M939 series power loss on inclines and pulling.

[sollobo1]

This question may have been addressed but I have had no luck in my quest to address my truck issues. I have several M925A1 and a 1986 5 ton wrecker all powered by the NHC 250 Cummins with the Allison automatic trans. Now the issue I'm seeing is that I lose power on inclines or when the vehicle is in a pull, it seems that the trans is not down shifting fast enough a I lose the rpms and I slow to a crawl. Any help with these issues would be very much appreciated.

Also I'm looking for a hood and hard top enclosure for a M925A1 if anyone has these parts available or info to locate. I live in Arkansas so the closer the better. Thanks Sean

 

[aheilmann68]

I end up down shifting manually in order to keep the rpms up. The truck drives totally different with weight in it.

 

[M35A2-AZ]

I end up down shifting manually in order to keep the rpms up. The truck drives totally different with weight in it.

I do the same thing.

 

[FormerNewMVGuy]

Anticipate the hills , Truckers know what I mean.....

 

[wreckerman893]

The 250 is a pig and has very little reserve power. Check the fuel filters first. There may have been a lot of crud in the tank if the trucks sat up for a long time. Manually downshifting does help some. If you ease off the throttle just a hair it will actually pull a little better (no idea why but it works). You can take it to a pump shop and have the pump settings optimized and then take it to an Allison shop and have the shift points adjusted.
As for parts CSMDavis in Hattiesberg, MS has parts for the 900 series and is very reasonable.

 

[73m819]

Most of the 939s do this unless the trans shift points are treeked and maybe a bit of rpm treeking, this has to be done on a machine or ???, doing by hand in the back yard will not get you what you want. I drove a 928 with three 1101s in the bed and towing my pu. to FL not to long ago, someone (military) did some treeking on this truck, the trans up shifted dead on at 2000, I have forgotten the down shift rpm but it was dead on the same rpm each down shift. the truck was pulling about 2250 in rpm steady. This was the nicest running 939 that I have driven, ranks right up there with my 819 and the 915 to NE.

 

[porkysplace]

The 250 is a pig and has very little reserve power. Check the fuel filters first. There may have been a lot of crud in the tank if the trucks sat up for a long time. Manually downshifting does help some. If you ease off the throttle just a hair it will actually pull a little better (no idea why but it works). You can take it to a pump shop and have the pump settings optimized and then take it to an Allison shop and have the shift points adjusted.
As for parts CSMDavis in Hattiesberg, MS has parts for the 900 series and is very reasonable.

It looks like CSMDavis is getting out of selling parts so you may want to contact him soon.
[h=3]CSM DAVIS closing the lot.[/h]

 

[jw4x4]

Manually downshifting does help some. If you ease off the throttle just a hair it will actually pull a little better (no idea why but it works).

The reason is that the RPM governor on the 250 starts to de-fuel the engine as it approaches the pre-set rpm (I am talking about an un-altered pump). I think its around 2250 rpm. Easing off as you mentioned just before that limit keeps it from de-fueling itself and does help, just as you have stated.