Pontiac GTO Forum banner

1 - 8 of 8 Posts

·
Registered
Joined
·
50 Posts
Discussion Starter #1
My vehicle has a 3 speed automatic, I believe the Turbo 400; had it rebuilt about 100,000 miles ago (25 years ago) and now having problems I will try to describe.

Accelerating up to about 30 to 40 MPH it will shift fine throughout all 3 gears; however, as I ease off the accelerator to cruising speed, the tranny seems to drop out of gear as if going into neutral. If apply a little acceleration, it will hit 2nd gear then go back into 3rd gear. If I am cruising at about 50 MPH or above, the tranny seems to stay in 3rd gear as it should.

A local tranny shop said $1000 to replace a part but I forgot what that part was. I’ve done almost all repairs to this vehicle since ownership in 1979 which included a complete engine rebuild. That said though, a tranny repair may be beyond what I can do. I’m not a mechanic by trade, just a mechanic hobbyist.

Any suggestions? If possible, I would like to try to repair myself and save $1000.

Thank you.

(67 GTO, 400, Auto)
 

·
Super Moderator
Joined
·
5,789 Posts
When accelerating from a stop using very light throttle, does it shift normally and at low rpm, or does it seem to stay in each gear "longer" than it needs to before up-shifting?

Bear
 

·
Registered
Joined
·
50 Posts
Discussion Starter #3
With little throttle (gradual acceleration) it will shift normally; it doesn’t hang in any gear. Just drops out of gear (like in neutral) when I’m no longer accelerating at about 30 40 MPH.
 

·
Super Moderator
Joined
·
5,789 Posts
From what you're describing, it sounds like one of the possible causes could be that "something" is happening to your transmission oil pressure after the 2-3 upshift that's allowing the 2-3 shift valve in the valve body to drop backwards in the valve body bore a little so that it downshifts back to 2nd until you accelerate some more and build the pressure back up. Automatic transmissions work on oil pressure that's supplied from the front pump, and routed through the various passageways to move valves, apply clutches, "and stuff" under the control of a rotating governor that's geared to the transmission output shaft - the faster it spins, it moves a valve that controls fluid path throught the transmission.

I assume you've checked the fluid level (engine/transmission thoroughly warmed up, sitting on a level surface, idling in Park) and it's ok?

One thing you can do yourself is change the fluid and replace the filter. If your transmission pan doesn't have a drain plug (most don't) the trick to doing it without making a huge mess is to remove all the pan bolts but 3: 2 on the front edge of the pan, 1 on the rear edge. Maks sure you have a -big- container underneath (larger in diameter than the pan by quite a bit), then remove the one bolt on the rear, slowly loosen the two on the front so that the rear of the pan tilts down, allowing the fluid to spill over the edge and into the container.

Remove the small bolt(s) going through the filter that hold it on, remove and replace with a new filter, reinstall the pan with a new gasket, refill with new clean fluid. Sometimes that's all it takes.

Bear
 

·
Registered
Joined
·
50 Posts
Discussion Starter #5
Yes, I have checked the oil level warmed and on a level surface. I replaced the original pan with a larger one to accommodate additional fluid and it came with a drain plug. Prior to replacing the pan gasket and dip stick O-ring-seal, it leaked quite a bit. I may have caused damage operating this vehicle with low tranny fluid.
 
1 - 8 of 8 Posts
Top