8.1/Allison swap 1999 Tahoe

[b454rat]

Local junkyard has this combo already out. ECM and harness is there. It was a running driving truck, the t-case took a dump. I'm pretty sure a standard 241 will bolt up, the pattern looks the same. I haven't found much on swapping this combo in a truck, would think Tahoe would be the same. I can get the setup for a decent price I think, just want to know how much of a pain it is, and don't see why it wouldn't be worth it. I know the tranny's are pretty big, but the hoe has a 3" b/l so clearance shouldn't be an issue.

[LockDoc]

Originally Posted by b454rat

Local junkyard has this combo already out. ECM and harness is there. It was a running driving truck, the t-case took a dump. I'm pretty sure a standard 241 will bolt up, the pattern looks the same. I haven't found much on swapping this combo in a truck, would think Tahoe would be the same. I can get the setup for a decent price I think, just want to know how much of a pain it is, and don't see why it wouldn't be worth it. I know the tranny's are pretty big, but the hoe has a 3" b/l so clearance shouldn't be an issue.

I have the same setup out of a motor home but I haven't done anything with it yet. I think it would be a great combo.

This may not answer any of your questions but it is a great thread on the 8.1s

http://67-72chevytrucks.com/vboard/s...d.php?t=374432

If you type "8.1 engine" into the google search box above (minus the quotes) and click on the circle in front of 67-72chevytrucks.com, then click on the "Google" button you will find a lot of info on the subject.....

LockDoc

[b454rat]

Yeah there's a lot of reading!!!!!

[ERASER5]

Our own DirtyLarry has some expertise on 8.1 installs.

[b454rat]

Ive seen his post, but his is in square trucks. I know the motor will bolt in, it's just the ECM and wiring, wondering what needs to be done, and about the stupid check engine light. Also if I'd be better off running the 4L80 over the Allison. Since the Allison uses 29 spline, and the '80 32. Kinda bass-ackwards to me. Not even sure this is ever gonna happen. Trying to get my old job back, and if so then it might become a distant possibility. But need to see how much I could get it for....

[Zoomad75]

Originally Posted by b454rat

Ive seen his post, but his is in square trucks. I know the motor will bolt in, it's just the ECM and wiring, wondering what needs to be done, and about the stupid check engine light. Also if I'd be better off running the 4L80 over the Allison. Since the Allison uses 29 spline, and the '80 32. Kinda bass-ackwards to me. Not even sure this is ever gonna happen. Trying to get my old job back, and if so then it might become a distant possibility. But need to see how much I could get it for....

Look, you kind of skimmed right over the detail of Larry's posts on the 8.1 as he goes into great detail on the PCM control systems in his swaps. It don't matter what the wrapper is you swap into a Square or GMT400 truck as the PCM still needs the same inputs, power and grounds and outputs. Keep this in mind, unlike the LS small blocks, the 8.1 shares the same mounting dimensions as any big block before it. So no adapters needed. As long as you have factory big block mounts it will fit.

Still the check engine light can be wired up for the 8.1 controller to light up. Larry's running a LED off of the ALDL port he has mounted under the hood, but it's just as easy to tap that output from the PCM to the cluster. My suggestion to you would be getting the correct factory wiring manual for your 99 and a custom harness for the 8.1 through Howell or another source and then marry the two together for things like gauge sensor inputs. The main difference on the 99 would be that it's an OBD2 setup already and not as simple as the Squares Larry's built. Because of that your gauge sensor inputs might be an output from the PCM to the cluster and not a direct signal from the senders to the cluster like the older stuff. The wiring manual will answer that question. Speak to howell about if they can build a harness that will integrate the system to provide the info to the cluster for the gauges and the check engine light.

[b454rat]

I've read thru more of his stuff, and pretty much boils down to wiring and ECM. Getting in the truck is easy enough, just the extras is what makes it not so easy.

[b454rat]

Went to the yard, I can get the complete engine, ECM, harness and Allison for $750. Or less tranny for $500. It's an 02, has the 0411 ECM (dunno if big deal or not, just happened to notice it) had about 200k on it, but did run and drive, just junk t-case. I start a new job in a coupe weeks, hoping I can put a deposit on it so it won't sell. Had someone want it but bailed last minute I guess. I don't wanna miss out on it, not very often can find these motors.

