MDT Allison 6 Speed - Diesel Forum

SilveradoHauler · 09-26-2009, 09:39 PM

Interested in hearing about the 6 Speed Allison used behind CAT 3126 Engines in medium duty Freightliners and Petes. (Not the 6 speed used by Chev/GMC in the pickups). And in IH's behind the 466 and 530.

Namely torque converter lockup and unlock with heavy loads on hills, heat build up, etc. Does this transmission have a good reputation for long life, or is it heat stress prone like the ER100? My first guess is it is a stronger transmission.

I an looking to buy a Freightliner SportChassis or Peterbilt, most of the ones I have found have the Allison 6 speed. I prefer a manual, but they are hard to find in the SportChassis or Western Hauler package.

Birken Vogt · 09-26-2009, 11:11 PM

Probably a 3000 series from what you describe. They lock the torque converter in 2nd gear and it stays locked the rest of the time so heat buildup is not a big deal and they seem to operate pretty well. They certainly don't have the failure rate of the smaller transmissions, take a look at one and you'll see why (size of the transmission and the cooler hoses are 1" or bigger).

You say 3126 so I assume used chassis. They also come as a 5 speed but it is a matter of reprogramming. However the 3126 and C7 are not so well loved in their ability to stay together and not break down all the time.

Birken

SilveradoHauler · 09-27-2009, 03:20 PM

Yea, Birken, I am not all that thrilled with the 3126, but regretfully it has been the engine of choice for the SportChassis builders. Mercedes are starting to show up in the SportChassis Haulers, so I guess Freightlliner has had second thoughts on the 3126. And some haulers I have seen use a 8.3 Cummings, I don't know anything about this engine. The 3116 in our TopKick is getting tired after 297,000 miles. Don't think it is worth a rebuild, isn't this a sleeveless engine?

I would love to have something with a 3406 and a 10 speed, but we are getting into class 7 trucks over 26,000. I would like to stay below 26,000.

Anyone make software for 3126's, similar to AutoEnginuity's for the Powerstrokes? Or a dedicated palm pilot sized code reader? I recall seeing a palm pilot type code reader/diagnostic unit used by a Class 8 mechanic on Freightliners. Sure would be nice to check out a used truck before buying, or having to drive 100 miles or more to a Freightliner Dealer and drop a few hundreds for a code read.

The big Allision might be OK, especially with the TC locking and the two top gears being overdrive. 75 MPH at 1950 RPM would be nice on the flats.

Birken Vogt · 09-28-2009, 01:39 AM

The 3126 is a higher RPM range engine, and I don't think it wants to be lugged at 1950. But I could be wrong on that. People seem to like the 8.3 Cummins, I know I do. I think it gives the 466/530/570 good competition in the longevity and power department, it just runs. I know the injection pumps are sometimes problematic on the ISC and also some turbo failures but no major meltdowns that I know of personally, 3126/C7 is a different story.

Birken