Dead TH350 transmission - rebuild, or junkyard swap?

We had an open house at work last Saturday, and I wanted to bring my truck there to showcase a couple parts it's running. On the way, the transmission started slipping, and finally wouldn't pull at all in any forward position. (Reverse still works now that I checked it today.) So, this leaves me with a couple options on how to get the truck back on the road. Something inexpensive and bulletproof are the top two goals, but I wouldn't mind adding an overdrive gear, either. I plan on staying with an automatic to avoid having to hunt down extra conversion parts.

So, the main options I'm weighing:

1. Rebuild the TH350 I've got. I have never done a transmission rebuild, but there's a first time for everything. Plus, it's likely to be the least expensive option, and I'll know exactly what I've got in there.

2. Find a used TH350 and swap it in. This could be the quickest way to get it back up and running, but do I really want to trust a secondhand transmission that hasn't been made new since 1984?

3. Swap in a used 4L80E. I like the idea of a beefy overdrive. The electronics aren't a big worry for me, but a lot of the used ones seem to be from high mileage trucks and probably have been hauling around a lot of weight for a long time.

4. Used 4L60E instead. Not quite as strong as a 4L80E; should that be a concern?

What would you guys do here?

There are no bad options there, buying a replacement would be ok, they are cheap and plentiful. Rebuilding yours would be ok, easy rebuild, and parts are cheap. Upgrading to overdrive, anything ending in E is electronically controlled so stay away from that unless you want to come up with a computer for it, or switch to a manual valve body. A 700R4 is the overdrive version of your TH350 and isn't computer controlled. You could also consider a TH400 if you want bulletproof, they are mondo bulletproof.

Pretty sure the OP can come up with a controller for either of the "E" series transmissions.. The 4L60 and 4L80 are pretty much electronically controlled evolutions of the TH350 and TH400 with an overdrive gear. Failure areas for them are similar to their predecessors. Sounds like you have a broken sun shell, which leaves you with only Reverse and 2nd in the newer 4L60 or 4L80 (700R4 as well). Like gearheadmb says, any of the options you pose are pretty good.

Ninja Edit: If you do go 700R4 or 4L60 or 4L80, understand that the modulator function is changed from vacuum to cable operation for the 700R4, and is electronically controlled in the 4L60 and 4L80. Proper adjustment of the TV cable is crucial for the survival of a 700R4..

gearheadmb wrote: Upgrading to overdrive, anything ending in E is electronically controlled so stay away from that unless you want to come up with a computer for it...

Yes, coming up with a computer for it was part of the idea. Glad to hear none of the options sound like truly bad ones. Any tips on what to look out for on used transmissions?

Depends what your end goal power and usage is.

I think the worst of your options would be the TH350 junkyard search- too damn old. Overdrive is the bees knees, and first gear is way shorter in a 700 or 4L60 than the 350, so it will launch out of the hole like nobodies business. Only downside would be if the diff already a 2.73 ratio or taller- highway driving around at 1200rpm is a bit boring.

700r4 is actually becoming pretty hard to find now, too. They only built them for a few years before they went electronic, and the early ones had a couple of weak spots. I've seen tons of controllers advertised for e transmissions, but have no personal experience.

I'm kind of in the same boat. The TH350 in The Crusty Chevy may be on its way out.

In my research, most tranny guys suggest it takes about three or four cores to rebuild one good TH350. My guess is fresh parts in a ratty case are probably better than ratty parts in a ratty case, so it can't be all loss.

BUT

If I'm going to rebuild anything, why not rebuild a 700R4 and get overdrive. Or a 4L60 or whatever falls into your lap. I happen to have a 700R4 on a bench, so that's likely the route I will go.

If only there was a DIY transmission controller, like MegaShift or something coughcoughcough I'd be all over that!

Considering how hackable GM PCMs allegedly are, and how much standalone aftermarket controllers cost, I'm surprised that nobody has hacked and published a rom for that yet.

Is your trans a long, medium, or short tail?

I believe it's got a 9" tailshaft.

My Dad is sitting on a well built TH350, but I believe its a 12" tail. Was working fine, but was swapped out for a 700R4.

I'm pretty sure I can swap the tailshaft with other TH350s no problem.

You have a PM

Kenny_McCormic wrote: Considering how hackable GM PCMs allegedly are, and how much standalone aftermarket controllers cost, I'm surprised that nobody has hacked and published a rom for that yet.

A ROM for what?

IIRC, MegaShift already can do 4L60/4L80. Before I switched directions, my plans involved a Chevy 3400 and 4L60 controlled by a MegaSquirt/MegaShift combo.

Anyway, people already have figured a way to use a GM PCM to control a transmission standalone, but I think it still requires a TPS at a minimum, possibly a MAP sensor.

Knurled wrote: Anyway, people already have figured a way to use a GM PCM to control a transmission standalone, but I think it still requires a TPS at a minimum, possibly a MAP sensor.

Link me? I'd think you would only need VSS and TPS, maybe RPM, which I thought you would need to properly run any fully electronic automatic. I was just thinking if you could do it with a $40 junkyard PCM and a reflash, people who only need trans control would be.

VSS is integral to the transmission, and you can probably get RPM from the input speed sensor, although it might affect TCC strategy if you don't have access to engine speed.

WHAT?

"MegaShift" actually exists? WOW! Now I am SUPER stoked. This I didn't know - I had just hoped the folks at MegaSquirt would do something like this. Oh this is so cool!

Thank you thank you thank you!

Yes, it's currently just sold in kit form. There's also experimental code to use a MicroSquirt for a transmission controller.

The last I saw of it, the details were "We have GM 4 speeds figured out. If you make something else work, share the knowledge."

IMO, you can get standalone trans controllers for cheaper than buying an MS2 and converting it, so the only real reason to use it is for CAN communication with an existing MS install.

Yeah I looked into it as there was an option to run the 41te chrysler but it seemed like no one outside the GM 4L area had actually had any huge successes getting everything ironed out.

Picked up a 4L60E today off Craigslist. We'll see if I made the right call.