C6 No Reverse

[jon schapiro]

A friend of mine has a 1978 Mark V with a 460 engine, and his C6 began making funny noises in reverse, and now has no reverse whatsoever. He changed the fluid and filter and no change. Thoughts?

[1Bad55Chevy]

Step 1... Verify the linkage is adjusted properly and all the bushings are still intact.

If everything looks good move to step 2.

Step 2... Rebuild transmission...

The C6 does not use a reverse band like a lot of other transmissions. For reverse to work it applies the direct and the low clutch simultaneously to spin the planetary gears in reverse. If you care to test the clutch packs this is how you do it.

Put the trans in drive and it ensure that you feel it shift into high gear. If it successfully shifts into 3rd the direct clutch is good. Next while driving SLOW manually down shift the trans to 2nd and 1st with your foot off the gas. You should feel the engine brake engaging slowing the vehicle down. If you do not feel the engine brake engaging the low clutch is gone.

Remember the fluid dosen't have to be smoked for the trans to fail. If the fluid looks good the piston seals have failed in the clutch pack.

[TonyC]

Well, I'm not certified on transmissions, but doesn't the intermediate band the C-6 does have also play a role for reverse? That's what I read in the shop manuals. Not looking to argue, just to get clarification.

Otherwise, I agree with Bad's Step-2. Having been through that myself, I can tell you that's a sign for a rebuild. That was one of the signs I got in 2004, along with a loss of automatic shifting, prompting me to relent and get my first rebuild (after I got the engine running again, which is another story, not the same one from two years ago...but that was the reason I couldn't get the first rebuild until early-'05). I'm not sure if the design flaw in my C-6 was carried over to later variants; but if that is the case, that is a thing to talk about. The last rebuilder of my transmission (the second rebuild, in 2013) told me about this flaw: The clutch pack was not given a steady source of lubrication; the engineers designed it to receive splash-lubrication only. The thought was that, since the C-6 was eventually meant to go into trucks, that splash-lubrication would suffice when said trucks run on rough terrain as they were perceived to do. But, on a car that spends 98% of its time on paved roads, especially a luxury car, that just does not cut it. My clutch pack literally ground itself up because of that. The rebuilder engineered out that flaw at no extra charge during the rebuild, by drilling a hole inside the casing near the spot where the clutch pack would be, tapping it to a fluid passage; now my clutch pack has a constant supply of fluid going to it. Clearly the first rebuilder either did not know about that flaw, or he didn't care to do anything about it. But, I must say, it took 8 years and 200 kilomiles before the second rebuild became necessary.

I don't know how many other transmission rebuilders may know about this trick, however. If they want clarification, they can contact Smith's Transmissions in DeRidder, Louisiana (my last rebuilder).

Pricing? Not sure of the differences today. But, in 'Texas in '05, the first rebuild cost me $1,400. In Louisiana in 2013, cost was again $1,400, maybe a few dollars more. Nowadays, I can't tell you...but a C-6 rebuild still has to be the cheapest rebuild one can hope to get.

---Tony

[Lee]

I believe Chevy is correct. The intermediate band is released, and only the two clutch packs are applied in reverse.

[1Bad55Chevy]

Tony I am far from an expert on the C6! I learned about them years ago when my buddy was racing a notch back with a turned up 347 on 3 kits. We use to drop it and replace the clutches about every 50 passes. I did rebuild the c6 in the Mark and apparently I did such a good job that now it won't stop leaking from the tail shaft seal that i replaced. I need to get it up on my lift and figure out why but I haven't had time.

I am not sure what you mean by splash oiling on the c6. The valve body is bolted to the case directly under the transmissions rotating internals which blocks fluid from being able to splash up but still allows fluid to return to the pan. How the clutch packs are lubricated is through a hole in the apply piston. If the clutch is being applied then the clutches are soaked in fluid.

[frasern]

The clutch packs have an outer spline which wears, on my truck the peaks were worn to points, whereas they should be rounded. It sounds to me like they wore till they started to skip, that was the funny noise, they will then wear off very quickly. The hub they ride in may now be damaged.

I like step 1 above, too many people just go straight to the transmission and don't even look at the linkage and mounts. I would also check the band adjustment, I like to count the turns in and out and compare.

