overdrive opinion

[HO455]

Originally Posted by MikeB

Yes, they are good for dropping RPM by around 10-15%. Can't remember if they only engage in top gear, but as with a 700R4, a manual switch can be used. .

GM installed them in different configurations in the first years of use. Mostly depending on brand. Some had lock up in 2nd and 3rd gears. There were issues with the 2nd gear lockup driveablity so GM quickly dropped that part.
And as far as I know the lockup clutch is completely electrically controlled in all Th350Cs. Unlike a 700r4 that hydraulically prevents lockup below 40 to 50 MPH.

[MikeB]

Originally Posted by HO455

GM installed them in different configurations in the first years of use. Mostly depending on brand. Some had lock up in 2nd and 3rd gears. There were issues with the 2nd gear lockup driveablity so GM quickly dropped that part.
And as far as I know the lockup clutch is completely electrically controlled in all Th350Cs. Unlike a 700r4 that hydraulically prevents lockup below 40 to 50 MPH.

Just read where some versions of the TH350C had a 3rd gear pressure switch and road speed sensor that a computer "reads" to determine when to lock/unlock, along with a brake pedal unlock switch. But I don't think my 81 had a computer of any sort. I do remember the wire that locked up the clutch originated from the valve body, but I don't know which parameter "closed the switch". It was more than just 3rd gear pressure, I would assume.

[HO455]

Unfortunately there isn't a lot of good information on these transmissions and GM seemed to have a different set up for each vehicle model.
The one I installed was from an 83 (I think) Monte Carlo and it's lockup is electric with the 4 wire plug using only 2 of the pins. 12v positive and 12v negative. Internally it has no pressure control of the lockup. If the circuit is energizing the valve in the valve body the converter stays locked up.
Which at 30 MPH causes detonation issues. I currently only have manual control of the lockup, but I have been researching a more automatic set up with some sort of speed sensor.

[MikeB]

HO455 -- I'm now wondering if a TH350C would be an inexpensive alternative to a OD trans or GV unit. My 69 C10 with TH350 slips around 200-300 RPM on the highway (depending on engine load) with a Hughes GM11TOW converter.

It would give me the equivalent of a .80 -.90 OD, meaning a 3.08 axle would effectively become 2.46-2.77. I'd probably only engage it above 65 mph.

Gotta do some research on this!

*** Just spoke with a guy at Phoenix Transmissions. They have 4 different TH350s to choose from and they can build any of them as a C model. Even their entry level trans is a step up from stock. That one would work great, because the truck has been running a stock one, except for a TransGo kit, for 30+ years, with nothing but fluid/filter changes and new seals.

The lock-up converter stall speeds are 1600 to 3500. I could probably be all-in for less than $2K and not have to relocate the crossmember or shorten the driveshaft. And my 3.08 axle will work just fine.