LML or L5P swap into '56 GMC rollback wrecker? - Chevy and GMC Duramax Diesel Forum
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post #1 of 8 (permalink) Old 12-18-2019, 12:30 PM Thread Starter
 
Join Date: Dec 2019
Location: Boulder, Colorado
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LML or L5P swap into '56 GMC rollback wrecker?

New to the site but already getting lots of good information.

I have a 1956 GMC F375 "short conventional" (ie, the bulldog nose trucks called low cab forward in Chevrolet) rollback wrecker that I'm hoping to start working on in late spring. It's going to be moderately over the top getting axles from a 2012 Isuzu NPR for better brakes, steering and gearing and I'm looking now towards either an LML or L5P and Allison to swap into it. The original Oldsmobile V8 was replaced by a tall deck 427 sometime in its past and it seems a diesel swap would make this an amazing car hauler.

I've just started looking for donor trucks (mostly at Copart) focusing on 2015 and 2016 trucks, but every now and then a newer truck will pop up that might fit the bill. In addition to the engine and trans I'd like to put much of the drivetrain and body electronics in the truck. I spent my career in industrial control and computers so doing the wiring swap and modifications doesn't intimidate me. I know it's a lot more work than doing something standalone but it's a challenge I think I'm up for. Because the GMC is over 60 years old, the emission requirements here in Colorado will allow me to do EGR and DEF deletes legally. I'll also need someone to reprogram the PCM (don't know about the BCM or if that's even possible).

So my question is would there be any benefit in this case for choosing the LML over the L5P or vice-versa? I've been twisting wrenches on gas engines since the 1960s but these new diesels are unchartered territory for me and your knowledge and opinions will be greatly appreciated.

- Glenn
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post #2 of 8 (permalink) Old 12-27-2019, 08:52 AM Thread Starter
 
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I've done some more digging and it looks like I'm headed the LML route for reasons of cost, configurability and aftermarket support.

1) While reports of the increased power and torque of the L5P over the LML would certainly be desirable, it seems a mild tune would wake the LML up more than enough to make my truck perform more than well enough for what I need it to do. It won't be a working truck but will be used to facilitate my needs in acquiring and moving projects so even the stock numbers would probably be sufficient.

2) Currently, the only PCM solution I can find for the L5P is the HPTuners swap which is both more expensive than having the LML PCM modified, and I haven't been able to learn if some of the changes I would want to make for my older truck would be possible.

3) Since the LP5 trucks are usually more expensive, even in salvage form, the difference in cost could easily cover the parts necessary to do the deletes, add a lift pump and probably change the injector pump as well to eliminate those issues with the LML.

4) There have been many LML swaps and a lot of folks have described and discussed the process in detail so the amount of technical information is pretty amazing. A lot of what I will need to know is already out there and I can learn from that.

So bottom line is that I'll be looking for 2015 or 2016 Sierra or Silverado with (hopefully) under 120K miles over the next 4 or 5 months that's in my budget. While I would like a 2WD, they're rare so I'll need to do the Allison tailshaft swap which I've yet to see described in a step-by-step manner, but I'm sure that information is out there somewhere.

Comments and suggestions are still welcome and I'll be back once I get the project started.

- Glenn
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post #3 of 8 (permalink) Old 12-27-2019, 09:17 AM
 
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I have a 2015 4500 express that was an ambulance I would consider selling if the price is right. I think it would be great donor. It has the heavier suspension, the longer frame, and a more forward looking motor location. PM me if interested.


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post #4 of 8 (permalink) Old 12-27-2019, 09:35 AM Thread Starter
 
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I think the Express has the 6L90E which doesn't have the PTO provisions of the Allison. I need the PTO for the hydraulic pump for the bed. If I'm wrong please let me know.

Thanks!

- Glenn
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post #5 of 8 (permalink) Old 12-27-2019, 09:48 AM
 
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Not sure on the pto. It's a 159" wheelbase,
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Last edited by RickDLance; 12-27-2019 at 09:51 AM.
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post #6 of 8 (permalink) Old 12-27-2019, 09:49 AM
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Sounds like a fun project! Keep us posted. I think EFI Live and a V2 will allow you to tweak many of the parameters on pre-LP5 trucks.

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post #7 of 8 (permalink) Old 12-27-2019, 09:59 AM
 
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You could do an electric pto.


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post #8 of 8 (permalink) Old 12-27-2019, 10:21 AM Thread Starter
 
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Quote:
Originally Posted by RickDLance View Post
You could do an electric pto.
I could, but I already have the pump for the bed and winch and would prefer to use it. It also wouldn't put an additional electrical load on an engine operating at slightly above idle. The Allison has the SAE 6 bolt PTO covers on both sides so moving the pump from the Saginaw is a direct swap. I don't know if I made it clear in my initial post, but I intend on keeping the chassis but doing the drivetrain and axle swaps to it. I know that everything fits now and the solid axle front end is in keeping with a more traditional look.

- Glenn
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