Disclaimer, these are just my thoughts, the others who have responded have more experience than I with automatic transmissions, and they can be a little mysterious.

[1Bad55Chevy]

Tony, got it..

The hole he drilled was for the sprag. Apparently the sprag was splash oiled up until 69. Ford offered a kit which came with a new sprag race and instructions on where to drill the hole. I have never heard of this probably because I have never worked on an old C6. I have never wrenched on a c6 with a screw-in modulator meaning all of them are like mid 70s and newer possibly.

[TonyC]

Okay, that's very good info there! It helps to clarify more what Smith's did to mine. He never actually showed me the job he did, because when I checked back he had already completed the job. He did, however, show me the pile of ground-up metal bits that used to be the clutch pack, so I took his word for what he did (being that the owners were Mennonites I felt their word was good enough). It's been 10 years now, and the only thing I have had to do to the transmission since was replace the pan gasket which was leaking, last year. I haven't even done any adjustments to the band. Well, I just remembered that I did have to do a tweak of the vacuum modulator about a week or so after I got the car back because the engine was revving really high before upshifting; but that was an easy job.

They did a very good job, I think. They even had contact info for some of the Usual Suspects, including Baker's Auto, so I didn't even have to tell them that. They knew where to source the unique parts they needed.

I'm going to download those shots you shared, for future reference. I'm pretty sure many other owners of C6-fitted clap-doors will also benefit from this when they need transmission work done.

---Tony

[1Bad55Chevy]

For reference

The TSB required replacing the sprag inner race (#28 in the above diagram). The hole would have to be drilled from the inner case towards the output shaft.

If you were going to rebuild a C6 that was not yet modified I doubt you would be able to buy the old version of the inner race (but I have been known to be wrong before).

[TonyC]

Very good reference, I have to say again! All this will have to be useful to newbies who get clap-doors from the last 4 years of the '60s! Somehow I doubt that all transmissions of those years were upgraded with this fix, just like not all engines had their OE timing sprockets or barely-adequate oil pumps changed out.

As for availability, well, getting the service-replacement sprag may not be easy-peasy, but I think it should be obtainable. Apparently, Smith's Transmissions was able to acquire one when they did Frankenstein's transmission. On the other hand, that was 10 years ago...

So, this design flaw affected only the first-gen C6s and not the later ones? Lucky for the '70s owners.

---Tony

[frasern]

Good info for anyone with an early C6, I will download this for my own records, but it really should be in a pdf in the documentation section, before it gets buried.
Among others, It will apply to all with the slanted bell housing, I don't know if the first Marks used that or not, but my '68 sedan with a 460 does.

[1Bad55Chevy]

Very good reference, I have to say again! All this will have to be useful to newbies who get clap-doors from the last 4 years of the '60s! Somehow I doubt that all transmissions of those years were upgraded with this fix, just like not all engines had their OE timing sprockets or barely-adequate oil pumps changed out.

As for availability, well, getting the service-replacement sprag may not be easy-peasy, but I think it should be obtainable. Apparently, Smith's Transmissions was able to acquire one when they did Frankenstein's transmission. On the other hand, that was 10 years ago...

So, this design flaw affected only the first-gen C6s and not the later ones? Lucky for the '70s owners.

---Tony

What is really strange about that 1969 Ford document is that it's specifically saying that some of the cases were manufactured with the hole in the case. I pulled that document from a Ford enthusiast forum from a post dating back to the mid 2000s and the guys were arguing about what cases had the hole pre drilled. Many of the people were saying they had the holes starting in 68 but did not have the inner race and many were saying that they were not implemented until mid production 69 so who really knows! I guess we need a time machine.

[TonyC]

Hmm...that is interesting . That tells me that the engineers got wise to the design flaw fairly quickly, about '68-ish. I suppose, after decades passing, the best way is to examine each individual car's transmission when it goes to a shop for repairs. The shop should be notified of that possibility and what to do about it, if in fact the running fix hadn't been done to the car in question. Perhaps if I had known about that in 2004, I could have warned the shop in Killeen about that, and maybe they could have fixed that then...but that's as if I had known. At least the shop in DeRidder knew, and they did something about it.

---Tony