Allison 6sp: 6 speed C2 piston

[THEFERMANATOR]

Could somebody out there post up a pic of a C2 apply piston out of a 6 speed? I'm brainstorming on some ideas to try and correct my C2 bleed off problems, and wanted to see a 6 speed piston to see how ALLISON did theres(and the pics in there manuals have very little detail of it).

 

[Cougar281]

It's been almost 2 years since I had mine apart, and I don't think I have any pics of the 6-speed assembly, but the entire C1/C2 assembly is totally different in the 6 speed. I don't think anything Allison did in the 6-speed would in any way relate, work or help with a 5-speed. The whole thing was re-designed, it comes apart differently, and there's no balance piston.

I don't think there's much you could do other than a transgo. You sig only mentions clutches. Did you do anything else while you where in there? If you didn't already install a transgo kit, you might start there.

 

[THEFERMANATOR]

I've come to the conclusion that my problem started with the TRANSGO kit. I'm not saying that it is the problem, but it has caused issues in mine. My C2 piston doesn't get enough backfill oil charge to keep it filled now and it is causing a harsh delayed engagement after periods of not being engaged(say 30 seconds in neutral or 2 minutes in gear). So far I've called Mike several times and had him thinking on it for awhile, called George at Suncoast who is also trying to help me figure it out, and the local Allison shop was baffled when I ran it by him. I was trying to see if I could compare how ALLISON redesigned teh C2 and how they put the bleed holes in it VS how the TRANSGO kit does it. I'm pondering making some radiocal one off changes to mine to overcome this issue, but trying to see how the OEM did it first to let that play into it.(mostly considering relocating the bleed holes more so towards teh center to reduce centrifigal release of the C2 pre-charge.

 

[Mike L.]

Been tried twice. Suncoast and DTT. Both failed pretty bad. I still have the DTT custom C2 piston and mods and Suncoast still has their C2 piston and mods.

 

[JoshH]

Why not get a 6 speed rotating assembly?

 

[Sledhead]

opcorn:

 

[Mike L.]

Why not get a 6 speed rotating assembly?

Very good idea.:thumb:

 

[THEFERMANATOR]

Why not get a 6 speed rotating assembly?

I've been looking close by to me for the last year for one, and have yet to find a 6 speed for under $1500. If I could find one that is what I would run as that is what I wanted to do to it. I'm pretty sure I have figured out what the problem is with my shifts, it is the why that is puzzling me.

 

[Mike L.]

I'm pretty sure Fingers enlarged the retricter holes a little bigger in his truck.

 

[MaxPF]

I've been looking close by to me for the last year for one, and have yet to find a 6 speed for under $1500. If I could find one that is what I would run as that is what I wanted to do to it. I'm pretty sure I have figured out what the problem is with my shifts, it is the why that is puzzling me.

Too bad you don't live in AZ. Most yards out here try to hit customers with a "core charge" to get the old trans back. At least one yard has several core 6 speeds that would go for ~$400 or so. Do the yards in your area do the same thing maybe?

 

[THEFERMANATOR]

Too bad you don't live in AZ. Most yards out here try to hit customers with a "core charge" to get the old trans back. At least one yard has several core 6 speeds that would go for ~$400 or so. Do the yards in your area do the same thing maybe?

Most of em charge $450 around here, and then they promptly sell em off for scrap for about $50 each. Unfortunately ALLISON stuff around here is few and far between. I've been studying the ALLISON books and I can say one thing, these trannies are pretty bass ackwards in how they work from a common sense stand point. ESPESCIALLY the 3-4 shift with the A trim releasing the C3 and tehB trim bringing in the C2 and then the E shift valve doing a hand off of teh C2 B trim to the C1 and then the C2 to main. No wonder it's such a touchy shift.

 

[Fingers]

My TransGo was locking up bad on the 3-4 shift. (c2 apply, Bang!)

The problem appears to be that the transmission computer does not modulate the C2 apply. It just turns it on assuming the balance piston is there. The result of blocking and venting like the TransGo does is the transmission is in two gears at once.

My solution was to drill a tiny (#60 drill IIRC) hole through the Transgo plug in the piston and then plug the vent hole they had you drill and drill a new one through the plug, but smaller. I don't recall the size, but ~ 1/2 the original hole. This hole is also about twice the #60 hole.

What this does is allow the balance side to flood with fluid when the piston is not applied. (there is always like 30PSI in all pistons when not applied) During the apply, the fluid exits out the vent hole, with restriction, which provides some dampening during the cycle. After the piston has gone full stroke, the vent hole allows 100% clamping since the vent is larger than the bypass hole.

The result has been very good. No harsh lock ups. The shifts are not perfect though. I think the vent hole needs to be just a pinch smaller, but I am very happy with the way it is.

 

[THEFERMANATOR]

30 PSI will apply the clutches I know that much as about 20 PSI of air pressure will apply them, and if you get close to 80 or so to it it is ridiculoius how much the stock C2 set-up flexes(no wonder they limp so easy with the stock plates). I'm still waiting for another book from ALLISON for mine to go back together, but I'm going to try something completely different. The TRANSGO orifice plugs as they come out of teh pack have about a .012 to .013 diamter to them as I couldn't get a .015 diamter torch tip cleaner to go through them. So I tapped the transfer ports with a 6-32 tap and installed some allen head set screws where the orifice plugs would normally go. Then I went and opened the factory 01-02 bleed hole back up which is .040.

