Help with tranny problem on '77 305/T350

[baradonai]

Hi all.
I'm hoping someone can help with a problem before I spend a lot of money I don't really have. Also, I know just enough about car mechanics to be dangerous.

I bought the ole girl about 4 years ago from a guy in Alabama. It was a farm truck. She's a Blue 1977 GMC Sierra with a 305 V8 and what appears to be a T350 transmission. It's in fair shape for it's use and age but runs good. Perfect for hauling stuff on the weekends.

Weekend before last I decided to replace the valve cover gaskets. They were older than dirt an leaked alot.

When I was finished, I noticed a green wire had come loose from somewhere. Couldn't tell where it was supposed to go. It comes out of the firewall on the driver's side




There's also a hose on the back of the carb that goes to nowhere and a nipple above it that has nothing on it. Don't know if I knocked something off there or it came that way.
http://67-72chevytrucks.com/vboard/a...1&d=1319034068

The biggest problem, though is that it no longer goes into any forward gear. Just reverse. No grinding noises. Nothing seems to engage going forward.

Originally I thought, great! the transmission has finally gone out. But then after reading some posts here, maybe I hooked something up wrong on the vacuum or the green wire has something to do with it.

I moved this thing when i replaced the valve cover gasket but didn't remove it. It looks like maybe this is a vacuum line that goes to the transmission.
It hooks into a tree like adapter on the front of the intake manifold.
If this was bad, would it prevent the transmission from engaging? How would I know it was bad?
http://67-72chevytrucks.com/vboard/a...1&d=1319034723




I've posted pics.

Thanks in advance for any help and advice!

[makman1297]

That green wire looks like the one that goes to the temperature sensor, should be on the lh cylinder head by the exhaust manifold, the one on my 77 just goes over the valve cover to the sensor. The TH 350 transmission uses a vacuum line, there is a canister on the rh side towards the back of the transmission. It has a vacuum line, just follow it up and make sure it is connected to live vacuum.

[rfmaster]

The vacuum port at the base of carburetor is used to activate air cleaner themovac door - there should be a vacuum hose connecting thermal switch in the intake air cleaner assembly and carb vacuum port.

The last photo with a large vacuum canister mounted on the passenger head is called EFE = early fuel evaporation valve. The vacuum operated canister pulls on a rod the increases passenger side exhaust restriction with a flapper valve (known to break with age and cause a rattle like sound). It is a classic 70's lame solution that GM came up with to improve engine warm up time. Thermal switch in the thermostat housing shuts of vacuum once coolant temperature is above threshold. Lame lame!

//RF

[baradonai]

Made sure it was full of fluid and checked the vacuum line. Put a little duct tape on the end at the engine just to be sure and no go.

I have a friend who has a t400 from a '76 Chevy truck. Anyone know if the bolt pattern is the same between the 400 and 350? Can I swap?

Thanks!

[Your41Plague12]

The bellhousing pattern is the same and yes it will bolt up. Try to get the crossmember too, as well as the driveshaft.

[baradonai]

Thanks! Say, while we're on the subject of adaptability..
If I go poking around a pull-a-part or buy one off Craigslist, will all t350's have the same bolt pattern, connections and swap out, or do I need to stay within a certain year range?

[lindstromjd]

All TH350's have the same bolt patterns as long as they're Chevy/GMC (I think the oldsmobile ones are different?). The tailshaft housing is what you're going to want to look out for. Just measure how long yours is, and make sure you get the same size as a replacement.

[Your41Plague12]

Yep, as long as it's not from a Buick, Oldsmobile, Cadillac, or Pontiac, you're good to go.

[baradonai]

Excellent! Thanks guys.
I don't really have the $650 the guy at the local transmission shop wants to rebuild mine. I may have to try and do this myself in my driveway.

[lindstromjd]

Originally Posted by baradonai

Excellent! Thanks guys.
I don't really have the $650 the guy at the local transmission shop wants to rebuild mine. I may have to try and do this myself in my driveway.

3 speed automatics are fairly simple and straight forward to rebuild on your own. Just make sure you're really comfortable with what you're doing before you tear into it. I know you can get a manual to walk you through it if you wanted to that's supposed to be really good...