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Hi I'm new to the forum. I've scoured the web and can't find a lot of info. I have a 2008 Chev Avalanche (I know its not the focus vehicle here, but the truck forums don't seem to have many transmission savvy people) with the 6.0. I get an rpm flare at downshift from 3 to 2 under load only (pulling my boat up a hill, or if I'm getting on the throttle) no other issues and no codes. I dropped the pan last night, no black stuff, no burned fluid, only a little fine metallic "slime" on the magnet. I was going to just replace the fluid, filter and the 3-2 downshift solenoid, but it was such a pain and mess to drop the trans pan, I want to make sure I get this thing solved the first time around. So what other parts should I be looking at? Upgrades I should do?
What is your band clearance?
What servo do you have?
Those 2 things are likely the cause of your issues.
On a 3-2 downshift, the 3/4 clutches need to exhaust the fluid from the 3/4 piston.
And the "3rd accumulator" oil...which is oil that is pushing the band OFF...needs to also exhaust. As the 3rd accum oil exhausts, the servo travels into the case and engages the band.
A flare in this scenario would mean that the 3/4 clutch is exhausting faster than the band is applying.
A slow band apply can be the result of excessive band clearance.
a larger ratio 2nd gear servo (like the corvette servo) will also help apply the band faster.
What is your band clearance?
What servo do you have?
Those 2 things are likely the cause of your issues.
On a 3-2 downshift, the 3/4 clutches need to exhaust the fluid from the 3/4 piston.
And the "3rd accumulator" oil...which is oil that is pushing the band OFF...needs to also exhaust. As the 3rd accum oil exhausts, the servo travels into the case and engages the band.
A flare in this scenario would mean that the 3/4 clutch is exhausting faster than the band is applying.
A slow band apply can be the result of excessive band clearance.
a larger ratio 2nd gear servo (like the corvette servo) will also help apply the band faster.
I don't know a ton about transmissions. I assume its a stock servo. Im not sure how to measure band clearance. If I recall correctly from some of my research, the access is behind the heat shield on the passenger side right? Servo comes out with a retaining wire?
Should I be able to change the servo with the trans installed? Can I check band clearance from the servo opening?
Is the transmission completely stock?
Have a shift kit?
We should really know what the transmission line pressure readings are to start.
Also having a look at the transmission data... even without any DTCs is helpful as well.
The 3-2 control valve and solenoid change the timings of the 3-4 clutch and 2-4 band.
The 2-3 up-shift and 3-2 down-shift both require specifically timed apply or release of the 2-4 band and 3-4 clutch.
The clearances of this clutch pack and band are critical in maintaining a proper range of timed shift events.
When the timings of the 3-4 clutch and 2-4 band are incorrect... a flair or bind can result upon the shift/ gear change.
The 3-2 control valve solenoid when turned on, will move the 3-2 control valve and close the passage in its path.
This path is for 3rd accumulator/ band release... slightly slowing down the re-apply of the band for 2nd gear.
This timing encourages the 3-4 clutch oil to exhaust slightly before the 3rd accumulator/ band release oil.
If the 3-2 control valve solenoid did not turn on...
It leaves the 3-2 control valve open providing a path for 3rd accumulator/ band release oil to exhaust slightly more quickly.
In a sense the opposite of the 3-2 control valve solenoid being turned on.
Just like MaroonMonster asked... the servo size, band clearance, and 3-4 clutch clearance are major factors for...
Properly timed and executed 2-3 and 3-2 shifts. These clearances should be kept at the smaller side of spec.
0.060" clearance for the 2-4 band; and 0.030" clearance for the 3-4 clutch... are the smaller side of spec. and provide excellent shifts.
If the transmission is not completely stock; then:
-We need to know if using a Sonnax 3rd accumulator check-ball capsule check-valve
-We need to know if the 3-2 control valve is blocked of functional
-We need to know orifice sizes for 3rd feed, 3rd exhaust, and 3rd accumulator/ band release... are the #2 and #4 check-***** active?
