1981 F150 RC LWB, 302/C6, 9" 2.75.

(Yep I used the search before making this topic...)

As of late my C6 has been on the fritz. In D it shifts good some of the time...other times it has a funky hang-up trick it does when it needs to shift to 3rd. It will force a shift to 3rd if I let off the throttle when this happens. If I shift manually it has nice firm shifts but sometimes in 3rd I feel it act as though it's trying to come out of gear. No change in engine RPM when it does this but it's noticeable to me.

I originally wanted to take it in and have the C6 rebuilt and have a shift kit put in, but that's ~$800 I don't have. I'm not going to even attempt to pull the thing apart myself.

Another idea was to swap in a ZF5, but that'd be even more than the rebuild, plus I'd have to modify a bunch of stuff to make it work with my truck. So after some thinking I started entertaining the idea of swapping in a T18 since it's what could have came in my truck had it been optioned with it (whereas the ZF wasn't offered 80-86).

What will I need to address in order to do this swap? Will I need to have my driveshaft shortened even if I find a parts truck optioned similarly to mine (cab, bed, axle, wheelbase) that has a good driveshaft?

I'm hoping I can pull this off without too much custom work. I really don't want to park my truck and have it sit up for a long time.

 

You'll need a whole new set of pedals, linkage, flywheel, clutch, hydraulic master cylinder, cut the trans tunnel for a place for the shifter to stick through, disconnect or swap the steering column etc...

Definitely more straight forward and less expensive to fix what you have.

 

I am not sure why the zf would be so expensive, and the t18 swap would not? And your shifting problem on the c6 sounds like a vacuum line problem to the modulator on the pass side rear of the tranny.

 

^^^ What he said. Check the vacuum line from your manifold down to the modulator. See if it fell off or is cracked and or broken. If it's okay replace the modulator. About $15 and 30 minutes.

 

I thought it would be more expensive to run the ZF since I recall Gary Lewis saying he spent several hundred on the trans and parts...I was thinking a T18 would be the cheaper way to go since I would be looking for a parts truck that was already junked, wrecked, etc. that I could get for real cheap, but one that had a good trans.

I replaced the vac line just last week with reinforced fuel line of the correct size. It had plain rubber line before. I was going to replace the mod valve but when I saw the price and didn't have enough (my check was short this week), I took an old vac hose and hooked it to the mod valve. I sucked on the hose to apply vacuum to see if the mod valve was good, and it's fine.

I don't know exactly what to think about this C6 then since it's not the mod valve. Once I get a little extra cash I'll run some Seafoam Trans Tune through it to see if it helps.

 

Sam,

The Ford book says the only adjustment available on a C6 is the Intermediate Band, you might google that and see whatchya come up with.

Converting to a stick shift isn't a quick, Saturday-afternoon job.

 

About the zf, just get a cheap donor truck that has the zf, just like you where going to do the t18. Most people think the 87-up trucks are ugly anyway, and you probably can get one cheap.

It's good that you replaced the vacuum line, but it still sounds like a vacuum problem. How is the engine running? Where does the vacuum line go to? Does it go to the tree on the firewall? If so, do you have good vacuum up there? If the engine is running lousy, it won't have good vacuum and could affect the shifting of the trans also.