Hello, New to the forum so apologies if my queries are redundant. I am working on a new to me 55 chevy with a 265 backed by a TH350C. These are new to me, and I'm told I can run them without worrying about the lockup function, and have read that they have more converter slip. I have read that they worked with a vacuume switch to unlock the converter under load, and a brake switch to unlock when the brakes are applied, and they ran through a neutral safety switch for, well I don't know. Now that I'm dizzy from conflicting info, here's some questions. 1) Why would I want it to unlock if I'm on the brakes? Seems like engine braking would be better locked up. 2) Why the neutral safety switch? If it's in neutral or park what's the issue? 3) Does the trans put pressure to the lockup clutch only in 3rd gear? So I would not have to worry about how it works in 1st and 2nd? Or is it applied in all 3 gears? My thinking is power to the lockup solenoid through a vacuume switch that will unlock at what ever GM thinks it should, 10" of vacuume or so, and maybe a toggle switch if I want to turn the function off. Like if I'm really flogging that high torque 265. Yes I realize there are some kits to basically disable the function, but I kinda like the idea at cruise. No it wouldn't be my first choice of a trans to swap in. Yes down the road it will get re-powered and a true OD trans, but for now I'm kinda working with what I've got. Thanks in advance for advice from those who know these gearboxes.

 

all GM lockup transmission of that era use a brake switch and vacuum switch to control lockup. i believe one of the reasons for the brake switch is so the vehicle wont stall when coming to a stop with converter locked.. personally I don't like these transmissions that don't use ECU control. With a computer lockup is setup to engage a certain speeds. I daily drove a car for 5 years that locked up immediately upon shifting to 4th, it just sucks IMO. I think TCI makes a kit to control lockup with a speed sensor but it is expensive.

all automatics should have a neutral safety switch, but it doesn't have anything to do with lockup.

 

So, does the lockup not function in gear at an idle when you take your foot off the brake? Does it not apply in low gear? What keeps it from going into lockup if the vacuum is high and no brake is applied? I know there's a trans guru out there. Thanks.

 

It uses a pressure switch in the 3rd gear oil passages to activate the lock-up along with a vacuum switch and brake switch. For the lock-up to apply the brake switch has to be closed, the vacuum switch has to be closed, and the pressure switch has to be closed. Anyone of those 3 switches opens and the converter will not apply, or it would unlock.

A long time ago I experimented with a pressure switch in the 2nd gear oil passages along with 3rd gear. As tayto says it was too clunky for my liking. I never could get it just right. I tried different pressure rated switches but that didn't help much.

 

Could you just put it on a manual switch?

 

You could but, why? It is easy to wire up with the switches.

 

I was just curious as I also have a th350C. Thought about using it on my next swap and for me I would rather manually lock the converter.

 

https://www.summitracing.com/parts/bmm-70244

The kit is made by B&M, kinda pricey but the only way to go IMO. This eliminates the vac switch, still need brake switch.

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Without the vacuum switch the converter won't unlock when the engine is under load. I like that the kit uses speed instead of a pressure switch, but without a vacuum switch I can see where it could lockup at full throttle. (exp lockup speed set to 50 mph and revving out 2nd gear.)

 

that is a good point!

 

So a simple switch to operate the solenoid won't work? I'd rather have it a user manual operation myself.
Why have automatic cycling especially for high traffic areas. Only lock the clutch when needed.

 

FYI Actually the TH350 TCC switch is in a Governor pressure port that way it can close at a switch determined mph. Governor pressure builds at approx. 1psi per mph. That's why they had different psi ranges of switches. If it was in a 3rd port it would get no delay to the TCC apply just like most do with a 4th apply on a 700R4.

 

A simple switch can be used to activate the clutch.

 

That may very well be why I couldn't get it to work the way I wanted. I was a lot younger back then and was just learning automatics.

 

Sooo, thanks for all the info. Some was right on point. After some study I realize that the Governor pressure switch is installed in the valve body and has been accused of having a low pressure setting that can cause in and out lockup at 40-45 MPH, so I have sourced a 52 (or maybe it's a 54, I forget) PSI switch. That should eliminate the erratic lockup in town traffic. The power to activate lockup goes through a brake light switch that doesn't exactly work in my 55, but since I have a dual M/C with a simple pressure switch inline, I can tap power off that to activate a relay wired to be normally closed and that will effectively cut power when the brake is applied, and I have sourced an adjustable vacuum switch to cut power at a pre-determined pressure, (or lack of). Then there's the connector issue, it seems the first couple years had an oval 2 pin plug in the trans case that I can't find a connector for. Fortunately the later models 4 pin plug has connectors available, even though only one wire is connected inside (note here for those who think one terminal is hot and the other ground) the transmission and fits in the same hole, at least on mine.
This is relatively inexpensive to do, +/- a hundred bucks or so, and should allow lockup to function as intended and with known issues addressed.
I can (and probably should) make a list of the part numbers and sources if anyone needs it but it's all pretty straight forward once you know how it's supposed to work.
Of course you can just wire it to a toggle switch, but as I understand it the whole reason for the vacuume cutout is to keep from smoking the converter's clutch. Not much of an issue with my 265, but I like the idea that it's automatic. Kinda like you really want in a daily driver type car.
That's likely to change in the future with a 500+ HP mill and an O/D auto turning a 4.11 limited slip, but for now....
Thanks SO much again for the willingness of knowledgeable individuals to educate the rest of us!

 

I did my first OD conversion when I was 18 on a 64 Impala. An 80s GM brake brake switch bolted right in which gave me the NO contacts for the brake lights and NC contacts for the TCC. You could also invert the current brake lights switch with a Bosch automotive relay and achieve the same thing.

 

I'm guessing you didn't read my last post.

 

i did. seems like a silly way to do it, especially considering you can prob get a proper brake switch for next to nothing, if not free from a junkyard. i've never seen or heard of anyone using a brake a pressure switch to control TCC. I assume since you mentioned a master cylinder you are talking about the pressure switch that turns the dummy light on the dash?

 

Some brake systems use a pressure switch to turn on the rear brake lights. Using a relay will be the easiest way to wire up the TCC if your brake pedal lever and bracket is not set up for a stock type switch.

 

i've only ever seen brake pressure switches used on industrial/off road equipment. not saying they're not on cars, just doesn't seem like a good idea to me for a number of reasons. while the vehicle description is vague I am going to assume a tri-5 chevy, which as far as i know have some type of mechanical brake light switch. It's probably been 15+ years since i've worked on one and I cant remember if the switch is similar to the 58-64 chevs or the lever type switch. either way, OP can do what he wants. I regret trying to help only to be accused of not reading while sharing real-world info that only benefits others on this forum.
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