transmission issue troubleshooting

[kevinvinv]

uhlhazard, I feel your pain. I had to rebuild my 454 twice in a row... never found the problem... just changed a bunch of parts out and re-did it all... very frustrating.

buddrow, Not sure... I just have never had that sort of behavior before. I mean... if this is how all tranny's worked then how would your average person down shift going down a mountain? In my case, if I do that, downshift going down the mountain... the freaking rig speeds up (coasts faster)!!! I mean - I've driven a lot of mountains in a lot of cars and never had this happen. :)

[buddrow]

Does it not try to shift until 72mph? It should shift into 3rd under light throttle at about 25mph give or take a few mph.

Need to perform circuit integrity checks using compressed air. Assemble the input drum(clutched installed), and pump. Place the pump with the converter seal facing down towards bench, insert the input drum into the pump with all washers/bearings in place. Hopefully the pump is emptied of fluid :).

You can leave out the sprag for this test. Apply air pressure via air gun to the feed holes in the pump. Moving from one hole to the next(some holes are not associated with the input drum) you should see and hear each clutch pack apply. You should be able to see the coast/overrun clutch(bottom), the forward clutch(middle), and the 3-4 clutch apply by applying air to different holes in the pump. I dont have a pic of which holes are which so just trial and error.

If you never see the 3-4 apply then you have a massive hydraulic leak in either the pump(not likely but possibly the stator tube) or the input drum. Inspect all seals on the input shaft for any signs of distortion or damage. Also disassemble the input housing removing all clutch packs/pistons. The last out/first in piston is the 3-4 apply piston. Is it a bonded/one piece design or does it have replaceable lip seals? Replace any damaged seals.

If all that checks out ok make sure you have all 7(seven) checkballs installed in the correct spots in the valvebody, should be a chackball location chart in the rebuild kit as well as the ATRA book. A missing checkball could also cause various issues. Time to check the valves in the valvebody for free movement in their bores. Be careful not to nick or damage the valves as that can cause the valve to stick.

If all that checks out, reassemble the transmission except for the valvebody and seperator plate. If you have a long skinny air gun then you can apply air to the input and reverse input circuits ad listen for a "clunk" type sound indicating that each clutch has applied. You can use a paper towel or even a shop rag as a "seal" on the tip of your air gun The feed holes are located at the front(pump end) of the worm tracks in the case. If you look you will see the roughly 1/4 inch holes placed parallel to the case surface. A few of the holes you will hear "hissin" which is normal, but you should hear 4 clutch packs apply, reverse input, overrun, forward, and 3-4. if you don't hear all 4 then you may have forgotten the pump gasket :).

You never mentioned how far south you were.

Buddrow

[buddrow]

Kevin,

this behavior is inherent in several transmissions. When in 4th gear there is no engine braking due to the overrun clutch overrunning or not being applied. It is applied in 1-3 gears to aid in engine braking, effectively using the engine to help slow the vehicle. When coasting downhill, downshift to third and you will feel the vehicle start to drag and engine speed increase, same thing for second and first gears depending on vehicle speed though I wouldn't recommend speeds above ~35mph or so for 2nd and ~25 for 1st.

Now, take all this info with a grain of salt as not all transmissions operate the same during engine braking, it's just a generalization.

Buddrow

[kevinvinv]

Buddrow, thanks for helping both of us, and uhlhazzard, I am sorry if I hijacked your thread a little... tell me to butt out if need be :)

I paid a little more attention on the way home tonight. Here is what I notice,

4L65E

4th gear, cruising at 60MPH let off gas.... engine braking happens (doesn't coast very well) - RPM stays up a little as the engine is dragging the truck down
Shift down to 3rd gear (normal "drive") -- engine RPM goes DOWN and the truck starts coasting much better... like it is in Neutral
gently rev the motor- up to 1500 or so... yeah- it seems like it is in Neutral.... can rev up to 1500 and down all day.. seems in neutral...
rev the motor higher to 2500, the trans shifts finally into 3rd and engine braking occurs.

If you've got the patience to comment on this sequence... I'd sure appreciate it. If you still think it is normal.... just let me know :)

Thanks man!!

[buddrow]

It almost sounds like the sprag is installed upside down but that would be noticeable as you would have to place the shifter into 3rd or lower to take off from a stop.

[uhlhazard]

Buddrow: Awesome info. I believe you are referring to "air checking" the drum assy. I'll get er done tomorrow. You motivated me to clean up my shop so now I got an np241 I just went through for craigslist!

The molded vs lip seal piston you speak of. I believe you are referring to a piston that my new manual is saying to trash the lip seal aluminum one for a steel molded piston. Evidently those can have invisible cracks and cause crossleaks. That part is what stopped me from continuing the project. This transmission keeps soaking up $$.

After my first rebuild when the trans goes for 3rd it just freewheels. If I romp on it from say 40mph , she slams into 2nd and whoops it... till she redlines at I think 72mph. If you let off of the gas the engine drops to idle despite doing 70mph. Once you slow down to say 30 or so it grabs 2 and is back to normal. (its a 3.73 geared 94 bravada with 235/75/15's and a very good running 4.3 cpi)

Trans history: The old trans came to me with no 1/2. Just reverse. They already did the solenoids thinking it was that. (I can't ever seem to have a failure that's that simple)
I tore it out of the vehicle, made a 2 page list of broken stuff on the truck. The motor mounts on 4.3's are garbage and the prop shaft was slamming into the body. Had a good 4" diameter punch under the driver seat.
Dropped the pan to find a TON of clutch material. Started puling the pump off and that is where the drama began. Once I was inside of the drum there was black burn marks everywhere.
I believe it is called the "forward sprag" the little one. It would spin both directions, had slack and the clutches teeth on the sprag were rounded off and warped. The inner went in and the outer went way out, say 1/2 an inch.
The 3/4 clutches did look good but burned (all clutches/steels were replaced) in fact nearly all of the internals were replaced with a 700r4 gm goodwrench. This definitely became a frankenstein project.

