Riddle me this..

THEFERMANATOR

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Feb 16, 2009
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How do you figure? Why does the transmission care what the rail pressure is?

Just like if your fuel filter is plugged it will hang shifts. The TCM looks at alot of variables, not just RPM, speed, and APPS to shift. If anything with the engine or it's tuning is afoot the TCM will not shift or allow it to shift unless you let out of it.
 

JoshH

Daggum farm truck
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Feb 14, 2007
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Just like if your fuel filter is plugged it will hang shifts. The TCM looks at alot of variables, not just RPM, speed, and APPS to shift. If anything with the engine or it's tuning is afoot the TCM will not shift or allow it to shift unless you let out of it.
I disagree.
 

05smoker

I'm officially done!
Mar 30, 2007
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I disagree.

I agree with him. Had an 03 lb7 that would hang 3rd gear until he lifted a bit. Talked to Brayden and he recommended we start with a fuel filter and that was it. If its commanding pressure and not getting it, weird shit happens!
 

Dan@PPE

Diesel Enthusiast
Aug 8, 2006
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How do you figure? Why does the transmission care what the rail pressure is?

Not sure, why dont you tell me??? Anything useful you would like to add about his issue????:hello:

Just like if your fuel filter is plugged it will hang shifts. The TCM looks at alot of variables, not just RPM, speed, and APPS to shift. If anything with the engine or it's tuning is afoot the TCM will not shift or allow it to shift unless you let out of it.

BINGO!

I disagree.

Disagree all you want. I have seen this issue HUNDREDS of times....;)

I agree with him. Had an 03 lb7 that would hang 3rd gear until he lifted a bit. Talked to Brayden and he recommended we start with a fuel filter and that was it. If its commanding pressure and not getting it, weird shit happens!

:thumb:


Just for shits and giggles im going to change my fuel filter tomorrow :roflmao:

Currently has 7k miles on it.

:thumb::woott:

Not saying 100% this will fix it but its worth a shot....
 

whitetrash21

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Apr 29, 2008
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My 05 used to hang 2-3 and 3-4 at the track bad on a big tune.... Put a lift pump on and it shifted like a dream. It'd do the same thing when the filter was dirty. I could literally tell when the filter needed changed by how the trans shifted.
 

DBUSHLB7

Team DMAX
Mar 9, 2012
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I almost feel unworthy of entering this conversation as some smart guys are already on it but when my trans was stock it hung shifts... After a BD triple disc didn't fix it a changed fuel filter did.....

Brian you're pushing fuel through how many filters? Lift pump filters AND Cat 2 Mic or? By pass OEM filter head worth a shot?
 

hondarider552

Getting faster
May 28, 2008
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Brian you're pushing fuel through how many filters? Lift pump filters AND Cat 2 Mic or? By pass OEM filter head worth a shot?
I have tts dual pumps so no filters there. I run stock fuel filter head with UDP adapter and 2 micron cat shorty filter.
 

JoshH

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I've seen bad fuel filters hang shifts a bunch of times too, but it wasn't because the rail pressure was low. It was because the RPM wasn't reaching the point needed to command the shift. I have never seen a pure low rail pressure issue cause a no shift condition. I've seen plenty of trucks that lose rail pressure but still shift fine.
 

fishsmith

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May 14, 2008
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My 05 used to hang 2-3 and 3-4 at the track bad on a big tune.... Put a lift pump on and it shifted like a dream. It'd do the same thing when the filter was dirty. I could literally tell when the filter needed changed by how the trans shifted.

My 04 was just like this. I could tell exactly when my filter got just a little dirty.
 

THEFERMANATOR

LEGALLY INSANE
Feb 16, 2009
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I've seen bad fuel filters hang shifts a bunch of times too, but it wasn't because the rail pressure was low. It was because the RPM wasn't reaching the point needed to command the shift. I have never seen a pure low rail pressure issue cause a no shift condition. I've seen plenty of trucks that lose rail pressure but still shift fine.

My bURB would hang at 3400-3500 where my fueling drops off when my fuel filters got plugged from a bad tank of wal-mart diesel last year. 2 filters later and it is back shifting at 3250 just like it is supposed to.
 

Mike L.

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I think plugged fuel filter and low rail pressure act differently. On my LB7, my rail pressure would drop to 7K before lift pump and shifted fine. After lift pump install my rail pressure came up to 15K and never changed the shifts but it sure ran a hell of alot faster.
 

JoshH

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My bURB would hang at 3400-3500 where my fueling drops off when my fuel filters got plugged from a bad tank of wal-mart diesel last year. 2 filters later and it is back shifting at 3250 just like it is supposed to.

Well, in all the clogged filters I have seen, I have never seen one that would hang a shift past the RPM and/or MPH it was supposed to happen. Every time I have seen it, the RPM would stay at or slightly below about 3000. So how much rail pressure loss does it take for this hung shift to occur because I'm sure there are plenty of trucks that run much more than 3000 PSI below commanded and still shift fine.