GMTH250 /350 transmission trouble

[naplesdean]

I have a trans question. In my 78 El Camino, I have a 350 engine and auto trans( it has a 13 bolt pan so it is a TH250 or TH350). A few weeks ago the trans would not shift from a dead stop unless I revved the engine up to get the rpms going and then the torque converter engaged to move the car. I drove the car home and it is up on my lift. I started to take the trans out but I have never done this before.

So my question is, do you think it is the torque converter that I need to replace or would you think it is the pump inside the trans?

I thought I would just buy a new trans and torque converter as long as I am taking it all out but thought it might just be the torque converter, which I can change for a lot less.

Is there a way to tell what is broke or not working. Please advise.

 

[Hokiefyd]

I had a TH-200C in an '84 Cutlass and it would develop that same problem going in to reverse. It wouldn't shift into reverse unless the engine was revved. The transmission shop speculated that it was a problem with internal transmission seals, and not enough pressure at idle speed to execute the shift. I had it rebuilt.

 

[eagle23]

It sounds like the pump isn't getting the pressures up to where they need to be. I doubt the TC is the issue.

I had the TH350 in my old '77 Chevy pickup redone a few years ago. The clutches were fine; new seals on the pump and internals made it a lot more efficient.

A rebuild is fairly inexpensive for the TH350. I'd go that way and not look back.

 

[The_Jeff]

Could also be a valve body issue. If you can get the trans out you can have it rebuilt for short cash. If you're daring you can get a parts store one for under $1000 as well, though it was probably rebuilt in the dankest darkest 3rd world country with a dull screwdriver, vise grips, and three sizes of hammer.

You'd want a new TC, even though your old trans hasn't grenaded the TC holds lots of fluid.

 

[supton]

Is this your daily driver? I'd be tempted to rebuild it myself. I mean, if I had the means to pull it, and could have it parked in the meantime, why not? I would "trust" my work; and push come to shove, I'd know how to do it a second time... Or get a rebuild at a later date.

 

[KB2008X]

No, it's not the Torque Converter and I'd bet you don't have the tools, knowledge nor the experience to diagnose exactly what is wrong with that transmission. The car will need to go to a good transmission shop for that. Once it's diagnosed properly you'll still need to have it repaired, and once again you most likely do not have the tools, knowledge, experience, etc. to handle that repair by yourself. Pulling the trans is one thing, repairing it correctly is something entirely different.

Your choices are to have the trans repair shop that does the diagnosis do the repair, also, or buy a rebuilt trans from a reputable rebuilder and do the R&R yourself. Either way you'll need to replace the torque converter and replace or flush the cooler and lines.

This an opportunity to upgrade some of the internals to better match the usage of the vehicle versus the factory parts and tune. I'd buy a rebuilt and do the R&R myself, so the trick then is to pick the right rebuilder. I'd advise you to consult a few of the many Chevy forums as to the best rebuilder, parts, tune, etc.

Keith

 

[naplesdean]

Thanks for the input, it sounds like I will just have it rebuilt + a new torque converter.

 

[naplesdean]

Not my daily driver, just a weekend "Home Depot" runs and the like. Thanks for the comments.

 

[naplesdean]

Yes I have a lift in my garage and am capable of removing the trans. I won't be able to rebuild, so I will take it to a reputable shop for rebuild or buy another trans+converter. Thanks.

 

[naplesdean]

I agree. That seems to be the consensus here. Thanks.

 

[naplesdean]

Thanks for the response.

 

[TooManyWheels]

Any chance it is simply low on fluid due to a ruptured diaphragm in the vacuum modulator?

 

[zzyzzx]

I also think you have low transmission oil pressure. On that car it's likely to be adjustable via an external cable. How many miles are on that transmission?

 

[The_Jeff]

Even if he has low pressure per the TV cable it should kick into reverse at idle.