'82 Chevy Shift Problem

I recently acquired an '82 Chevy C20 with a 6.2L NA. The original transmission was a 700R4, but someone along the line has replaced it with a TH350. It shifts fine from 1st to 2nd, but seems like it slips really bad between 2nd and 3rd. If you shift it into 2nd at a stop and then pull it into drive at around 35-40mph it seems to do fine. At first I thought the tranny was just going out, but then started thinking that maybe it's a vacuum problem since it shifts fine manually. I've never dealt with diesels and I'm not really sure how the modulator gets it's vacuum signal. The vacuum line goes up to one side of a switch near the throttle lever and then the other side of the switch goes to a T. One leg of which goes to a vacuum resevoir and I haven't found where the other leg goes yet. I'm just lost on where to even start diagnosing the problem. Any help would be greatly appreciated.
Thanks in advance.

Welcome to the board!

You should have a diesel/TH400 VRV (Vacuum Regulator Valve) mounted to the fuel injection pump, that is used to emulate manifold vacuum produced in a gas application. The TH350 requires it. This also means that the vacuum pump must be working correctly. Aside from the VRV, vacuum is necessary for the cruise control system and the HVAC controls inside the cab.

The first 6.2 diesel I owned came with a TH350 (bought it used), which the original owner had installed in place of the TH700R4 the truck came with from the factory. At one point, I experienced the exact same shift problem with my TH350 that you're reporting. Getting the vacuum system operational solved the problem.

However, I eventually re-installed a properly built TH700R4, which was the single best upgrade I made to that truck. I consider an OD trans more necessary than the Banks Sidewinder turbo I added later on. Having OD is essential and will make it so much more pleasant to drive. Been there - done that! :)

Jim

Thanks for the quick and helpful response. So i guess my first job is to ensure that the vacuum pump is creating vacuum. How much vacuum should I be looking for? I'm assuming that the VRV is the switch I was referring to by the throttle lever. If I have vacuum at the switch how do i determine if the switch is bad? Thanks again for the help.

The vacuum is definately a serious item andf needs to be correct.
The vacuum at the modulator should be around 18-20"Hg at idle.
The vacuum at the pump should be around 25 or so. If the vacuum is low or the needle on your gauge wobbles all over the pump is shot.

The vacuum switch is a GM part #14057219 and is available from Ma general or an AC delco distributor.

Low vacuum at the modulator will cause very later and hard shifts.
Too much vacuum at the modulator with too light of throttle will cause early and soft shifts.

Hook your vacuum gauge into the modulator line with a T fitting and run the gauge up to the winshield wiper area and watch it while you drive.
As the throttle is depressed and the vehicle gains speed the vacuum should drop off the harder you press the pedal.

If the vacuum does not drop off this will make for soft sloppy shifts.

The high clutches in the TH350 are a sore spot and if they have been let slip they will glaze over and the condition will continue to worsen until they quit altogether.
Yank the dip stick and give the fluid a SNIIIIIIIIIIIIFFFF
If it smells like a burnt pot roast the game is over for the tranny without some internal rework.

Let us know

Robyn

Went out this morning and put a vacuum gauge on the pump. Looks like i'll be getting a pump. Gauge was bouncing from 5-15". Hopefully that'll take care of the shift problem. Thanks a lot for the help. Now I know where to come for future help with my diesel problems.

Rotty,
Welcome to TDP. If you can add some info to your signature it will help in future diagnosis of your problems. :D Look at some of the others and you can decide what you want to add.