Downhill Coasting Problem

[HotRodTractor]

Ok - long story made short - I've owned the truck since new, beat on it, towed heavy with it numerous times, had the transmission built with ATS parts 80K miles or so ago - things were OK. January of 2012 I took the truck off the road, stripped it to a bare frame and did an SAS swap with a 2005 Ford front axle, 2004.5 Dodge AAM rear axle, it has 4.10s and 37" tires. I installed an NP273 tcase at this time as well. I JUST got it up and running and have started driving it around a little bit locally trying to shake the bugs loose and tackle them one at a time. I might have 200 miles on the build at most. So that brings you up to where I am now.

Right now when I coast downhill at a pretty decent incline and I let it downshift to slow me down - it starts to GRIND! It sounds kind of like something is trying to catch, but it just clicks. The conditions have to be just right - and I have figured out how to reproduce the problem - it has done this to me now maybe 8 times. I fear I know the problem, but really want some expert opinion on it as a confirmation. I see no issues with it under a hard acceleration in a 400hpish tow tune or under any other driving conditions except down hill under a coasting condition when it down shifts.

 

[Mike L.]

Maybe transfer case?

 

[HotRodTractor]

Maybe transfer case?

I'll say maybe.... its definitely something I want to check. I had it out again tonight just to put some miles on it and I came up to a stop sign pretty hard and it did it..... It shouldn't be the tcase because the one I installed is a fresh build just to install into this truck - but I can't rule anything out with changing so much stuff.

I'm not sure what the best way to check the tcase is without just pulling it and having it over with..... sounds like fun....

 

[HotRodTractor]

Well just for giggles I test drove it in 4hi - it still does it, although it appears to be much harder to do..... I was manually downshifting it "early" trying to load it up hard to make it slip/grind and it wouldn't do it. Then later when driving it easy it did it once to me.... slipped enough to put the trans into limp mode - but I assume that is because the output speed sensor is on the tail housing of the tcase - so if it slips there - it would cause a speed mismatch and a limp.

This sucks. fml.

 

[HotRodTractor]

Well - I've been busy and haven't had much time to really diagnose this - but I have been forced to drive the truck for a bit while I do some repairs on my fuel sipper. This doesn't occur all that often, and it definitely can cause the trans to go into limp mode if it occurs long enough. The real odd thing is that if I hit the throttle it will re-engage - further pushing my concerns away from the trans and to the transfer case. I did manage to catch it on video tonight on the way home from work - it had just happened to me and popped it into limp mode so it was stuck in 3rd gear - no downshifting. I decided not to clear the code because there was a steep hill after it happened and I knew I had a good shot of catching it on video because of it.

https://www.facebook.com/photo.php?v=523588544362528&l=7147273876335427290

 

[Burn Down]

Wow! I would think that it will show up pretty fast making that kind of noise. Sounds like Toby has some ideas. Good luck.

 

[jraymer]

Sounds like a t case chain to me, sounds terrible.