[wildwilly4x4]

I would skip out on the Allison for your Tahoe swap. Its not gonna fit without doing a body lift or rebuilding your trans tunnel. Mount up a 4l80e behind that 8.1 and it will bolt right into your Tahoe. Im running a 4l80e behind one in my 90 V3500 and I love it. Pulls my trailer/ jeep combo really well ( @8-10,000#s), and my truck is sitting on 35's.

[b454rat]

I do have a 3"b/l. Gonna put a deposit down on it so it wont go anywhere. I'm gonna get the Allison anyways, if I don't use it, maybe I'll put it behind the Cummins I'll eventually build....or maybe sell the tranny, everyone keeps telling me it's worth a chunk...

[BR3W CITY]

You'll need the TCM for the Allison as well, the LSx ECM's won't have the provisions to run the 5 or 6spd Allisons.

[b454rat]

The ECM is there....

[DirtyLarry]

Personally, I wouldn't use an Allison in a Tahoe. Too big and robs a lot of power. A 4L80E or NV4500 is much better performing behind a L18

[BR3W CITY]

Originally Posted by b454rat

The ECM is there....

? The TCM, not the ECM. The ECM runs your motor, the TCM controls the trans. The ECM you have will not run the Allison. I didn't see any mention of the control for the trans in your posts.

[b454rat]

Talking to people bout LS's the ECM, tuning, turning things off, never said the ECM didn't control the tranny. Looking on Howells site, they list 4l80 and Allison ECMs for the 8.1. Never as anybody said there was another TCM just for the tranny. I'm getting the tranny, and if I don't use it, then maybe sell it or maybe I'll use it if I don't want to run the 4500. 4l80s are a dime a dozen, so getting one won't be an issue...

[BR3W CITY]

Ok I'm eating crow on this. I had to go and sit down with my damn PC open, and behold, there is a setting. Its the diesel ECM's that use the external TCM.

Back to my cave.

[b454rat]

Lol it's all good. But you got me going for a minute. The truck that the combo came out of is still there, but many pieces are missing, and would only guess that if it did have a TCM is long gone....Figured just one more thing I gotta get!!!!

[ls1nova71]

Just in case anybody is looking for a 8.1 swap oil pan...... http://www.gmcopo.com/engineswap.htm But at $800, damn!

[b454rat]

Holy ****, that's more than I paid for the whole deal!! $750 for ECM, harness, complete engine and Allison tranny...

[DirtyLarry]

Originally Posted by BR3W CITY

Ok I'm eating crow on this. I had to go and sit down with my damn PC open, and behold, there is a setting. Its the diesel ECM's that use the external TCM.

Back to my cave.

So, you're telling us an 8.1L backed by an Allison doesn't use an TCM?

BTW, regarding that $800 oil pan. That pan is not needed unless your swapping an 8.1L into car or some other odd ball recipiant.

[b454rat]

Good thing I don't need the pan, be scrappin' the thing. Also decided not to run the tranny. 4l80s are a dime a dozen so getting one won't be a problem if one goes bad or whatever. Also the size of the Allison. Right now my hoe has a body lift, and plan on taking it off after SAS'n it. Besides I think I can make some money on it....

[DirtyLarry]

Wise move, plus you WOULD absolutely need a TCM to run an Allison . Take that body lift off and throw it in the trash. Then when you become a thousandair after selling the Allison go buy a good suspension lift

[BR3W CITY]

Yall want me to be wrong no matter what;

I say they do need a TCM....told I'm wrong 3 times, so I have to go and check.
Here's the screen from HPt


And now that I do, you tell us they do for sure require it...so someone come up with a damn answer and stop making me feel like I need to question everything I see 5 freaking times.

[b454rat]

Have most the parts for an SAS, so that money would be put to good use!! So where is this extra TCM? Anything wiring was pulled with the motor, I can check next time I go there. But since not gonna use it, won't really matter.

[DirtyLarry]

Originally Posted by BR3W CITY

Yall want me to be wrong no matter what;

I say they do need a TCM....told I'm wrong 3 times, so I have to go and check.
Here's the screen from

And now that I do, you tell us they do for sure require it...so someone come up with a damn answer and stop making me feel like I need to question everything I see 5 freaking times.

You were right the first time(s) when you said the Ally DID require a TCM.