The Stovebolt Site Forums The Shop Area The HiPo Shop Here's another trans question. GM TH350C.

Hi folks,can any of you trans gurus help?

I've got a GM TH350C (lock-up converter) trans from an '81 Vette.

Would it be safe to drive without hooking up the lock-up function or will it cause any damage?

I'm talking low speed/short distances here, it's just to find out the general condition of the trans i.e gear shifts etc.
This is before I fork out $200 on a B&M wiring loom for the lock-up function.
I bought the trans from a wrecked C3 and so far everything checks out, the ATF was clean and smelled O.K. so I'm trying to avoid getting into a re-build.
The engine from the same car is in great condition so I have no reason to believe the trans is bad.
The car was burnt out at the rear,hence the reason it was in the junkyard in the first place!
I do not know the PO. so I can't ask him/her!
Any advice would be welcome.
Thanks in advance.
Jock.

No problem. You'll get better gas mileage with the lockup working, as it eliminates the 10% slippage that all torque converters have, but running unlocked doesn't do any damage. While you're pulling parts, get the vacuum switch off the firewall, and the plug that goes to it. It's a round black item with a vacuum line on one side and a 2-wire plug on the other. That's all you need, along with the plug that goes into the transmission, to make the lockup work. No computer hookup on the early ones!
Jerry

Thanks Jerry, unfortunately the rest of the car was 200+ miles away and has since been fully scrapped!
I didn't even know such a trans existed when I got the engine and 'box.
It was only when I investigated a bit further i found out about the TH350C.(It pays to do your homework!)
My C3 manual is a bit vague on the subject!
I'll contact a local 'Vette specialist and try and track one down.
Thanks again
Jock.
P.S. does the vacuum line hook up to the inlet?

Yes, the vacuum line connects to full intake vacuum. Voltage from the ignition switch goes to a closed switch at the brake pedal, then to the vacuum switch, and from there to the transmission. Inside, it passes through a governor pressure switch that prevents converter lockup below a set speed, usually somewhere near 45 MPH. Any time the engine gets under a load, such as going up a steep hill, or passing, the vacuum drop disconnects the power, causing the converter to unlock. The vacuum switch and the governor switch are in series, so all must be closed to power up the Converter locking solenoid, and opening either one, or all of the switches stops the lockup. Applying brakes, dropping speed, or low vacuum disconnects the lockup clutch.

That way, it's impossible to get the car stopped with the converter locked in, which would stall the engine like stopping with a manual clutch engaged. Thr toggle switch some people use isn't that idiot-proof!
Jerry

Jerry..I found the installation instructions for the B&M set-up on Summit racings site.
I've tried some of the local Corvette specialists here and drawn a blank.
I'll probably go for the B&M unit as and when funds allow. That way I don't have to rely on a vacuum source.
I simply preset my lock-up speed and it automatically locks and then disengages again when I apply the brakes.
As far as hill climbing I guess I'll just have to get used to manually switching it off!
Thanks again
Jock.

Check around they can be had for a lot less than that.do a google search and you will be amazed at the results.

Thanks 48GMC! I was just generalising around the $200 mark.

I've taken your advice though and will look into it.

I'm also waiting to hear from a 'dude' who's parting out an '81 C3 Corvette.

The GM part number for the vacuum switch is 14014519. I don't know if it's still available through the dealers' parts supply system, but I'll be glad to check on it.
Jerry

Thanks Jerry,....you've done more than enough 'ground work', I'm sure you've got better things to do with your time!

I appreciate all your help.

I'll get my buddy in Va. to check in his local GM dealer.I've been trying to brainwash him to liking Chevys anyway!

He's a diehard Mopar guy!

If I can get one his wife is coming 'home' to Scotland in a couple of weeks.
She can hopefully bring one back for me.

Jock

Just a bit more info- - - -the GM original equipment switch is available, but it's ridiculously expensive- - - -something like $85.00 USD. The aftermarket parts source is "Standard" brand, part number VX2. It's about $35.00! Here's a link from Rock Auto, if it will work:

STANDARD MOTOR PRODUCTS Part # VX2 {LOCK-UP TORQUE CONVERTER SWITCH}
Category: Auto Trans Lock-Up Torque Converter Switch
* Non-stock item--shipping delayed up to 2 business days
$35.79

I used a reference for a 1981 Impala, 305 engine, "automatic transmission parts" on the Rock Auto website.

Have fun!
Jerry

Hi Jerry, I'd done a bit of shopping around myself, the cheapest I'd found was $47 approx. plus shipping.
I just wasn't sure how many different models used this trans.
I'll check out that site myself.
Thanks once again.
Jock

P.S Link doesn't work....no problem I did the long way round!

If you don't have any joy with the US suppliers, an adjustable vacuum switch like this should do the job.
http://uk.farnell.com/jsp/search/re...&isGoback=false&isRedirect=false

If the link doesn't work, just search for adjustable vacuum switch on the Farnell Electronics site.

Hope the project is coming on OK, Stuart.

Thanks Stuart, I think I'm going to go with Jerry's suggestion,
plus my friend in Va's wife is coming back to 'Blighty' soon.
She can bring it home with her.
Thanks for all your help guys!

Jock.

P.S. The project's coming along slowly but surely!