Have a weird problem. 1980 Corvette, ZZ4 crate motor, stock TH350C transmisision. Just rebuilt the tranny, installed Trango shift kit and a high stall lockup torque converter.

When I first start the car and it's still cold, when I put it in reverse, it stalls. Then once it warms up, I can put it in reverse without stalling it, but once I start backing up, the engine surges badly. It wants to stall but will keep running.

I'm guessing vacuum leak, but checked all vacuum lines and haven't found anything loose or leaking. Could a faulty vacuum modulator cause this?

It doesn't stall or surge in Drive or Low gears, and it seems to shift fine, although it does shift a little soft. I expected firmer shifts with the shift kit.

Any ideas?

 

I was going to say that the issues you are having sound most likely related to the TransGo kit installation. If I recall correctly, there are a couple of check ***** in there, and perhaps one is sticking, or something else is affecting the vacuum internally. The modulator could be the culprit as well. Hope someone with some transmission experience chimes in, because I am now curious as all get out! Hope you get it ironed out though.

Deja

 

You need to look at the oil circuits - the differences between reverse and low gears.

It almost sounds like there may be a leak between the reverse circuits and the TCC apply. Admittedly, it's only when cold, but a leak, if the oil circuits are close enough to each other, may be allowing revese oil into the TCC and making it drag/lock. A missing check ball or a loose valve body bolt, mispositioned or a tear in the spacer plate gasket.
Since it works fine in drive/low, I really doubt a vacuum problem- conneect a vacuum gauge to be sure.
Just a guess, I don't have any diagrams or pictures to look at.

 

A modulator problem usually causes late and/or harsh shifts, because it won't operate properly with high vacuum (at idle and low throttle). So that doesn't sound like your problem. The actuator pin in the modulator could be frozen so that it won't move at all....that would give you too soft shift. But, that doesn't explain the reverse problem.

I would have to lean towards a problem with the build when the shift kit was installed. Most likely a check ball that didn't get installed or not in the correct place. If this problem did not exist prior to installing the shift kit, that is likely the area check for solving the problem.

 

Thanks for the input guys. I was hoping to dig into it today, but am in Hawaii and we're having Tsunami warnings due to the earthquake in Chile

Will have to wait until next weekend.

 

Take care of yourself and your family first. Stay safe.

Let us know how things go.

 

Well, the tsunami turned out to be a non-event. Minor tide flucutations, but no major issues.

Based on what you guys said I did some more research. Went to Transgo's tech library and found the following blurb in their tech article for TH350/350C


"350C kills engine in reverse all-time conv feed plugged with gaskets
bottom gasket 350 1-2 kit has hole open up if replaced without opening willl cause this complaint"

I'm assuming this means the 1-2 shift passage is blocked by the gasket that came with the rebuild kit I installed? (It was a transtec rebuild kit).

I'm not familiar enough with transmission and valve bodies to know where I need to make a hole in the gasket. (I had a friend guide me through the rebuild and he just moved out of state.)

Does anyone have a diagram or instructions to guide me through this?

Thanks for the help, guys.

 

http://www.txchange.com/tgpvid.htm

 

Thanks for the link. I actually already have that video. I followed it to a T when I installed the Transgo shift kit. I just watched it again to make sure I didn't miss anything.

I think the problem I'm having might also be from the Transtec rebuild kit that I used. Not sure which gasket needs to have a hole put in it, but that video doesn't say. Nor do the instructions that came with the Transgo shift kit.

 

i installed a transgo shift kit in my daughters mustang (4r70w), it was kind of doing the same thing, but the converter was staying locked up all the time. i called transgo, talk to a guy, found out i had left a small piston out. had to remove the valve body again to install it, worked fine after that. you probably did something slight lywrong on the installation.

 

Your next call should be to Trans-go to find out what needs to be done to the gasket and/or what else may be at issue.

 

Yep, that's the plan. They are only open M-F and am too impatient. Wanted to see if anyone has any advice for me. Will definitely call them on Monday ,though.

 

You'll need to drop the pan and then the valve body. At least you'll need a pan gasket and filter. Wouldn't hurt to have a new set of valve body gaskets on hand. seems like they're tougher than shoe leather when you want to replace them, and fall apart like tissue when you need to reuse them.

When you drop the valve body- and it's really not a big deal- just messy, you'll get the case check ***** and the spacer plate with it, and the accumulator and band apply piston may fall out too. NONE of this is a big deal- get a jar of vaseline to stick the check ***** and pistons back in place.

