Here is my story. I have an '81 with a 351W and the C6 transmission. Lately, it has started to slip a bit at low RPM (but it holds great at higher RPM). If I tip in to get the engine speed up above 1000rpm or so and the truck just rolling, all is well. It would also have a slow shift from 1 to 2nd if I was accellerating slowly. 2nd to 3rd was always fine and crisp. When I got into the throttle harder, 1st to 2nd worked fine.

As it has been a while (50000 miles if my memory is correct), I decided it was time to change the tranny oil. After the joys of dropping the pan to change the filter in 25 degree air and 40 mph, I decided NOT to check and see if the torque convert had a drain plug (and I hear it does). Either way, new filter from Pep Boys and 5 quarts of type F fluid later, it is off.

The initial test drive went great! The shifts were very smooth at all rpm's and there was NO slippage at all. I was happy. I drove it around for about 10 miles and went to eat. When I went to turn into the restaraunt, it didn't pull as hard as it should have and generally acted sluggish. After verifying the oil level (right in the middle of the do not add range), I went to eat. When I left, it did the same thing: sluggish response, but it drove absolutely normal when it got moving.

After playing with it, I found WHAT it was doing: It was starting out in 2nd. I could punch it and it would quickly drop into first and go. Or, I could pull it into first with the lever. After it was moving, it would shift into second or third just fine, but it was always try to start out in 2nd gear from a standing start. And I did check the kickdown rod. It was connected and I didn't make any adjustments to it.

Over Christmas, I will have access to a better shop, so I plan on changing out the filter again AND drain the torque converter this time to do the job right. Any other ideas on what to look for?

 

Well, number one, you should look for a rebuild kit for your tranny. All c6's built after '76 used Mercon/Dexron, and the type F will burn it out. After taking care of that, check the vacuum modulator line. It's probably off, or the modulator itself has gone bad. But, either way, it's kinda a mute point now. Sorry to be blunt.

Jared

 

[updated:LAST EDITED ON 25-Dec-02 AT 09:15 AM (EST)]What an idiot. I just "KNEW" it took type F. I'm not too sure the tranny is dead though. Other than the shifting, it is running great, no slipping, nothing. If the thing can hold together until April, I'll pull it and rebuild it anyway. If it can make it to April. It looks like I get to do another fluid change REAL quick!

 

Yes using type F fluid was a mistake but it's not likely it hurt the tranny, any worse than before. If it's taking off from a stop in second or third, it's because the governor and/or shift valves are sticking which means there's trash in the fluid. Normally this means an overhaul is needed but don't give up yet. You may be able to cheat your way until April by changing fluid again (that means the converter also) with mercon 3 and installing a cooler line filter which will catch the small particles that causes valves to stick. The cooler line filter does a better job than the valve body filter which has a coarse mesh. I saved the A4LD in my TBird with a line filter clearing out some trash caused by thrust washer wear damage. Ford sells a filter kit XC3Z-7B155-AA which includes a filter, hose and fitting but it's about a $100.00. You can make up your own fitting and use the NAPA filter 1-7825 which I thing is $20.00. Understand this is just a stop-gap fix if it works, because your C6 most likely has some internal damage.

 

Well, the rebuild is done. It was a lot of fun and I learned a lot about exactly what makes a slush box work. All went well, but I am definitely having problems with the governor at this point. It starts out in 2nd about 80% of the time, then shifts to third sort of normally (maybe a bit early). The manual valve works perfect. Each shift is crisp and clean. The kit I used (TCI truck master kit) had new springs for the valve body and a new valve. I used them, but I was a bit leary of making the shift too harsh. No fears, because it shifts perfectly (when the regulator isn't stuck!). I'll pull the tailhousing off tomorrow and go through the governor again and see what I find.

I definitely found the root cause of my earlier transmission problems: Forward clutch, forward drum clutchs shot (most of my fiber clutches and decided to cross the line and become steels! Only the reverse clutchs looked good, very good actually), the sprag clutch has 5 broken springs and most of the other springs where collapsed, and the diaphram return spring had shattered into a million pieces (not quite, it was one piece for each segment...) . I guess I was right on my theory of what caused the "racheting" slipping I saw from time to time.

All in all, this was a good project and a great learning experience. I am glad I did it myself and no just write a check. It wouldn't have been nearly as much fun.

 

I am glad you had fun with the rebuild- I just finished up the new one for my truck. And I thoguh my tranny was bad! The only things i found actually broken was the front selective shim was gone, causing the input shell to grind against the stator support. So, after a quick and frantic parts search, all is well in the c6 world again. I used a Napa rebuild kit, I didn't do anything in the way of a shift kit, as I plan on mostly shifting manually anyways. These are a really stout tranny, if taken care of. They aren't as forgettable as a manual, in that you actually have to do oil changes, and make adjustments, but they do last.

 

Another positive outcome of this rebuild: I will NOT neglect the oil changes again! *grin*

I pulled the governor out today. Sure enough, the small valve was stuck out. I pulled it apart and cleaned it and the valve was still sticking, so I used some 2000 grit sandpaper (from my chisle sharpening supply) to polish the valves and the bores. After cleaning it up again, both valves were still tight going in (they had to be aligned perfectly to go in), but they operated very smooth. I reoiled it and put everything back together.

And it shifted perfectly. For the fist 15 minutes. As soon as the tranny started to get up to temp, it started sticking slighted. Once, it was hot, it stuck solid. I guess I will bite the bullet and buy a new governer and be done with it. No biggie and it took less time to pull the tail housing off than I thought (with the tranny installed of course), maybe a little over an hour for pulling it, cleaning it, and reinstalling everything. The tranny mount sure does get in the way of the bottom housing bolts!