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04 Sierra, bought new, used for local maintenance, no towing, no heavy weight, one driver.


Have been through a number of tranny rebuilds, and now know enough of transmissions to be dangerous ;-)


After the last rebuild/replace (18 months ago), I had taken it back some 2 months later to complain about a harsh 1-2 shift. Plenty of discussions out there on possibilities and solutions (cracked piston, accumulator, worn seperator plate, alternator tsb). They suggested shift was normal and ignore. I also have 01 burb and it clearly shifts smoother so I tried to convince them otherwise - no avail.


Well, it went out again. Disassembly found a blown planetary. They accused me of racing, towing, abuse etc.


Has any one had a similar 1-2 / planetary blown problem? Any direct attribution of the 1-2 to a blown planetary?


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Mine broke the input drum. Also both sets of clutches were roasted. But I lost 3 and 4. I think trans number one had planetary issues. Sounded like rocks in the converter. Would shake and grind in 1 and 2

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Yea, prior two failures were band and shell failure leading to lost 3-4. The new symptom after the last rebuild was the harsh 1-2 shift that lasted for 18 months - then planetary failure. Just wanted to see if anyone else had the same pattern of symptom failure.


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Shop called, suggested the vette servo was the source of the 1-2, trying to recreate some history here.


I believe from way back, I was complaining about the late shift on high acceleration ( entering highway, stop and go lane jump), so we had gone with the vette servo.


It solved the high acceleration problem, but that's when the low acceleration 1-2 shift problem came up.


Rebuilder had suggested that they make an in-between servo, so we'll try that.


Anyone aware or try one?


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  • 2 weeks later...

Ok, got the truck back for good. Really fed up with the shop, won't be going back.


Been researching the problem, and remembering history as well (9 gm transmission rebuilds and a lot of personal experience with Toyota Std Transmissions).


Was researching the 1-2 shift bang problem. Gm had a tsb to replace the 1-2 accumulator valve. Transgo and sonnax each have shift-kits to address it. Stories of vette servo causing it (it causes the 2-4 band to clamp down hard so as to not burn up, leaking 1-2 accumulator, improperly matched accumulator valve spring to servo. Around and around we go.


While in have pulled a while auto transmission before, I have pulled add repaired components In valve bodies. So, went to wit to pickup shft kits, orings, accumulator-valve kits etc to enable me to drop valve and see just what's in there.



1) shift cable bracket not properly installed (missing a bolt and held with other bolt that was half way out - grrrr stupid shop)

2) dropped pan, everything looks great , 1 point my favor

3) disconnected harness and devoted valve body , so must be a relatively new harness as no tabs were broken , looking good

4) started pulling valves - wait, this isn't oem. With research, it's a sonnax shift kit. Oh yea, I had then put one in 2 years ago with the last rebuild , along with a vette servo.

5) pulled one of accumulator Pistons , plastic. Across the board videos and instructions push to toss these as they crack although they might not suffer add much from pin leakage. Strike 2 against shop.

6) pulled the 1-2 accumulator-valve (gm tsb and I had picked up a sonnax replacement with different springs). Found a stock dx sleeve with what appears to be a transgo red spring. Was going to put in the sonnax any way, but research first. It appears that gm had a number of versions of the sleeve for this valve with the firmest being dx the gm tsb/part description makes no distinction but sonnax does. They have a product that is the "only replacement" for a dx sleeve, but you must use the oem valve and spring. Transgo's shift kit includes a number of replacement springs, choice based on engine, sleeve coding and servo selection.curiously, the red would not be used on any dx sleeve no master what engine or servo. So

a) shop screwup?

b) is the transgo spring setup causing bang shift as the spring is too hard or soft? Transgo instructions suggest it is a hard spring. The sonnax has 3 springs with the firmest shift being the hardest spring.

c) can I use the sonnax 77777 kit in place of the oem dx transgo red setup?

d)Transgo has a chart tied to engine/sleeve/servo - sonnax does not , just soft, medium, firm.


I'm going to pull the other accumulator and the servo today to see what other surprises await.




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Ok, pulled the rest, put it back together, got rid of most of the bang. Actions taken.

1) while getting parts at wit (whatever it takes), found a sign for the shop looking for a rebuilder (as well as a sign for the original manager of the shop that when it had rebuilt the tranny 3 years ago)

2) took it out with a reader to get tps/rpm readings on where the remaining problem is most prevelant. Matched it back to specs in my 01 fsm. Shifts great at 12-25 tps and over 35. The big problem at 12-25 (light throttle) is gone, but there is still a bit at medium throttle.

3) went by the shop to ask about stuff. The new builder said he had taken over about mid way, the tranny was pulled and sitting in a pile with 2 others. He told me everything he had done - the valve body was a new refurbish build and came as is with sonnax kit and the older transgo style spring for the DX 1-2 accumulator valve.

4) I mentioned the sonnax kit I put in in place of the dx sleeve had taken care of some of the problem. He was a bit surprised, and said his next action item was to change out the oem springs in the 1-2 accumulator.

5) it dawned on me that its likely the last rebuild had the same refurbished valve body. I remember the discussion around adding a shift kit and was talked into sonnax so it's likely they put the same model refurbish, and thus we had the same 1-2 bang. I changed the valve so the accimulator spring is next

6) I'm sitting on a transgo spring set that is stiffer than oem. From my research, and transgo, this will help with the bang. The accumulator piston is positioned next to the separator plate. Yes?

7) he also recommended against vette servo etc - claiming the leaking valves were likely the cause of the slipping band. Vehicle is a daily local driver with once in a blue moon long highway run.


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  • 1 month later...

You go through transmission rebuilds faster than I change my underwear. You must be really frustrated. I don't know how much more reliable the upgraded or "HD trans' rebuilds" are compared to factory; as you have found, everything must be compatible or you end up with major reliability & performance issues.

Your research is paying off & you have provided some good points in regard to diagnostics. With that being said, i my advice to you is register with other forums & repost your finding elsewhere as it does not appear you will get any help here. I was browsing a forum when I was thinking about upgrading some trans' components in my full-size. I was finding a lot of helpful info over at 1 of the performance forums. There were a lot of members over there who had knowledge & experience with the GM trans/kits/compatibility



LS1tech.com is where you should be looking for info. Good luck

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