Help! Smoked 4L70E Post-mortem Analysis (W/pics)
Thread Starter
Teching In
Joined: Feb 2017
Posts: 45
Likes: 0
Help! Smoked 4L70E Post-mortem Analysis (W/pics) Backstory: 100k miles on a bone stock trailblazer SS. Started slipping in 3-4 and 4-3 downshift. I attempted to overhaul the trans, install shift kit, and new 3k stall converter. Truck drove fine afterwards. I then cracked the Tcase adapter while launching the truck. i drove it about 5 miles home in hopes it would be okay. i drained the fluid to see how much was left, and i received about 2-3 quarts back, and it was totally black. (there was approx 100 miles on trans at this time). I replaced fluid and filter in hopes everything would be okay (after replacing Tcase adapter). The truck was actually running and driving "okay" except after a couple trips to work, it started slamming really hard during the 1-2 shift. So hard, that i kept trying to reduce the shift pressure in the tune. The trans felt "soft" to me while driving on the freeway, but i wasn't sure if maybe its the new feel of the converter, since i had never driven a vehicle with a higher than factory stall. I put a pressure gauge on it, and in every gear @ idle i had 200psi.
I stopped driving the truck after about 300 or so miles on it. Checked the pan again, alotttt of clutch and bushing in the pan. I parked the truck and tore the trans down.
Here are the results...
So my main question is, assuming that the loss of trans fluid was the ultimate cause of the clutch failure. Why would that be the only clutch pack with damage. All other clutch packs looked brand new. What else could cause that damage besides low fluid?
I stopped driving the truck after about 300 or so miles on it. Checked the pan again, alotttt of clutch and bushing in the pan. I parked the truck and tore the trans down.
Here are the results...
So my main question is, assuming that the loss of trans fluid was the ultimate cause of the clutch failure. Why would that be the only clutch pack with damage. All other clutch packs looked brand new. What else could cause that damage besides low fluid?
Last edited by jthomas; Sep 30, 2017 at 04:05 PM.
Thread Starter
Teching In
Joined: Feb 2017
Posts: 45
Likes: 0
front pump bushing is trashed
tailshaft bushing is also heavily worn. Both into the babbit
During the rebuild, i replaced both of these. I remember when installing the torque converter, it felt really tight... but i assumed thats just because its a brand new converter and brand new bushing. Whats the clearance spec on that?
i feel like maybe the bushing and or converter hub were not correctly sized???
I installed the bushings with a driver, so id assume they were installed straight.
tailshaft bushing is also heavily worn. Both into the babbit
During the rebuild, i replaced both of these. I remember when installing the torque converter, it felt really tight... but i assumed thats just because its a brand new converter and brand new bushing. Whats the clearance spec on that?
i feel like maybe the bushing and or converter hub were not correctly sized???
I installed the bushings with a driver, so id assume they were installed straight.
Last edited by jthomas; Sep 30, 2017 at 04:02 PM.
TECH Fanatic
Joined: Jan 2015
Posts: 1,114
Likes: 33
From: Fort Worth, Texas
Fluid loss/Low fluid level can smoke the 3-4's really quick...... The Forwards & Band can show distress but it's not a given if you were just limping it back to the house.
I would replace that Input Drum as cheap as they are from GM. ($125)
The case bushing wear could be from the T-Case adapter cracking & causing the output shaft to run off center.
The pump bushing should be removed & reinstalled from the PUMP POCKET as there is a small lip to prevent bushing walk-out. If you beat it in from the SEAL SIDE.....The converter hub will be tight on the bushing, There will be aluminum debris under the bushing, The bushing will be distorted.
100% check the bushing on the converter hub before assembling the pump.
Beating bushing in with a hammer is a poor practice in my opinion.....Use a Arbor Press!
At idle with no load.....You should have around 75-80 psi (Depending on the PR Spring) & 230-240 psi at full EPC/Torque signal boost (Depending on Boost Valve)
I would replace that Input Drum as cheap as they are from GM. ($125)
The case bushing wear could be from the T-Case adapter cracking & causing the output shaft to run off center.
The pump bushing should be removed & reinstalled from the PUMP POCKET as there is a small lip to prevent bushing walk-out. If you beat it in from the SEAL SIDE.....The converter hub will be tight on the bushing, There will be aluminum debris under the bushing, The bushing will be distorted.
