to soften the seals. I did (about 4 ounces) and about 50 miles later, the slipping stopped completely and it seems the thing keeps working better every day. Today at over 83K miles now, the transmission seems perfect. Starts off great in the morning (even after sitting several days) and shifts better than I've ever had it in over 5 years. Maybe too good to be true but this truck has been amazing to me. As old as it is, it only seems to exhibit problems once it's parked in my driveway; never on the road and usually easy fixes (like a blown water hose, leaking (rusty) brake line that fell apart when I touched it, loose wire to the ECM that killed the engine as I got home). When (or if) the tranny goes, I'll definately get back to this same shop in Plano TX.
(another shop told me there was no such thing as a c-5!).
Norm, Plano, TX

 

Thats a old trick to get them to work again. Yes the brake fluid went right in there and made that old hard worn seal swell and expand to seal and cause the drum to apply when cold. The only problem is that the brake fluid also did the same to all the seals, it will make them soft and over sized and this could cause them to blow out and fail. It is a temporary fix, we use to call that a "transmission builder in a bottle". It will buy you some time though.