Is the Allison the only tranny that will bolt up to the 8.1 and hold up to it? I know someone with a messed up tranny and he doesn't want to pay $4000 to rebuild his Allison. Does anyone have a better deal on an Allison if it is the only one that will hold up? TIA

 

The 4L80 shouldnt have any problems bolting up to a Vortec 8100... As far as i remember the Allison was only available as a premium option as opposed to the standard 6 speed manual, a 4 speed auto was also an option which i would assume it to be a 4L80. Then again is from memory when i was thinking about getting a Sierra a few years ago.

Besides the stock 8.1L is not as powerful as some of the trucks running a 4L60 here anyways

 

Is the 8.1 have a different bolt pattern than the other engines? I thought it was the same, which if it was then a 4L80E would work good.

 

How did he manage to grenade the Allison? What is wrong with it? A DIY rebuild can be pretty reasonable on parts as well.....

 

Well, it was a police vehicle and it was given to the county to use. My father is a commisioner and he called me and said the tranny was gone. They are looking for the cheapest way to get it running again and everyone they have talked to is wanting $4000 to rebuild the Allison that is in it. I would think that the 4l80e would be more than enough to get it down some county roads but I wasn't sure if it would bolt up or not.

 

It will bolt up.

 

Cool, thanks for the info!

 

8.1L Vortec Big Blocks in the 2500 Suburban, Avalanche, and Yukon XL are all backed by a 4L85-E. The 85 has external dimensions and bell housing the same as any 1997+ 4L80-E.

 

Cool, good to have a little confirmation. I will let him know that the 4l80e will work. Thanks for all the help guys!

 

Well... not so fast...

The Allison is controlled by its own computer or "TCU". The trucks wiring harness is specific to acommodating that setup. In order to install a 4L80-E you're going to have to choose one of two paths...

1. Reprogram the factory PCM for a manual transmission and install an aftermarket TCU, such as that made by TCI (about $700) and make the connections with the wiring harness that comes with it.

2. Reprogram the factory PCM with a 4L80-E program, possibly from a Suburban, and modify or replace the truck's wiring harness to acommodate the 4L80-E.


There are other options for getting the truck going again:

3. Find a used Allison 1000 out of a wreck.

4. Reprogram factory PCM for a manual transmission and install a manual transmission. If it's 4WD you can do the ZF 6-Speed which will adapt to the transfer case, or if it's 2WD you also have the option of doing a T56 6-speed from an LT-1 or LT-4 if the truck is not used to tow. The parts to install the hydraulic clutch are available.



Quite honestly, the least-headache solution is to just get the trans rebuilt. You can save some money buy removing it yourself (with help of course) because then the trans shop you take it to does not have to dedicate a bay to your truck.

Sorry, probably not what you wanted to hear, but this is how it is.