I've been in search mode for awhile and didn't see any other questions on this so I thought I'd ask.

I've got an 03 Dura max allison with no mods, 100,500 miles on it. When coming off of the icy side road, onto the highway, my tranny/front end won't drop out of 4hi to 2wd after I push the button. The light for 2wd comes on but the shift doesn't actually happen. Sometimes stopping to put it in park will make it shift, sometimes lower speeds (on the way to the above stop )causes the shift, and sometimes, I have to turn the truck off. Obviousy, this isn't how shift on the fly is supposed to work.

Temps are, of course sub freezing, and I'm thinking it may be ice on the shift linkage?

Tires all the same size and have the same wear, 2 very important points, new tires in the rear and bald tires in the front will cause this for example, a 1/8 inch in dia difference is enough to cause a issue. also make sure you take the load off the driveline when performing a shift. You could also run synth. fluid in the front diff, there was a bulletin on that as well it will help for cold weather...

Tires may be an issue. I'm getting ready to have the studs put back on for the season, so we'll be able to prove that theory. I always unload the drive train, so I don't think that is it. I do want to shift to synthetic fluids, but I'm not sure when I'll get to that. I'll be changing the tranny filter this weekend. I should look at switching fluids as well. Do I need to do more than drain the tranny via the plug in order to go to a synthetic? Which synthetic?
Thanks for the reply!

Your actuator could be going bad too. they are a thermo actuator so they use heat to engauge and disengauge if im correct. so in theroy with temps as low as they are it could cause this. will it go into 4hi from 2hi no prob?

The acctuators are electric, they have been since 98 or so..., no more thermo accts, they were and are a p.i.a.....

The acctuators are electric, they have been since 98 or so..., no more thermo accts, they were and are a p.i.a.....

You sure cause i remeber reading that on here somewhere about a prob with the 98 and up acctuators?

I agree that the thermo actuators are a PIA, and that they are gone on the current Silverado's.

Would hate to be wrong about that !!

jb

Switching to Synthetics is pretty straight forward for the axles (the front axle will require a different vent cover) and transfer case (I use the appropriate wieght of Mobil 1). But I'd highly recommend going to the local Allison dealer (assuming they have flush capability)and getting it flushed with Transynd (my guys insisted on cleaning out the pan and internal filter, too). My tranny, previously slipping a bit, holds more power now and the change interval is up to 50,000 miles...