pulling a38ft 5th wheel from alcoa tenn to dayton on my way back to michigan going thru the mountains i kept getting a roaring noise like my tanny had down shifted no change in rpms and it would stop on is own most times it would do it in and out of cruise and od up and down mtns. also temp ran 185-190 saw no problems, no warnings , a couple times breaking down hill it would down shift to low once it red lined 3100 rpms just for a second the outside temp wes 85-91 everything is stock .it did it once last oct. same area heading south. never a problem m/t any ideas?:help:

Fan clutch?

fan clutch

Sounds like a broken record on this one, but Fan Clutch. When the fan clutch engages, it will make a very notable roaring sound exactly as you have described.


John

Whew! I had the SAME thing happening to me when towing (especially when going up hills and in the down shifted 2800 RPM range). Was almost like what I figured limping the tranny would do, or a downshift, EXCEPT there wasn't any change in the RPM's.

THANKS!!! This really makes much more sense and I feel a LOT better knowing what it is. So, I take it the fan isn't running all the time (or is is more of a hard/soft clutch than engaged or not at all)?

-JM

thanks guys except fore the odd down shift when brakeing i did"nt think it was bad will keep an eye on it

thanks guys except fore the odd down shift when brakeing i did"nt think it was bad will keep an eye on it

The down shifting is normal part of the allison transmision's automatic engine grade braking (don't think you can disable that, and it will only down shift out of OD unless you are in tow/haul).

See <a href="http://www.allisontransmission.com/service/faq/index.jsp?CategoryID=11#87">here</a>

-JM

http://www.dieselplace.com/forum/images/smilies/agreed.gif :lol:

it's all explained in the manual... you know that big thick book thingy that is in yer glove compartment with the words READ THIS FIRST on it? Just kidding, LOL!

it's all explained in the manual... you know that big thick book thingy that is in yer glove compartment with the words READ THIS FIRST on it? Just kidding, LOL!

Where? I've read through most of it, and never found the part about the increased fan noise. Of course, I spent most of the time reading the Duramax and Allison addendums rather than the pretty useless and over worded main manual. How can they put SO many words into a book without actually TELLING you anything?

-JM