1986 turbo 400 shifts hard

[chevy86]

In my 1986 Chevy truck, the Automatic Turbo 400 transmission shifts really hard into second gear. In fact it bangs into second unless I allow it to shift while under 20 MPH. I had it serviced and the mechanic stated there were particles of something in the old oil. He thought it was parts of the band itself and he said run it to failure. I asked about the band being somehow adjusted but he was almost scared to mess with it, being not adjustable from the outside of the tranny. Currently it still "bangs" into second with any sort of throttle applied. It seems to be vacuum shift only - no visible shifting cables showing coming down from the carb, only a vacuum modulator of some sort on the right side.

What's going on with it?

thanks
Sam

[75K30]

Welcome to the site:
Been a while since we discussed a good old TH400.
Those rough shifts are often related to vacuum problems. Either too much, or not enough. Both have the same result, but usually at different times. Since it is hitting hard, this would actually indicate operational parts. Bad bands, or clutch plates would have the opposite effect.
I believe your troubles are in either a stuck governer, or bad vacuum modulator, or both. You may have a governer that shifts the vehicle by itself (even if the detent is in low, ever try that, man it shifts hard) and a faulty modulator that is allowing, or telling the trans to shift, and if it is governed mode, it bangs pretty hard. I mean like crase cracking hard. This is tough with a stock converter, and very hard on the drivetrain. There is an adjustment inside of the modulator itself, running the screw in decreases the effect of manifold vacuum. try to play with this adjustment after you have verified good vacuum at the modulator. Could have a leak in the line up near the firewall, and could cause the same issues.
After you verify good vacuum, (it has to match the vacuum at its source) than make adjustments to the small screw in the modulator, if you do not see a change in shift points, try replacing the modulator, this one might be bad. They are only a few bucks. Good idea to look inside of the vacuum line, usually a bd modulator will leak under vacuum, and leave oil in the line. This is a sure sign of a bad modulator, and leads to small puffs of white smoke.
Give it a try, and good luck. Let us know how it goes, we like to earn too.

[fyremedic115]

I have a 88 C1500. Just did engine swap 305 to a 350. Not sure of History of tranny but fluid is clean and smells ok. Looks like pan gasket has been replaced recently. Up untill a few days ago it shifted fine except being kinda slow to shift into reverse. I noticed the otehr day that the truck is starting off in 2nd gear most of the time. When it does start in 1st gear it will max out at about 40MPH before it shifts to 2nd. Regaurdless of monement of the gearshifter. Will not shift into 3rd gear hardly at all unless at higher speed like about 60 or 70. Then it will still down shift back to 2nd anytime there is anykinda of load put on engine. Modulator looks new, is vaccume to modulator. (Unsure how much) All connections are clean and look ok. Speedo works good. No leaks. Fluid level is perfect. Is engine vaccume possibly a problem? I don't klnow what is normal but ECM says that 12 mm/ug at idle and about 8mm/ug at about 2500 RPM. Is this about right? Have had problem with EGR in past is why i ask.
If anyone could help and email me at fyremedic115@yahoo.com I would really appreciate it. Don't want to have to rebuild if I don't ahve to. Ohh and Tranny has ( I think) about 230K miles.
Thanks in advance

[Stitz Love]

You are describing some of the symptoms I described in my post... "Another Transmission mystery. TH400".
Check out that more current thread and the various replies for some ideas. I am waiting for a governor-with-gear and gov.-cover gasket to come in from an online transmission parts dealer.
I hope you find some of the info useful.
Let the forum know of the solution when you get it.