Transmission slips, vacuum pump?

[Mark Krieger]

My 1988 USPS Stepvan with a 6.2L and a TH-400 Automatic has an unusual problem when I first start it up.

When I first start it and throw it in Drive immediately, it won't move. If I push on the throttle it'll make a squealing noise like a belt slipping and still won't move.

If I let it idle for a minute or more instead, I can throw it into gear and it'll move right away with no signs of slipping.

I'm guessing that it's taking some time to build up vacuum when I start it. When first started, there's not enough vacuum to let the transmission do it's job, right?

I guess there could be a pump problem in the transmission instead.

Does anyone have any clues on whether this is a vacuum pump problem or a hydralic pump problem in the automatic transmission?

It can't be good a good sign. I'm not sure if it's normal for it's age or if it's a sign of an impending failure.

Any help would be appreciated!

[john8662]

I don't have any experience with the TH400 specifically, but it sounds like the trans fluid may be low. Always check these transmissions hot with the engine running and the transmission in Park. If you check it while the engine isn't running or is cold you'll get a reading as if you are full of fluid, when you are not.

The squealing is concerning though, but could be unrelated to the trans, like Power Steering or a belt? Anyone else got any ideas?

[DmaxMaverick]

The vacuum modulator receives vacuum input from the throttle valve. The valve simulates throttle position vacuum seen on a gasser. It should have no effect what so ever with the tranny not going into gear. It could, however cause delayed/early and/or too hard or soft shifts.

I would suspect torque converter drain back. It can be caused by higher mileage trannies that have some build up in the passages and valves. Best remedy is to use a tranny additive, like Trans-X or TransMedic for a short period. If it continues, just allow more warm up time. Unless your needs fall into the emergency vehicle category, the cost to repair is not worth it. It may have no effect on the life of the tranny at all.

[Robyn]

Dmax is real close to the mark. I am also leaning to the idea that the tranny being cold is having seal leakage problems in the forward clutch. If this box has high miles on it you probably need to start thinking about rebuilding the little critter. The vacuum will have zero effect on how it goes into gear. The modulator controls TV pressure and with no vacuum you would have high TV and shift real late and hard. Either you are having drain back or hard seals that dont work well when cold. Check the oil and if its brown and smells like old varnish you need to be getting the tranny apart. If the oil smells burnt its really time to fix things.
Good luck
RC

[Robyn]

How long has it been since the tranny had a filter change?? Cold oil and a plugged filter can do wierd stuff too. Actually one should check the oil level in neutral with the engine running to get the most accurate reading. have someone hold the brakes or use the park brake and chock the wheels. Safety ya know :0)

[Mark Krieger]

The truck was built in 1988 and has less than 80,000 Miles. I don't remember anything unusual about the transmission fluid level or color but I didn't look very hard either. Time for a second look!

I'd like to do a filter change and a fluid flush but now I'm wondering if that'll screw things up more by causing leaking seals. Decisions, decisions...

If I let it warm up for even 30 Seconds (at 60 Degrees Farenheit) it seems to be fine. I've driven it for more than 1,000 miles since I got it and I can't find fault with anything else the transmission does.