th400 worked perfect, lost 3rd gear out of the blue....

i have a 67 skylark with a 70 455, 71 th400 bb code, and 3;55 posi chevelle 12 bolt.
i was driving 2 days ago, everything worked fine, have had one tranny issue a while ago with shifting, ground governor weights and that fixed it, been pounding gears and down shifting etc great for years.
i was leaving a light and it shifted 1-2 great, shifted 2-3 like normal but slowed down....i pulled over and checked for something obvious, all looked well. figured maybe i hit shifted into neutral with hand resting on it....pull out on road, shifting great, 1-2, shifts to 2-3 and same thing, feels like brakes are dragging, put shifter into 2 and goes great, move it back to 3 and feels like brake dragging, shift to n and car rolls smoothly/faster, move back to d and feel drag again.....put gas pedal on floor takes off and runs great,(downshift) when it shifted on its own felt drag and would not go anywhere.....ended up having to keep shifter in 2 and drove it home....shifted and worked great on way home in rev, 1 and 2. down shifts great, smoke the tires off the line still.....just no 3rd gear....

been looking online for hours and keep finding "wont shift into 3", my car will shift into 3 its just not there. before i think about a rebuild i figured id see if it could be something stupid. i see the r and 3 are on the same drum and when that brakes you lose both? this is where i feel like maybe its not something horrible since i still have rev.....

what do you think?

 

If it hasn't been rebuilt or refreshed in the last 2 decades, you are overdue. Perhaps blew a seal.
Bruce Roe

 

Check to make sure your vacuum modulator isn't leaking. Easy test first- take the vacuum line off at the modulator. If fluid runs out, you have your culprit. Verify fluid level, as well. If the modulator is leaking, you may be low on fluid. You won't necessarily see smoke in the exhaust, either.

 

idk if it has ever been rebuilt. guessing you are talking about an internal seal? if so, does it have a seal that could go and only lose 3rd, and everything else work perfect?

 

i was thinking this too! have not checked other than to make sure it was still hooked up. ill check in a few days, got a surprise 8" of snow out of nowhere! was in a tee shirt washing car at night a few days ago, drove it and lost 3rd gear. parked in front of my garage and came out next day with it covered in snow!!! did not see that coming.....

i did notice when i got it home, shifting with brakes on through gears it sure did seem to slam into gear and rev harder than normal....thats what got me thinking modulator....just hard to think its something major with it not slipping at all and shifting perfect up and down etc. just no 3rd!

 

That may not be the problem, but it is an easy check.

 

just checked vacuum mod, dry when i pulled hose off. checked with vac gauge, held vac, checked steel line(few years old) still all good.

test drove, shifted 1-2 great, 2-3 was like going into neutral. shift to 2 and drives great. back to d, shifted great but felt like neutral, while going about 30, held constant, light throttle (not idle) with foot, i shifted 3-N and it felt like i rolled better, back N-3 and still nothing but feels like slight dragging....
got home and it hits pretty hard from N to rev or drive. car always had light bump when putting into gear but now it slams pretty good. rworks normal, no slipping or having to give it lots of gas, but to test, i held the brake and put in rev, gave it gas and pretty much power braked it...in rev, got tires to spin no problem in Rev....

from what i read, rev and 3rd run off direct clutch so if i can spin tire in rev im guessing that not my problem?

i did grind the governor weights last year and changed shift points, thats the next easy (clean) thing to check, but i think if something let go on the governor the shifting would be affected, it shifts perfect, just no 3rd gear!

not finding much in my searches and have limited knowledge of autos. surprised how hard it is to find basics i can learn about a super common tranny online....

 

It's an unusual problem in a 400, since the 400 is a 'stacking clutch" transmission.

That simply means that as you shift up automatically, you simply engage another clutch pack to change ratios, as opposed to having to "turn off" an element (clutch pack or band) that is holding part of the gear set.

Now with manual range shifting, you do actually add more elements. D1 adds the low/reverse band in the mix, D2 adds the intermediate band. When you click up to drive that intermediate has to release, to allow the direct drum to spin.

If you were shifting manually, and this happened, I would look into the intermediate band apply piston, up under the valve body. See if the band is stuck applied (piston up in the bore, as you looking at it with the trans in the car) It's possible the return spring broke on the piston.

If this happened while shifting automatically, I can only assume you developed a cross leak somewhere.

Drop the pan and check the valve body bolts, they may have come loose.

If not, then you will have to R&R the trans and determine which friction element is smoked, the intermediate or rear band, and go from there.

JW

 

thank you for the reply! i thought it was a very odd problem, i have been through a bunch of 200r4's (used and junk yard) years ago in a monte carlo ss i had, ive seen and killed a lot of trannys and felt the problem coming on for days/weeks etc, but never had one shift perfect, pound gears, never felt a slip ever in it, and just had it shift into a gear and it not be there! thats why im hoping and really thinking it is something stupid.

i cant remember if i was shifting or not, i remember my hand was on the shifter because i wondered if i had hit it into neutral by accident, if i was shifting it was not wot, i was just putting around a small town. that def sounds like it could be it! ill pull the vb and check it out.....im in the process of moving and gave all my garage stuff away a few weeks ago, dont even have a jack stand at this point, ill have to see if i can do it at my friends garage....will keep you guys updated.

