One of the side effects of having to remove my transmission is finally seeing what I need. I have made quick checks before and did not see the electrical plug. I noted it does not have the external band adjusting screw, eliminating it as a 250C, and all appeared to be a and older TH350 transplanted in the last 39 years. However, once out I found the connector on it. It has not had a connection since I bought it and shifted/worked fine. There is evidence it was overhauled and "dynoed" in 2008 by the signature on top. My question is; can a 350C be rebuilt to TH 350 specs and operate as the old version? Since it has not been electrically hooked up, does it operate like the pre-electrical ones? Is the torque convertor different?

 

TH350 transmissions from the early 1970s also have a one-terminal electrical connector. This was part of the transmission controlled spark system. Check the tag or VIN derivative stamp to find out exactly what year trans you have. If it really is a TH350C, the "C" just means that it has a lockup torque converter.

 

Hi Joe, This one has a square multi prong plug, but it has nothing plugged in and no plug is anywhere to be found. It has never had a plug since I have had it. The pics are just for reference. I see that it is a 350C, but without the electrical feature, is it acting like a 350 now using vacuum to decide up and down shifts? The lockups I have felt before will not shift down under certain conditions and keep the rpm constant. All this does not matter to me because I have no budget to work on the transmission as long as it functions correctly enough With the underpower V6, it shifted down on rises and upshifted early/ What was the function for the plug that is gone?

 

Yes, it is a TH350C, and again, all the computer does is control the converter lockup. There is no computer control of shifting, only the converter lockup. Everything else in the trans is the same as on every other TH350 - hydraulic shift control through the valve body, vacuum modulator and governor to control upshifts, and cable to control kickdown. Your first trans photo even shows the vacuum modulator. With the plug disconnected, it works just like any other TH350, as you've found up to now.

 

Thanks Joe! Since I have to lift the car 3 feet off the floor to get under it, I paid attention to the info people were supplying me, but since I saw no wires and no band adjuster, I assumed it was a TH 350. Since it functions like a Th 350, and seems to work fine, It will go back in as it is. I think there is a leak at the bottom of the circle thing on the passenger side. Technical terms are lost on me haha, I appreciate the info you give, even though it may seem I'm a bit dense. My working on cars ended in 1974, before the metric, 200, 250 and 350C versions so I had no clue. I also did not have spark controlled or ECM engines, so when your knowledge ends in the 70's it is all just new to me. Thank goodness this will be my last project!.

 

Be warned that the torque is different than a regular TH350, won't interchange unless the regular TH350 pump assembly is swapped in, different spline counts. The stock stall sucks, mine is 1700 rpm, it needs to go. The lock up converter lowers highway rpm by 300 rpm.

 

The cases are different, as is the pump and input shaft and housing. B&M use to sell a conversion kit to convert the 350C to a conventional 350, as plentiful as the 350 is I can’t imagine it was a popular item.

If your just just needing a trans for a cruiser, or just a street car, the 350C will work fine. With the exception of the input shaft, pump and valve body, all the internals are basically the same and have the same weaknesses as a older 350. The hydraulics are different, they don’t respond the the dual feed modification.

You can can get a custom converter made that retains the lockup, and a higher stall speed. It’s not difficult to wire the lockup to work on a older car.

 

That's good information, thanks! I do plan to just cruise it with an occasional exhibition of speed, There are many upgrades I would make if I win the lottery, all of which have $500 price tags. I am old so "don't fix what isn't broken" has to be my choice. When it breaks, it will get what it needs to be modern and reliable.... short of a LS engine!

 

Chuck, research what some of the g body folks have done with the computer. The e quadrajet was a novel idea that worked great when it worked great. However, an e quadrajet that runs like shinola is worse than a normal quadrajet, so a lot of them get off the computer and use a controller for lockup control.

Something like a certain number of seconds after shift to 4th lock up. Unlock on braking and down shift or something like that.

 

That sounds good to me! I like the idea of controlling it. I really don't mind an on/off switch for the long cruises

 

Good to know about the dual feed, so what does the TransGo kit for it actually do? I have never been able to get this one to lock up. It is acting funky so I will pick up another one from my wrecker and add the Transgo kit. I am also planning on a 2500 stall Hughes converter. If my wrecker doesn't have another one, I will go 4L80E. I am thinking the 403 stroker will put out 500+ ft/lbs. I just want a core I know the lock up will work on, realize it may not live long. Also Boss Hog, Hayes and Torco make higher stall converters for the TH350C. Even Rock Auto had a cheap 2025 stall reman converter for the TH350C but sold the last one. I

 

I haven’t put a TransGo shift kit in a 350C, so I can’t tell you what they do. I’m guessing it includes the stiffer pressure regulator spring, the spacer to allow manual low at any speed, and the stiffer shift valve spring. It’s been a while since I worked with a 350C, and even longer since I compared hydraulics, if I remember right some of the passages normally used for the dual feed are used for the lock up converter.

If your our going to use the 350C in a heavy duty application, about the best you can do is to machine the direct piston for a additional clutch and steel plates. You can still drill the spacer plate holes, and you can delete the 3Rd gear accumulator by putting a 3/8 freeze plug in the valve body, under the accumulator piston. BTW, if your trans has the plastic accumulator piston, and it hasn’t broke yet, consider yourself lucky! Replace it with a aluminum piston from a earlier 350

 

I have taken out anything that resembles computer controlled or ECM. Mine will have the HEI, and will run as a 1968 Cutlass with upgrades; 7111, W-31 valves and springs; 9.5 :1 pistons, W-30 Auto cam, dual exhausts, and high oil volume! I wanted simpler, not technology dependence!

 

It’s easy enough to get the lockup to work with a simple switch. With the W-30 cam your going to need a custom converter made. I seriously doubt your going to find a off the shelf converter with enough stall speed to make the engine remotely drivable at low speeds.

 

Drivability issues are phase two, lower ratio, limited slip and convertor to match. I will have to get a new wife if I get it all too fast

 

Hayes makes a 9.6" 2800 to 3200 stall converter.
https://www.holley.com/products/driv...parts/97-1C28Q

 

I stand corrected. I guess there is some performance aftermarket stuff for the 350C

 

Haha...where there is a will there is a way. Thanks for the info!

Chuck