Allison 5sp: Pops out of 5th

[lil bleur]

Allison 5spd Pops out of 5th

Hello every one, I am having a little problem with a stock 5spd Allison. While under WOT at anywhere between 90-120 MPH it pops out of 5th then almost instantaneously drops into 4th, if I shift it into N then back into D it immediately is back to normal. Someone thought maybe manifold pressure switch but I changed that and it still does it. Anybody have any idea what PIDs and stuff to watch when it does this? The truck does have a 90Hp "stock trans" tune, but the trans holds solid until bang it lets go at 90+. One of these days I need to monitor it when it happens, but I almost hate to take it up to 90+ MPH were it does it. If only I had a dyno...LOL

 

[skintback]

From what i have seen and been around that's the c3's not liking life. But the weird part to me is you say when it does it you can shift it into neutral and back into drive and it goes back into 5th

 

[Mike L.]

90 HP tune is still not safe for a stock trans. I would be watching turbine and output speed sensors. Could be play in the transfer case causing incorrect speed sensor output.

 

[lil bleur]

Ok...its 2wd so no transfer case, I thought that if the C3 would be slipping it would put it in limp mode till it you manually clear the code? Yes every time so far I drop it into N then back into drive and it clears up and drives like normal. It has 4.10 Rears so its been in 5th for a couple sec b4 it drops it

90 HP tune is still not safe for a stock trans. I would be watching turbine and output speed sensors. Could be play in the transfer case causing incorrect speed sensor output.

 

[Mike L.]

Was the 4.10 reset in the computer?

 

[lil bleur]

Was the 4.10 reset in the computer?

Um no but I was trying to dial in my speedo to no avail. It's always about 4 MPH off. When I was doing that I did not change my rear ratio. I was changing pulses per km I think.

 

[Henry95]

Mine does the exact same thing.


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[lil bleur]

I can't help you out much at this point, but I'm working at figuring it out. I'll let you know what I find out. I think I'm just do a trans build, but I want to figure out why its doing what it is b4 I go in hook, line, and sinker.

Mine does the exact same thing.


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[THEFERMANATOR]

Don't need to change the tcm for gear ratio changes. I've been running 3.42's in mine sonce day one of my swap with no problems. If you mess with the speedo settings in the tcm, it can cause this issue as I've helped others fix similiar issues because they changed the tcm gear ratio or tire size settings. Leave the tcm alone, speedo settings are in the ecm.

 

[lil bleur]

Thanks for the info, the thing is I reflashed the TCM back to stock and it still does it...well, let me reflash it again just to make sure!

Don't need to change the tcm for gear ratio changes. I've been running 3.42's in mine sonce day one of my swap with no problems. If you mess with the speedo settings in the tcm, it can cause this issue as I've helped others fix similiar issues because they changed the tcm gear ratio or tire size settings. Leave the tcm alone, speedo settings are in the ecm.

 

[Henry95]

Only does it when I don't get it to drop a gear to pass


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