Just wanted to post a follow-up on my transmission problem.

Background:
My C6 mated to a 390 has been shifting early for quite some time. Vacuum modulator adjustment helped none. It shifted into second around 15mph and third around 30 mph....not matter whether coasting or 3/4 throttle. Shifts were soft and mushy. Throttle kickdown and manual shifting were fine. Rebuilt transmission about a year ago. Installed shift kit, but did not do any drilling mods....Drilling made no sense/scared me at the time.

Today:
Today I pulled the valve body again. Took it apart. Drilled the passages per the instruction sheet. Kit is a Transgo SK-6, by the way. I adjusted the intermediate band per the shift kit instuction sheet, not Fords. I also removed two of the internal servo accumulator springs. The kit calls for all three springs for stock shifting...Large and Small spring for increased firmness...and Large spring only for very firm. I installed only the Large spring. BTW: I learned in my shop manual that Ford didn't even use the internal servo accululator on some models. Hence, my decision to only use the large spring. I also found that I left two valve body bolts loose. The two Boost valve/Regulator valve cover screws were loose. I found out when re-assembling that they wern't on the torque sheet sequence when I re-assembled the valve body. So that's why I missed them the first time.

Results:
Awesome. That's all I can say. Vacuum modulator can actually be adjusted with results now. If I turn the screw all the way in it won't even shift. Or maybe it will at 10,000 RPM or something. If I back the screw out it shifts soft, but not too soft. Mid-way...perfect.
Engine now shifts according to throttle position. At closed throttle...1-2 @ 15 mph, 2-3 @ 25mph. With it floored, but not hitting the downshift linkage 1-2 @ 35-40, 2-3 @ 60-65. Both shifts are very firm, but not by any means harsh. I would compare them to a police package cars shift. I've driven a few police cars and that's what the shifting reminds me of. Just a split-second bump.
If you install a kit. Do the drilling. Follow the directions. I did...the second time. This worked for me. For you? Maybe. No guarantees. But I thought i'd post my results after searching through countless unsolved threads on this issue. Hope this helps someone.

 

Good job 71. I remember a schematic that showed where to drill and size of bit to use. no hard parts but a band adjustment that made a 71 LTD do some amazing things with a couple of engine bolt on mods. Put me in the top 3 of a 10 car street drag club. when I was a whole Lot younger.

 

The whole problem was probably caused by the loose screws.
What pressure was being produced was dumped.

 

I'm 99% sure the problem had nothing to do with the loose screws. Ford actually did not use a Boost Valve cover past '71. Just a clip. That's why the cover was not shown on the valve body assembly sheet. Cover is not sealed. Trans was shifting bad before rebuild also. Didn't seem any worse afterwards. But she's right now!

 

Yes but.
Pressure was being lost by the screws being loose.
Valve bodies changed. That's why plate was removed.

Try leaving a few head bolts loose.

 

I understand your train of thought. But why did it do it before the rebuild, also? Maybe Ford left them loose?

 

That could very well happen.
I have also seen the banana plate loose or not aligned properly.
That's the longest plate on the valve body.

 

Yeah, I made sure to tighten the "banana" plate last per the instructions. I guess the two halves have to align first, then the "banana" plate aligns to them. I have a friend with a C4 and his shifts early, too. Noticed that a lot of posts on here about shifting early with the C4/C6 trannies. But I really never found any information on the problem being solved. So I hope what I went through will save someone else some trouble. I guess after they're fixed no one posts. BTW: What does drilling the hole through the valve body wall do? The hole was in one of the chambers near the boost valve/regulator valve. I just find it hard to believe that this is the same transmission!! Shifts awesome!

 

Changes to the shift timing and feel can also be made by installing a longer pin in the modulator.
There are 4 different lengths.
I use a chunk cut from a metal coat hanger.

 

Cool! I never thought of the coat hanger. I used welding rod. But, you know, it never helped mine. The throttle modulator was unresponsive. My dad bought it new. He says it had always been like that. Now if I turn in the screw all the way it won't even shift out of first! Back it out all the way and it shifts a bit early. Mid-way is perfect. Just as it should be. I guess removing two of the accumulator springs helped give the shifts that firm feeling. No matter what the cause, I just enjoyed actually solving it myself. Not having to fork $$ over to a trans. shop. Thanks for the coat hanger tip, though!

 

It was unresponsive because of the loose plate.
Like trying to fill a bathtub with no plug in the drain.

 

Transgo hands down makes the best kits for C-6's

 

Hey I will second and third that.

 

I would recommend to anyone. Very happy with my C6 now. Just follow the instructions!! And for pre-71's make sure to tighten the Boost/regulator valve cover. It's easy to miss if you are following a torque sheet for post-'71's that don't use the cover. And I would highly recommend only using one accumulator spring. The firm shifts have to be better for the transmission.

 

heres what ive found with my c6 shifting late from 1st-to-2nd and 2nd to 3rd adjusted the band problem solved just remember to replace the locknut or you will adjust it more than once