Bench testing with air shows it to engage slower than a stock one does, but I realize air is different than fluid when it comes to apply rates. I'm pretty sure that my problem was air holding inside the C2 piston area as the orifice plugs are about half out in teh piston fluid area. So once air gets in there, it can only backfill to about half capacity as the orifice plug location will only allow this much. By plugging the 2 orifices and opening up a bleed hole closer to the center this should allow the C2 piston to completely fill up like it did in stock configuration but still allow excess fluid to bleed off and give some extra cushion. I know this is the way that the origanal patent from TRANSGO describes how to modify the C2 piston area(with the exception of the .040 bleed hole in the center as 03+ did not have this). I'm hopeful that this will work as it appears as though my balance piston has been riding close to the bottom of the C2 piston jusdging from the lines on it, and once the C2 has been applied initially to purge it it would shift very soft afterwards. And I know my problem was not that bad before when I had one of my orifice plugs blocked and only one fuctioning. After making both work it got MUCH worse.

And the C2 piston is pulsed on by the B trim valve, and after the 3-4 shift is completed the E shift valve is engaged and transfers the C2 to main and the C1 over to the B trim valve. And I was getting the bang as well(but it had a delay to it as I could feel the defuel come in and go away before teh shift would bang in), but only on the initial 3-4. If I slowed down and went back to 3rd and immediately did another 3-4 shift it was PERFECT. This is teh main thing that leads me to think my problem is a C2 bleed off problem. This is why I wanted to see how ALLISON dealt with the C2 orifices.

 

[MaxPF]

OK, this question may SOUND naive, but I will ask it anyway: What was the original "problem" that the transgo kit was supposed to "fix"? It sounds like it may have created more problems than it solved...

 

[rcr1978]

Ferm did you put a wave plate in this last go around on the C2?

 

[Fingers]

OK, so it's not 30 PSI, but there is always static pressure. I just don't remember how much.

Though the computer CAN modulate the apply via the trim valve, it does not appear to. Software thing I guess.

 

[ripmf666]

I've been looking close by to me for the last year for one, and have yet to find a 6 speed for under $1500. If I could find one that is what I would run as that is what I wanted to do to it. I'm pretty sure I have figured out what the problem is with my shifts, it is the why that is puzzling me.

This is still in his sig for sale and the ad is not closed was pending sale but I dont think it sold since its still in his sig.
http://www.duramaxdiesels.com/forum/showthread.php?t=18871

 

[THEFERMANATOR]

OK, this question may SOUND naive, but I will ask it anyway: What was the original "problem" that the transgo kit was supposed to "fix"? It sounds like it may have created more problems than it solved...

Basically the TRANSGO kit eliminates the C2 balance piston from the C2's operation. The C2 piston has 2 different apply sides in stock configuration. The bottom portion is the main surface area apply side and then the balance piston which is an opposing apply. By eliminating the balance piston from the apply it has taken away the opposing apply which increases the effective apply area of the C2 piston. From what I can tell 06+ ALLISON made the C2 piston so that the portion of the piston that contacts the clutches was larger in diameter to give a more even apply to the clutches and eliminated the balance piston all together(they did though add in some tangs onto the piston as to keep it centered in the rotating assembly which is what the balance piston was for). Also TRANSGO adds in stiffer return springs to the C2 piston to help slow the apply of teh C2 since the mods changes the apply rate without changing teh TCM's shift strategy(which has been found to not really be possible with the software that most of us have available to us).

Ferm did you put a wave plate in this last go around on the C2?

Yes, and it didn't help the bang into gear on the initial shift after the wave plate flattened down from some applications. It did make the shifts VERY soft after the initial apply though, but I never had problems with that one. My 3-4 shift was very consistent and predicteable in what it would do, and always had a slight delay in it on the initail apply after being in park or neutral that was causing a VIOLENT bang into gear. It was always the worst at light to no throttle as WOT was always pretty good. Iit had gotten to where if I was doing a shift I knew would bang, I would floor it before the shift so as to not get the slam into gear(not reccomended in a congested town though). This last go round with both orifice plugs working though, it would bleed off if I went say 2 minutes with engaging the C2's.

 

[THEFERMANATOR]

OK, so it's not 30 PSI, but there is always static pressure. I just don't remember how much.

Though the computer CAN modulate the apply via the trim valve, it does not appear to. Software thing I guess.

The backfill circuit is supposed to hold less than 10 PSI from what I have gathered, but 2-4 is what it is supposed to be. Also the C2 gets some prefill VIA the C1 in stock form, and the C1 is backfilled Via cross leakage from teh C2 for a 5-4 downshift. Heres the ALLISON literature listing how the 3-4 shift takes place.

 

[MaxPF]

Basically the TRANSGO kit eliminates the C2 balance piston from the C2's operation. The C2 piston has 2 different apply sides in stock configuration. The bottom portion is the main surface area apply side and then the balance piston which is an opposing apply. By eliminating the balance piston from the apply it has taken away the opposing apply which increases the effective apply area of the C2 piston. From what I can tell 06+ ALLISON made the C2 piston so that the portion of the piston that contacts the clutches was larger in diameter to give a more even apply to the clutches and eliminated the balance piston all together(they did though add in some tangs onto the piston as to keep it centered in the rotating assembly which is what the balance piston was for). Also TRANSGO adds in stiffer return springs to the C2 piston to help slow the apply of teh C2 since the mods changes the apply rate without changing teh TCM's shift strategy(which has been found to not really be possible with the software that most of us have available to us).

Hmm, interesting.

So, you said you could get a good 6 speed parts trans for $450? Seems like that's not a bad deal to potentially fix your problem, as well as giving an input assembly that fixes many of the drawbacks of the 5 speed assembly. I'm sure it's a lot cheaper than the parts would be new...