Tranny won't shift into gear until warmed up

[arveetek]

My father-in-law has an 85 C-20 with the TH400 tranny. He has a problem that just cropped up recently. If he starts his truck in the morning and puts it into reverse, nothing happens. He has to wait a few minutes until everything warms up and then it will go ahead and shift into reverse and operate perfectly normally. I don't recall if the problem also happens if he tries to shift into drive as well, but it probably does.

He recently had the fluid and filter changed, and this problem didn't occur until a while after that.

I haven't worked on too many 400's, so I'm not for sure about this one. He wondered if the vacuum modulator or a vacuum supply problem could be the culprit. I don't think so, because that should only affect shift points, and not prevent the tranny from engaging.

I'm thinking maybe a clogged filter. But, since the filter was just changed, that must mean there is extra junk in the pan, perhaps from clutches failing? Whatever the cause, it doesn't sound too good, does it?

Any help would be appreciated!

Casey

[Robyn]

The 400 is a very durable tranny but in time they all get tired.
The symptoms you describe are classic of a tired box that is seeing the seals in the clutch drums getting hard. The oil cant build pressure untill the thing warms and the seals soften a bit.
Unfortunately the only real way to fix it to drop the box and go through it and put in new seals and clutches.

[Mark Krieger]

Wow, my stepvan has a similar problem. With mine it only lasts a half a Minute and not much more. If I try to move it too quickly I'll get a slight squeal like a belt slipping. It works fine otherwise.

If your's has the same problem after a fluid flush, it might mean it won't help flushing mine either.

I wonder if a bottle of conditioner would do any good.