Crazy c4

[Joshpow]

My c4 in the 71 just started acting up. It slips on the 2-3 upshift and 3-2 downshift. After it is in gear, it pulls fine. When it slips, it makes a high pitched squeal/whine noise. I can tell it is from the tranny area, but not exact location. 1-2 shifts fine, and no other oddities. I checked the fluid, no burn and full. I adjusted the bands on both to no avail. Any ideas? Please help! Thanksa in advance for any input.

[BobbyFord]

Check the modulator vacuum lines. Disconnect the modulator at the vacuum source (intake) and hook up a vacuum pump. The modulator should hold vacuum. Also, pull the vacuum hose elbow at the modulator valve and check for trans fluid seepage out of the valve.

[Joshpow]

Does it sound related to modulator valve / valvebody or with clutches or torque converter? Would I need to rebuild you think?

I appreciate the input Bobbyford. I'll try to check it out tomorrow evening.

[Joshpow]

Further update. I tried the bands for no luck. The test drive after adjustment proved the same results as normal, slip on 2-3 3-2 shifts. I was going to move it to change the modulator valve today, and no movement at all. Just whines and sits still. No reverse or forward gears. It only gets worse.

[Joshpow]

The band adjustment I used was wrong. Re-adjustment gives me motion. A new mod valve helped a little, but still slips. Also tried to lock up on me going 35 or so in 3rd. Stepped down to 2nd and went on. Tried 3rd again, and it still pulls. I'm puzzled.

[BobbyFord]

You can screw the little screw inside the modulator clockwise 1/2 - 1 turn at a time to increase overall line pressure and lengethen the shift pattern.
Front band adjustment 10 ft. lbs. and then back out 1 1/2 turns.
Rear band adjustment 10 ft. lbs. and then back out 3 turns.

[Joshpow]

I got the bands at 1 1/2 front and 2 3/4 rear. They seem OK. The slip and lockup worry me. The lockup skidded the rear wheels!

[willowbilly3]

I am not a tranny man by any means but it sounds like it's getting in 2 gears at once. Maybe a sticking servo or some FOD in the valve body?

[Joshpow]

I kinda wondered about that two at once thing. I had the same idea in my head about a servo problem. Anyone know of a way to check it in the truck? I was afraid I was way off, so I was embarassed to even mention it. I know very little about transmissions.