 

i pulled the pan and valve body, nothing sticking, all looked good. had a few very thin, pieces of metal in pan. like paper thin, sliced metal, not the regular crap in the bottom of the pan. fluid did not look or smell that bad.....looks like shes coming out. its a bb code, ive been told it was a 71 gs tranny? thinking i might try to rebuild it. always wanted to do an auto tranny, i have done a bunch of standards in front and rear wheel drive and big trucks.

summit sells one for $850, kinda feel like im better off doing it myself, looks pretty straight forward. i would love to get o.d., but cant spend much over 1000 at this point on this car.

 

The model year and donor is designated by the tag on the passenger side. The model year is also designated by the partial VIN on the driver's side just above where the pan mates.

 

FOUND THE PROBLEM! or a few...hahaha.
pulled tranny apart, blew my sprag, broken rev servo spring, worn servo housing, drum has heavy wear where bands run.....

see the wear? piston just falls in, zero resistance

broken spring......

heavy wear marks







yup....thats the paper thin metal i found in the pan!

 

so last weekend i found a used bop tranny for sale, guy said he pulled it from a 70 gsx, i drove 2 hours to ny, got there and guy said it was a gsx convertible(did they even make one?) he pulled motor and tranny and scrapped car! so now i kinda think hes full of it, look at tranny and looked strange, checked tag and saw it was a 71 bc code, said screw it, i already came this far, got it and checked codes while driving back....b= buick, c= all other models....oh boy, hope it will work. got home, took it out the next day and finally noticed the long tailshaft! duh! knew it looked different. feel stupid i did not see that, but i was beat 2 hours from home and like 11pm with another 2 hours home, and excited just to get my car going again.

so i cleaned it up a few days ago and was going to swap, build my tranny and swap back, but don't want to change driveshaft or cross member, was going to try to sell it......but now that i need hard parts im thinking im going to rip it apart and steal hard parts.

ordering overhaul kit and all the goodies now. this is my first auto rebuild, after taking it apart it looks super easy to do.(at least to get it working again, im sure you have to have a bunch under your belt and years experance to make one to handle the track).....wish me luck, im sure ill be back asking what i did wrong. hahaha

 

mine was the bb code, guy said it was a 71 when i got the car, tag said 77. (did not know the gs went all the way to 77).
new tranny is a 71 b/c code.

 

Get rid of that 9 element sprag, they fail a lot. The earlier version has 16 elements, and you can get
one with 34. Either requires a different clutch pack with a smooth surface for the sprag to go on.

I wouldn't worry a lot about band wear on drums; its only involved in engine braking and reverse.
Bruce Roe

 

pulled the "new" tranny with the long tail shaft apart and rebuilding that. it is a 71 and has the 16 elements.



clutch plate and springs look different than the 77 trans has, they are stamped 71, so must me oem, i thought they were some kinda crazy aftermarket ones at first.


i steamed case off, then tried to get the crud off, then wire wheeled, starting cleaning the flashing off, got carried away....





i was about to bust out the polish and buffer and make it look chrome but could not get in the little cracks, and dont have the time, ended up painting the case. still looks good and no more sharp, knife edges when i install.

i got a b&m master rebuild kit, replaced every seal and bushing in tranny, got a tci 34 element and installed, got a transgo kit, installed springs in direct clutch as they wanted, also get a new 2000 stall converter from summit and a 2 qt chrome b&m tranny pan...so if all goes well and i did not mess anything up i should be driving it in a few days......if not all be asking you guys what i did wrong! hahaha. so far its a really really easy job, just kinda messy. build drums last night, putting together today....will update...

 

That looks nice; I usually try to scrape off enough of the mud so none falls inside. After my first, I found
several tools can make assembly a lot easier, shown on 2 TH400 trans albums on my PHOTOBUCKET.

http://s93.photobucket.com/albums/L71/bcroe/
click on an Album
click on a picture to enlarge + description Bruce Roe

 

thanks! its getting there. i def learned alot doing this. i think after a few it will be nothing, taking more time double checking everything and making sure its right. gotta get practice, i blew my 4l80e in my pickup truck on the way to my friends garage this morning. zero reverse and slipping in drive....not a good day.
i had clunking and rubbing in the rear of my car, found the rear control arm bolts loose and stock arms, said screw it and spent $400 on umi boxed lowers and tubular uppers and poly upper bushings.....of course my truck was jealous and popped the tranny 5 min later.

 

finished tranny, works perfect!!! shifts smooth....this was super easy to rebuild! so cool knowing i build it myself and every single bushing and seal is new and knowing exactly what is in it over "well i was told it has this or that". thanks for the help guys!

 

NICE! :TU: on the rebuild. That's one thing I've always left to professionals.