transmission shake

[red bandit]

85 RV on a Chevy 1 ton dually chassis. 350 engine, 400 turbo trans. I rebuilt the trans 2 years ago with a master kit. All new clutches, steels & bushings. Forward works fine. In reverse when backing up a hill (under load) it shakes. I don't use it much. Didn't notice it at first, might have not tried backing up a hill. Fluid is full and clean. Any ideas? Reverse accumulator?

[Old Red]

The rear servo piston seal could be leaking or the servo apply pin may be too short and not locking the low and reverse band tight enough. But I would first make sure the pressures are good. If the line pressure and throttle pressures are low, the first place it will show up is reverse. Low pressures will also give you early and soft up shifts. With it at normal temperature, do a test drive and look for transmission up shifts at minimum throttle. Pay close attention to high gear shift. Let me know if it is soft or firm.

[MPittma100]

Thm 400

Usually there is not good news when a 400 does that. Will likely be tearing it down sooner than later.

[red bandit]

Thought I might mention that when I did the rebuild I added a trans cooler 7.5 X 14. Flushed well before install. From trans to cooler to radiator to trans.

Fully warmed up, idle set at 600 rpm, 1st to 2nd gear shifts at 1200-1400, 2nd to 3rd gear shifts at 1600-1800. Once in a while I can get it to shift at 1500. All shifts are firm. I tried many times.

[Old Red]

If both up shifts are firm, it sounds like the pressure is OK. If the pressure is low most of the times the second shift (high gear) will be soft. Let’s try one more thing. Disconnect the vacuum line going to the vacuum modulator, this will raise the pressure. With the vacuum line disconnected and plugged, try reverse and see if it is better.

[red bandit]

Fully warmed up vacuum line plugged to the modulator it doesn't shake in reverse but it's slipping a lot. Drive seems fine.

[Old Red]

That doesn’t sound good. I was hoping that reverse would work good with no vacuum to the V/M. Its time to do a pressure test, if the pressure is good, then check the adjustment on the reverse band. (The one in rear of the transmission) It takes a special tool to measure to get the correct rear servo apply pin length. If the band has been run too tight or the servo return spring is broke, the band may be damaged. You can give me a call at 252-341-6426.

[Rasputin]

A Pooched motor mount will also give you a bad shake in reverse. Much simpler to diagnose and repair as well as a bunch cheaper. JAT.

[red bandit]

Motor mounts are good.

Pressures at pressure port are:
Park/neutral 60 psi. I thought I read somewhere that 50 was minimum.
Reverse 110
Drive 70

Old Red, I am still going to check/look at the rear servo per our conversation but with that much pressure in reverse I'm guessing it is ok.

Any ideas?

[Old Red]

Those pressures look about right. I will try to find my 400 manual next week and see what it says.

[red bandit]

From what I've red those pressures are in range. I will still check the rear servo. Other reads might indicate the rear band pin length might be off. Some place said the pin length can be checked with out the special tool but the link explaining the procedure wouldn't work. If you run across it would you please post it. Fixing this is not at the top of my priority list. The season is over. So it might take me a week or so to check it. But I would like to fix it this winter. I appreciate all the help.

[red bandit]

I pulled the pan today. A thin film of mud in the bottom. I'm guessing it's the rear band.
I can't see anything wrong with the rear servo, no cracks, all the seals look good although they are not a real tight fit when assembled. They don't slope around but they do move easily. I can push the servo in about just shy of 1/4 inch by hand which sounds about right from what I've read. Do you think I should try a little longer servo pin or is the rear band toast and time to take it all apart and inspect???

[Old Red]

Yes that would be my next step. If you use a pin that is too long it will lock up the transmission. You will not be able to turn the output shaft. It would be good if you had one just long enough to lock up the trans and then you could go with the next shorter one. The transmission output should turn easy in one direction and have more resistance in the other direction.

[red bandit]

Ok, it will take me a few weeks to try a longer pin. If that doesn't do it whats next??? Replace the rear band? That'll be my luck. Oh well, it wasn't that bad taking it all a part the first time, second time will go a little easier......I hope!

[Old Red]

Yes I am afraid so and hope the band has not damaged the drum.