I'm in Willis, TX so a good 5+ hours from ya! Hell the goodwrench trans came from oklahoma. Guy swore up and down it was for a 94. i get there and said "nah, its a 92 or older". He gave it me much cheaper for wasting my time.
E MEANS ELECTRONIC!!!

Anyways Kevin you aren't threadjacking anymore than I am. Ultimately this is just us 2 noobies getting some valuable intel from our superior on transmission issues.
I towed a trailer across town today for about 2 hours and a few times above 60mph I dropped into 2 and it doesn't slam into gear like I thought it did, was gentle and the speed does drop rapidly.

Kevin, can you datalog driving in your trans? I am certain watching your datalog through all 4 gears I can at least point you in the right direction.

[kevinvinv]

It almost sounds like the sprag is installed upside down but that would be noticeable as you would have to place the shifter into 3rd or lower to take off from a stop.

Taking off from a stop and any upshifting in general is basically perfect... just the down shifting is messed up (IMO).

[kevinvinv]

uhlhazzard,

I can datalog with TunerPro RT. What do you think would be good... just log the scenario I described? Maybe I can get that done in a day or so...

[uhlhazard]

A 10 minute drive going through all the gears under normal driving and possibly some WOT passes would be awesome. I'd like to see your 1-2 and 2-3 shift times and when you go into lockup how it holds.

[buddrow]

Kevin, get a log, as short as possible but with the problem happening and Ill have a look. Need to see if the pcm is commanding the downshift.

uhlhazard,

Willis, TX is a bit more than a 5 hour dive for me. I'm just north of Tulsa, OK and its about a 4.5 hour drive to Dallas from my house. I'll be in Dallas briefly not this weekend but the following weekend helping my mother move back to OK. If I wasn't flying down I'd just build you a trans and meet you in Dallas. :)

As for using the aluminum piston Ive only seen a few cracked in the 20ish years Ive been doing trans work and it would likely go into 3rd but maybe slip under load, if it air checks it should be ok. You can also air check the drum apart from the pump using the feed holes in the input shaft. The swapping of the internals shouldn't be a problem unless there is a slight mismatch in the pump/input shaft feed holes. If it's an early 4l60e it should be ok, I'll look in my parts book in a bit and see to be sure.

NOTE: When air checking always remember safety first and keep your head away from potentially flying parts, i.e. clutch plates, etc.

[uhlhazard]

I read to set the air regulator to 40psi to reduce the flying parts phenomonon. Thanks a ton! Ill check the feed holes. From everything i can see the r4 is a 92/93 and the bravada is a 94. Both caees have the 4 bolt output instead of the newer 6 bolt. I gave a friend of mine the valve body which got his suburban up and going. He was thrilled. Will report back asap!

[buddrow]

The 6 bolt tail housing didn't show up until the 3 piece case with the bolt on bell housing on the later units. 93 was a transition year. early production units were 700R4 while later production were 4L60E's. Some 92 cars got the 4L60E. But you obviously know how to tell the difference. :)

Buddrow

[uhlhazard]

Hey Buddrow you might know the quick/dirty answer for this: My brother gave me an np241 tcase.

History: He bought it and a transmission that had 20k after reman on them from goodwrench. He just needed the trans so he kept his tcase.

Research: I googled the difference and it's a pretty sweet unit. It has a higher sustained tq value from the engine side than my np233. Reading online the 241 has a 6 pinion planetary, a 1.25" wide chain, and a stronger 4 series case. I have an np241-4l60e mount/adapter from my brothers old trans and was wondering if I could upgrade to the 4 series case.

Results: After some mocking up I realized that the largest issue is the 4 series doesn't seem to have a vacuum actuated front diff like my S-series and I don't want to convert over to electric. If I can find a vacuum switch for the 4 series tcase, then I can proceed with the mod. That doesn't account for yolk differences and shift solenoids either but gotta start somewhere.

To consolidate my overly verbose question: Is there a vacuum switch application for the 4 series np transfer case that you are aware of?

[kevinvinv]

Kevin, get a log, as short as possible but with the problem happening and Ill have a look. Need to see if the pcm is commanding the downshift.

Buddrow, on the log file, do I need to log it with a "trans" adx file or does that matter? - I assume it does matter.... There are two different trans adx files if so... so I am not sure which one to use really. This is for $0D 6395 PCM. I'll try to figure it out but in case you have a comment off-hand... thought I'd ask.

[buddrow]

Hmm, I cant think of any right now but it's prolly because my brain is mush from writing code all day. You could try installing the vacuum switch from the 233 into the electric switch(similar position on tcase) spot on the 241 and see if that nets u the results u need. The switch on the 241 turns on the light for 4HI/4LO on the dash/console. Both switches are basically the same aside from one being a vacuum control switch and the other an electric switch. It's worth a shot.

Buddrow