100% check the bushing on the converter hub before assembling the pump.
Beating bushing in with a hammer is a poor practice in my opinion.....Use a Arbor Press!
At idle with no load.....You should have around 75-80 psi (Depending on the PR Spring) & 230-240 psi at full EPC/Torque signal boost (Depending on Boost Valve)
Trending Topics
LS1 Tech Stories
The Best V8 Stories One Small Block at Time
Gas Monkey Built a 6-Wheel Ferrari Testarossa With a Corvette LT4 Engine
Verdad Gallardo
7 Most Reliable High-Performance Engines GM Has Ever Built
Verdad Gallardo
Amazing '71 Camaro Restomod Is Modern Muscle Car Under the Skin
Verdad Gallardo
6 Common C5 Corvette Failures and What's Involved In Repairing Them
Pouria Savadkouei
Retro Modern Bandit Pontiac Trans AM Comes With Burt Reynolds' Autograph
Verdad Gallardo
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Pouria Savadkouei
Top 10 Most Powerful Chevy Trucks Ever Made!
Hennessey's New Supercharged Silverado ZR2 Has 700 HP
Verdad Gallardo
Coachbuilt N2A Anteros Is an LS2-Powered C6 Corvette In Italian Clothes
Verdad Gallardo
Thread Starter
Teching In
Joined: Feb 2017
Posts: 45
Likes: 0
Fluid loss/Low fluid level can smoke the 3-4's really quick...... The Forwards & Band can show distress but it's not a given if you were just limping it back to the house.
I would replace that Input Drum as cheap as they are from GM. ($125)
The case bushing wear could be from the T-Case adapter cracking & causing the output shaft to run off center.
The pump bushing should be removed & reinstalled from the PUMP POCKET as there is a small lip to prevent bushing walk-out. If you beat it in from the SEAL SIDE.....The converter hub will be tight on the bushing, There will be aluminum debris under the bushing, The bushing will be distorted.
100% check the bushing on the converter hub before assembling the pump.
Beating bushing in with a hammer is a poor practice in my opinion.....Use a Arbor Press!
At idle with no load.....You should have around 75-80 psi (Depending on the PR Spring) & 230-240 psi at full EPC/Torque signal boost (Depending on Boost Valve)
I would replace that Input Drum as cheap as they are from GM. ($125)
The case bushing wear could be from the T-Case adapter cracking & causing the output shaft to run off center.
The pump bushing should be removed & reinstalled from the PUMP POCKET as there is a small lip to prevent bushing walk-out. If you beat it in from the SEAL SIDE.....The converter hub will be tight on the bushing, There will be aluminum debris under the bushing, The bushing will be distorted.
100% check the bushing on the converter hub before assembling the pump.
Beating bushing in with a hammer is a poor practice in my opinion.....Use a Arbor Press!
At idle with no load.....You should have around 75-80 psi (Depending on the PR Spring) & 230-240 psi at full EPC/Torque signal boost (Depending on Boost Valve)
Is a new drum really necessary? Its a DD, not a racer.
why would i only totally destroy one friction?
I had a sonnax boost valve and whatever shift kit that came in my rebuild kit.
TCI kit that contains Raybestos BPZs
I dont remember the clutch clearance, but i did set it to the spec they recommended (i think it was something like .008" per disk???)
what causes the drum to get beat up like that?
the reason i ask all these questions is to simply better my education on automatics
Joined: Aug 2007
Posts: 24,241
Likes: 89
From: Turnin' Wrenches Infractions: 005
TECH Fanatic
Joined: Jan 2015
Posts: 1,114
Likes: 33
From: Fort Worth, Texas
If you can feel ANY harmonics in the truck with the TCC engaged.....It's real bad in the trans!
You can try to smooth the lugs out a little with some emroy cloth, But I would replace it, Make sure the 3-4 steels are indexed correctly also.
Thread Starter
Teching In
Joined: Feb 2017
Posts: 45
Likes: 0
Also, i had a random missfire from a bad o2, and you could feel it BAD when the converter was locked up.
So would the banging 1-2 shift have nothing to do with the drum damage?
TECH Junkie
Joined: Jul 2015
Posts: 3,012
Likes: 48
Why the hell not? After a few miles of easy driving/running through the gears.....its playtime. Not sure what caused his failure....but I've got money on it NOT being because he was at the track with a new transmission.