Allison 5sp: Trans acting weird??

[snowman]

Got a 2002 GMC with 238,000kms on it. All stock except for Hypermax tuner, 4 inch exhaust, and drop in air filter.
Keep getting codes: P0847, P0872, P0875

Been reading up on these codes from on here, and done some of the things you guys recommended. Trans level was low, but topped up.
Took for a test ride and this is what it does.
From a start the shift indicator doesn't seem to know whats going on. I put it in any gear and it tells me that i'm in third. It bangs really hard when i select a gear, reverse will just disengage, and then engage. As soon as i clear the codes they come back, sometimes it puts it into limp mode, and other times it doesn't. Then yesturday it worked fine until i stopped the truck and shifted into reverse. Then it started all over again!!!

So i have dropped the pan, fluid was dark red, but smelled ok. Nothing in the pan, magnet had nothing on it. Changed both filters and put in new fluid and still the same!! Checked harness in the pan, OK. Tore apart valve body, and all looked good!! PLEASE HELP!!!

 

[Darin13]

Time for some clutches and trans go mine did that a few years ago 3rd gear is your get home and fix it failsafe

 

[DAVe3283]

This may sound too simple, but my transmission was acting similar (slamming into gear, reverse not working, and intermittent shift indicator), and it was simply the NSBU switch. They are cheap at NAPA or a local parts house. I think I paid $35 for a new one. Took me all of 10 minutes to swap it out, as it is on the outside of the transmission (on the early ones).

 

[Mike L.]

Stuck E shift valve. Pull the pan and cut the filter open. Bet you find some crap in there.

 

[snowman]

Pulled pan, cleaned valve body, looked normal.

 

[snowman]

Changes nsbu switch about 4 months ago.

 

[snowman]

i'll cut the filter open tommorow and give you an update.

if it was the nsbu switch, then it would be different codes?? Right?

 

[Mike L.]

Pulled pan, cleaned valve body, looked normal.

Explain " clean vb "

 

[snowman]

I thought in one of the other posts it said to clean it? I pulled it apart and checked everything over. There was junk in trim valves and some of the other ones. I washed it all out and then lubed it up with fresh ATF.

 

[snowman]

It had some fine silver powder in a lot of the passages. Is that major damage?

 

[snowman]

so i cut the filters open and the spin on looks fine, just a little discolored. The one in the pan had some small filings, and some black junk on it. I don't think the one in the pan has ever been changed.

But just for the hell of it, i decided to swap the NSBU switch off my buddies truck. Low and behold, the truck worked flawless!!!