I have a C-6 behind a 6.9 diesel. It has an early upshift, no passing gear although it will downshift sometimes at part throttle if the load is just right, 2-3 shift flare and sometimes it takes off in second. When its in gear it's solid. This trans started out life in front of 3:55's in another truck. Now it's in front of 4:10's. Shift flare was not evident in the other truck. Other symptoms were but at a lesser degree. Seems worse hot. I have:
Installed a shift kit, When installing shift kit I found no intermediate servo spring in transmission pieces or otherwise. I installed the spring provided in the kit and adjusted the band, disassembled the valve body, cleaned and polished all the spool valves. Reassembled carefully paying extreme attention to dirt and torque specs. Have adjusted VRV and vacuum modulator all over the place and have slightly improved shift pile up but no affect on passing gear or 2-3 slip. It still starts off in second intermittently. Modulator holds vacuum and pin is present as is vacuum. My altitude is 6200 > feet and 15 inches of vacuum is all the pump will produce. I have not run pressure tests I have no gauges, Has MerconV in it. I need help from a Ford trans tech. I am a Ford tech but my specialty is not transmissions. Whats up with the missing servo spring? Servo concern? Governor problem? High reverse clutch? Reverse is solid. Thanks for reading my book..... HELP!

 

If the clutches are worn, you will get a delay and/or flare up during shifts. Looks like you have messed with everything else.

 

I thought about that but the erratic shifting and the fact that the flare wasn't present in the other truck still has me thinking that it's a hydraulic issue. Maybe I've got more than one issue? Probably should have just stuffed a kit in it when it was out.

 

You never stated wither that other truck was a gas or diesel, that the tranny came out of. If it was a gas and you didn't change torque converter's to a diesel converter there is a big problem. That diesel puts out more torque and a gas converter is not built to stand that. Plus If that tranny has alot of age and miles then putting in a shift kit only boosted the line pressure on all of the old rubber seals and you will have leak by. Major shift problems, A shift kit is good on a new rebuild not a used transmission it will cause more problems than it will fix.
I know this has not been much help at pin pointing a quick fix, because I don't think you have one.
My opinion is re-pull it, get it rebuilt with new or rebuilt diesel converter and put a transmission cooler on it if you don't have one already.

 

I haven't heard of the diesels getting a special torque convertor, but if they did, it would not matter since the diesel tranny will not fit behind anything but a diesel.

 

A gas takes a different torque converter, a diesel takes a different torque converter, you can not put a gas torque converter in a diesel or diesel in a gas. TCI Part #443631 This fits a 83 to 87 6.9L, Part #443632 fits 7.3L Diesel
Part #443630 Fits 71-91 All Gas 289 thru 460. He never stated what the transmisson came out of. So I take it came from a diesel. But more than likely he needs a rebuild to cure his problems.

 

It's critical on the diesel to get the VRV set exact. The adjustment only has to out a small amount to cause the problems you describe.

I have the C-6 in my truck. The VRV is not exactly the same as the one on a stock Ford diesel but functions the same. It took a dozen attempts at adjusting to finally get the trans to shift close to normal. Off just a bit and the flair would be bad, off a bit more an it was horrible enough that I manually shifted and let off the throttle a bit.

The pump in the diesel pulls around 20 lbs of vacuum, the max the C-6 needs is around 12-13.

Does the VRV hold vacuum? If not, it's going to be really tough to get the adjustment right.