Hopefully the 3rd accumulator check-ball capsule was not replaced with a cup-plug
I believe the transmission is stock. I did buy the truck used, but the previous owner didn't seem the type to do performance upgrades. The 3-2 valve is operating i believe. I pulled it and the spring out, looked clean. Put back in. I can push it in and out with my finger, so no hard spring like in some aftermarket kits.
Ill look into the measurements you are suggesting. For line pressure, it sounds like I'll need to put it back together and fill it up, so maybe this will be more than a one and done for dropping the pan.
One other thing, I have a line on a lower mile transmission with a 6 month warranty for $650, so basically if I have to drop the trans, i'll probably just cut my losses and pop in a different unit. I've swapped transmissions before, but time is an issue for me these days. I'm hoping to be able to get some more life out of the transmission already in there.
So it sounds like I need to figure out how to measure band clearance, and possibly adjust via putting in a compensating servo, then put the pan back up, fill it and then figure out how to get line pressure readings. Good approach?
The 2-4 servo is on the passenger side of the transmission. As pointed out by green arrow in below picture.
To check band clearance push that servo cover in and measure how far the travel is.
If you have excessive clearance ( .200" - .250" ) you can pull the servo assembly out and put a weld at the end of the servo pin and grind it down to get your clearance tighter.
I pushed on the servo a few times, I'm not sure how to get a fine measurement from the outside, but I think I'm seeing less than 1/4" of movement. Closer to 3/16". Here is a video what do you think?
I pushed on the servo a few times, I'm not sure how to get a fine measurement from the outside, but I think I'm seeing less than 1/4" of movement. Closer to 3/16". Here is a video what do you think?
That is way to much clearance and looks to be well over .250"
haha loud and clear! I'll need to shift some things around to make time. On getting another transmission to put in, I found that $650 one with 120k miles, but is there a better source n I should be considering?
If the 1-2 shift is still good. Then reducing the band clearance and installing the Corvette servo could buy you a lot of time. Since you already have the pan off, add the TransGo .500" boost valve and springs with spacer here. This will help when towing the boat. How did the separator plate look? Buying a used transmission, is a "crap shoot"...
If the 1-2 shift is still good. Then reducing the band clearance and installing the Corvette servo could buy you a lot of time. Since you already have the pan off, add the TransGo .500" boost valve and springs with spacer here. This will help when towing the boat. How did the separator plate look? Buying a used transmission, is a "crap shoot"...
I'd already put the pan back up. I don't want to put more money into a transmission on it's last legs. I also decided not to get a used transmission. I'll get a reman. Seems like they can be bought for around $1,400.
I wanted to see if I could milk a little more from this transmission until I have some more time to do a swap. I fully accept than with these miles and the wear it has, anything else this unit gives me is borrowed time. I didnt spend money to get the corvette servo, but I did lengthen the pin a bit more than 1/8". I just put a weld on the tip, ground it back into the cone shape and blunted the end. Now the servo only has between 1/16" and 1/8" of clearance (video here).
The shifting feels MUCH better. Firm 1-2, and more snappy 2-3. The downshift to 2 doesn't flare anymore (video here) . Again, I understand its just a bandaid, but it does feel good. Im going to stay easy on it and see if i can go a few more months into winter when work slows down. If it fails before that, well, it was borrowed time anyway.
Thanks all of you for your help and guidance on this!!
I genuinely hope you update the thread in like 18 months and go "hey guys, just checking in...trans still going great!"
I doubt it...but I hope!
I also doubt it haha... i took the boat out yesterday. No flares still, but I think i heard some slight pump whine while accelerating from stop. A new trans is definitely in my near future. I just hope to make it to winter, when I'll have time to work on it again. That would be fantastic.
06-17-2020, 10:50 AM
3-2 downshift flare. Any 4l60e experts on here? 