TIP- go get your own jar of vaseline, or replace the one you swiped from your wife with a new one. DO NOT take the jar back after you get trans fluid in it. It get real ugly real fast..

 

I've never seen a 'gasket' on a valve body for an auto tranny. They just bolt together and leak a bit at the splitline seams. The overspray just falls into the sump pan. Pressures are too high and gasket material would clog up orifices and valves.

Don't disassemble anything until you talk to Trans-go. The problem may be an easy fix.

 

Spacer plate gaskets? Just a thin paper gasket on each side of the spacer plate. Never saw any GM trans without them- This one looks like it's for a 440-T4
http://cgi.ebay.com/ebaymotors/86519...227142011r3400
and:
http://www.rmsautoparts.com/proddeta...4221350&cat=20

I forgot- the spacer plate gaskets come as a set of 2- one is for the spacer plate to case and the other one is for the spacer plate to valve body- and they are marked. Things get unhappy when those gaskets get swapped.

 

I've never seen a separator plate, channel plate, or valve body gasket. And I can't imagine anyone engineering one in there. Just my .02....

 

The Transgo shift kit came with different spacer plates and gaskets. As a matter of fact I didn't use the gaskets in the rebuild kit - they were replaced by the shift kit gaskets.

One of the modifications done for this shift kit was to drill a hole in one of the passages in the lockup valve body, and plug a hole in the stock spacer plate in the same place where the new hole is in the lockup valve body. Afte re-reading that tech note it seems to me they are talking about not having a hole in the gasket for that. I realized where it says "350 1-2" - that's the shift kit part number for a 350C transmission. So they're talking about the bottom gasket that came with the kit should have a hole. If the gasket was replaced without opening that hole, it can cause that problem

I used the gaskets that came with the kit, so don't know what I did wrong. The only thing I can figure is either I swapped the two gaskets (which I'm pretty sure I didn't), or a hole needs to be made in one of the gaskets, and the instructions don't say where to make it.

Hope to find out more tomorrow when I call Transgo.

 

Talked to Transgo yesterday and they weren't any help. Basically told me to pull apart the valve body and look for a slipped gasket. Didn't sound like the guy knew much about the TH350C.

Wasn't satisfied with that answer, so called back today. Got the same guy. This time he told me to check line pressure and make sure the tranny cooler circuit are not blocked. He sounded a little more like he knew what he was talking about today, but still not much help. I blew air through those cooler lines and through the cooler when I had the tranny out so I know they're not blocked. I also don't think I have a line pressure problem or that would cause more than just a sticking converter only in reverse.

I need a tranny guru familiar with TH350C to chime in here! This has got to be something simple.

 

You are on the right track, I'm sure. There are lots of possibilities. The gasket you mentioned could be manufactured wrong...missing something or have a hole or cut-out not removed completely. One of the punch-outs could have been stuck to the gasket (unobserved) and now be in the hydraulic circuit somewhere. Without having clear drawings/photos of what those parts should look like, and where the differences are from stock parts, you wouldn't be able to know if they were good or not. A ball check being put in the wrong spot, or not put it at all, or the ball could fall out as you are assembling it.... could also cause issues.

Trans-go owes you the technical assistance to figure out the problem. It sounds like their 'first line' folks aren't all that savvy on problem solving. Call again and ask to speak to someone more knowledgeable. The company can't afford 'experts' in all their tech positions, so they put the grunts in the first line and call in the good 'troops' when necessary. The old adage "the squeaky wheel gets the grease" is still true today. You don't have to get mean or nasty, just get firm and entrenched on them helping you solve this problem. Keep asking for better information and/or more knowledgeable tech help until you get what you need.

Your description of the symptoms needs to be thorough and clear. Ask if there is a sequence of tests that you can do with the vehicle to help them understand exactly what is not functioning correctly. Fixing an automatic tranny problem is not rocket science...but it's pretty close. There are hundreds of parts and if one [or more] are not right, it will act wierd. The Trans-go kit designers, techs with the detailed hydraulic schematics, and the problem solvers will have to get involved. Since most folks on this Forum only have experience with 'stock' units, they can only help to a point on your particular situation.

 

I hear you. I will keep bugging Transgo - and do it nicely. I had a buddy who knows trannies help me with the rebuild and he recently moved and took his knowledge and books with him. So, I just ordered the ATSG manual and will study up on it, then take the valve body apart to look for anything obvious. Then, I'll call Transgo back and go from there. Thanks for the encouragement. Now, I just have to find time! Wish I could be out hot-rodding it by now, but that's how it goes!