TH 350 shift issues

[Mlcdc1]

I have reviewed several posts on this issue and have found them to be a great value - unfortunately I still am having an issue with the 2-3 shift on my 1977 GMC heavy half 2wd pickup (1-2 shift is good). A little background- I recently finished a 1 year rebuild of the truck but did not touch the motor or tnsmission as both were working fine when I started the rebuild. I changed out the vacuum module and installed a new governor, replaced the filter and new fluid. I have unhooked the kick down cable and that made no difference. It does seem to be worse when the transmission is warmed up. Any suggestions before I breakdown and call the transmission shop?

 

[MikeB]

What are the symptoms?

 

[HotRodPC]

Sounds like he has no 3rd gear. And worse when warmed up would probably mean that same. Worn direct clutch pack or bad seals in the direct drum. Sounds like it's going to need a rebuild. If he comes back, we'll get more info to confirm.

 

[MikeB]

HotRodPC -- I'm impressed with your knowledge. The TH350 in my pickup was rebuilt by a friend who specialized in auto transmissions at GM dealers for nearly 15 years. I'd like to say I helped, but mostly I just watched in awe. The only things I did by myself were install a TransGo kit, the speedo gear, and tailshaft.

One of these days I want to buy a 87-89 700R4 and go through it completely by myself. I am familiar with throttle valve operation and cable setup, but other than that it will mostly be new to me.

Edit: Was just looking at 700R4 rebuild tips, and it looks like a lot of specialized tools are needed, say $500 at least. Ouch! OK for a shop, but hard to justify for a one-time rebuild.

 

[HotRodPC]

HotRodPC -- I'm impressed with your knowledge. The TH350 in my pickup was rebuilt by a friend who specialized in auto transmissions at GM dealers for nearly 15 years. I'd like to say I helped, but mostly I just watched in awe. The only things I did by myself were install a TransGo kit, the speedo gear, and tailshaft.

One of these days I want to buy a 87-89 700R4 and go through it completely by myself. I am familiar with throttle valve operation and cable setup, but other than that it will mostly be new to me.

I don't know near as much as I'd like to. The 2 biggest things about building auto transmissions is Cleanliness and Patience. If you're any kind of mechanical and have a good book or info resource then you can sure do it.

 

[Mlcdc1]

Symptoms - nice smooth shift from 1-2 at what I would consider normal RPM (no tach available so not sure of exact). Shift from 2-3 occurs about 1/4 of the time and at all other times, the RPMs are high and no shift. I can sometimes get a shift by backing off the gas and then reapplying throttle. Manually shifting through the gears from 1-2 is good but the same (high RPMs) shifting from 2-3. I don't have a vacuum gauge and wondering if it's loosing vacuum pressure at the higher RPMs? Checked the linkage and it is good. Visually inspected and blew out the the vacuum module line. Slightly varied the adjustment screw in the vacuum module. All to say no change. Wondering what effect, if any, that fact that it wasn't driven while doing the rebuild as to do with the what's going on. I've only driven it about 100 miles since getting it back on the road so is that a factor? I aprrediate your input and welcome any other suggestion you may have- thanks.

 

[74 Shortbed]

Pull the modulator and check the valve in the case make sure it's not sticking, and pull the governor make sure everything moves freely, and make sure you have full vacuum to the modulator and it's working smooth.

 

[HotRodPC]

Agreed, with that symptom, check the modulator valve. Pull it out of the case with some needle nose pliers, clean it and lube it with a little vaseline and work it in and out of the case a few times to remove any debris from the valve bore and reinstall it.

Just for ***** and giggles, I'd also check to make sure that detent cable doesn't have any kinks in it to where it's not letting the detent valve go all the way back home. Even if you disconnected it, if the cable was pulling on the valve and got hung up and hung the detent valve part way open, then it'll cause very late or even not shift depending how far open it got stuck. Doesn't sounds like yours would be stuck full open, but it could be why you're getting a late 1-2.

The late 1-2 and sometimes getting 3rd with letting up off the gas pedal and reapply really sounds like Vac Mod or Mod valve more than anything.

 

[Mlcdc1]

Thnaks for the advice. I will pull the vac module and the module valve and see what I find. Am I correct in that the valve will just pull out once the module removed?

 

[HotRodPC]

Thnaks for the advice. I will pull the vac module and the module valve and see what I find. Am I correct in that the valve will just pull out once the module removed?

Yes, it's just right behind the modulator. Either some kind of hook that will fit in there or a some small needle nose pliers and it should slide out. No keepers, snap rings or surprises.

 

[MikeB]

Also, with the modulator removed, suck on its hose nipple to make sure it's working.

 

[75Monza]

Also, with the modulator removed, suck on its hose nipple to make sure it's working.

Wow, suck and nipple in the same sentence. Are you sure your describing work?

 

[HotRodPC]

Wow, suck and nipple in the same sentence. Are you sure your describing work?

And last night we were talking about Hoes.

 

[Honky Kong jr]

HotRodPC -- I'm impressed with your knowledge. The TH350 in my pickup was rebuilt by a friend who specialized in auto transmissions at GM dealers for nearly 15 years. I'd like to say I helped, but mostly I just watched in awe. The only things I did by myself were install a TransGo kit, the speedo gear, and tailshaft.

One of these days I want to buy a 87-89 700R4 and go through it completely by myself. I am familiar with throttle valve operation and cable setup, but other than that it will mostly be new to me.

Edit: Was just looking at 700R4 rebuild tips, and it looks like a lot of specialized tools are needed, say $500 at least. Ouch! OK for a shop, but hard to justify for a one-time rebuild.

The presses needed to disassemble clutch packs can be made very easily.

 

[HotRodPC]

Yes there are ways to cut corners with special tools. I used a hole saw and made my own wooden bushing drivers for my first few trans builds. They don't last long, but for a 1 time build, it's all you need. I also used cheap China/Taiwan sockets as bushing drivers. I've bent and ground screwdrivers to make picks before. With some allthread and flat steel you can make clutch spring compressors, although that deep input drum with a shaft in it on the 700r4 you just about have to have a compressor for. You don't need all the ring resizers everyone says you need for the shaft sealing rings on a 700r4. You can use a 2 liter bottle and cut the plastic to a square and then use hose clamps around that to resize your rings. I've even used old radiator hose instead of 2 liter bottle plastic.

A good mechanic, especially a trans mechanic can get it done if he has a mind like MacGeyvor, which I guess I must have because the tow and recovery business is the same way. I normally get the job done with whatever I have at my disposale and make it happen. I've even ran a snatchblock off the Tpost of a barbed wire fence to upright a car before that was down in a creek bed and no trees or nothing around. Sure didn't use the top of the Tpost, but I ran a chain all the way to the ground around that Tpost and snatchblocked off of it. Yes, it held up fine.