I am looking for advice on the conversion of my gm gas motorhome to diesel. I currently have the gm 454 with the TH400 transmission and I have the opportunity to pick up a nicely built 6.5TD engine. The engine will fit on the current p-30 chassis and will bolt right up to the TH400. I have been looking at the 12 valve cummins but there would be too many customization. I know some people have their own opinions on cummins vs gm diesels and I personally am bias towards the powerstroke ( have a 1996 F250 diesel with 5 speed and a SAS that I love). Any advice both pro and con on the conversion. Thanks
Winnie 26' 4x4 class A motorhome
- Thread starter 864x4rv
- Start date
Cody1771
Explorer
TH400 doesnt need re programming, as they never had a computer. if i remember right the th400's that came off the 6.2 diesels the only difference was the torque converter. the 6.5 is probably your best bet as a cummins like you said would be a big job, the th400 wouldnt last long behind it if you decided to buy an expensive adapter plate.
Curtis in Texas
Adventurer
Several years ago I converted my 90 Fleetwood 460 EFI gas/ C6 to a Cummins 5.9 / with Allison 542 behind it.
It was a tight fit and did have it's challenges. But I went from 6 mpg to 10-11 instantly. I bumped up the Cummins to 350 HP. Too much HP for the Allison so I have to baby the throttle. But it does pull hills no sweat now!
Cost was and easy 10K.
I put it together to run it on bio diesel.
I put a limited slip in the Dana 70 and regeared to 3:56's.
Still not tall enough, so I got a rebuilt .74 overdrive 3 speed brownie box for it but haven't put it in yet. I'm thinking if I can get an overdrive in the .76 - .74 range I could see 14 mpg out of it. Not bad for a 37 foot Ferd RV.
Got a boatload of pictures.
864C4RV, I'll answer your question if you'll answer mine.
I'm curious about your front axle on your 86.
Is it a Dana 70 or 80?
And what transfer case is behind the 400?
Do you know what R&P are in your diffs?
My Class A has a tag axle but I'm still thinking if the front axle pullled I could get out of a lot of places I've been stuck at before. (The reason for the LS.)
Just with the LS and the dual wheels on the drive axles has shown a huge improvement on soft ground. We've been rained in at some RV spots where others were having tow truck drag them out of the mud and been able to just motor right on out. But, if I had a drive axle up front.... WELL!!!!
Would love to swap info with you.
Curtis in Texas
It was a tight fit and did have it's challenges. But I went from 6 mpg to 10-11 instantly. I bumped up the Cummins to 350 HP. Too much HP for the Allison so I have to baby the throttle. But it does pull hills no sweat now!
Cost was and easy 10K.
I put it together to run it on bio diesel.
I put a limited slip in the Dana 70 and regeared to 3:56's.
Still not tall enough, so I got a rebuilt .74 overdrive 3 speed brownie box for it but haven't put it in yet. I'm thinking if I can get an overdrive in the .76 - .74 range I could see 14 mpg out of it. Not bad for a 37 foot Ferd RV.
Got a boatload of pictures.
864C4RV, I'll answer your question if you'll answer mine.
I'm curious about your front axle on your 86.
Is it a Dana 70 or 80?
And what transfer case is behind the 400?
Do you know what R&P are in your diffs?
My Class A has a tag axle but I'm still thinking if the front axle pullled I could get out of a lot of places I've been stuck at before. (The reason for the LS.)
Just with the LS and the dual wheels on the drive axles has shown a huge improvement on soft ground. We've been rained in at some RV spots where others were having tow truck drag them out of the mud and been able to just motor right on out. But, if I had a drive axle up front.... WELL!!!!
Would love to swap info with you.
Curtis in Texas
bob91yj
Resident **************
Valve body and governor work and a new TC for sure on the TH400. With a gas engine it's set for 4500-5000RPM full throttle upshifts.
The biggest problem with the Allison 54X series transmission is the torque converter does not have lock-up, almost no engine braking, and it is not an overdrive transmission.
I used to be an Allison field service tech, had a customer put an exhaust brake on his Allison 54X equipped RV. Boy was he pissed when he realized the exhaust brake was all but worthless in that application.
The biggest problem with the Allison 54X series transmission is the torque converter does not have lock-up, almost no engine braking, and it is not an overdrive transmission.
I used to be an Allison field service tech, had a customer put an exhaust brake on his Allison 54X equipped RV. Boy was he pissed when he realized the exhaust brake was all but worthless in that application.
Curtis in Texas
Adventurer
Bob you sure got that right. That's the main reason I haven't put the Spicer brownie box behind the A 542.
It just wouldn't do what I wanted with the non lock up A542. So I've been thinking of an Allison 2000 series, but that may be overkill for my 16.5K lb. Coach. The A542 came as a package deal with the Cummins.
I have even considered an Isuzu MLD6Q 6 speed manual for it.
Since the Wife can't drive "The Big Girl", a standard shift won't be that big of a problem for me. And with the cable shift Isuzu Aisin tranny, it would have the overdrive ratios I need and a granny low for some serious takeoff pulling power, should I need it.
My Spicer 5831 was rebuilt for me when I bought it and has a 2.0 low; 1.0 second gear and a .73 overdrive. It would make a nice reduction / OD box for a 4 wheel drive Expedition Truck for somebody.
I had a complete take out Aisin MLD6Q found in South Carolina last year, but the guy sold it to somebody else before he got my money I sent to him. His takeout was a low mileage out of an FSR and had everything I needed to bolt it up behind my Cummins.
I'm still looking. An automatic would be nice but a standard shift will last me a lifetime.
And I could easily put a transfer case behind it and a front drive axle to really get myself stuck!
I do not want to take "The Big Girl" rock crawling, I just want to get back into the back woods a little farther than the big RV Campgrounds are.
Which is the main reason I responded to 84X4RV"s thread.
I have a rock crawler for an Expedition Toad and being able to get "The Big Girl" a little further back into Moab would be a big plus!
It just wouldn't do what I wanted with the non lock up A542. So I've been thinking of an Allison 2000 series, but that may be overkill for my 16.5K lb. Coach. The A542 came as a package deal with the Cummins.
I have even considered an Isuzu MLD6Q 6 speed manual for it.
Since the Wife can't drive "The Big Girl", a standard shift won't be that big of a problem for me. And with the cable shift Isuzu Aisin tranny, it would have the overdrive ratios I need and a granny low for some serious takeoff pulling power, should I need it.
My Spicer 5831 was rebuilt for me when I bought it and has a 2.0 low; 1.0 second gear and a .73 overdrive. It would make a nice reduction / OD box for a 4 wheel drive Expedition Truck for somebody.
I had a complete take out Aisin MLD6Q found in South Carolina last year, but the guy sold it to somebody else before he got my money I sent to him. His takeout was a low mileage out of an FSR and had everything I needed to bolt it up behind my Cummins.
I'm still looking. An automatic would be nice but a standard shift will last me a lifetime.
And I could easily put a transfer case behind it and a front drive axle to really get myself stuck!
I do not want to take "The Big Girl" rock crawling, I just want to get back into the back woods a little farther than the big RV Campgrounds are.
Which is the main reason I responded to 84X4RV"s thread.
I have a rock crawler for an Expedition Toad and being able to get "The Big Girl" a little further back into Moab would be a big plus!
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Curtis in Texas
Adventurer
All my research says the new Allisons just need the TCM and a TPS and CPS to work with my 12V Cummmins. But I do not claim to be any kind of an Authority on them.
Having not actually worked with one makes this makes this sounds rather simple.
I'm sure it isn't that easy.
Can't be any harder than fitting the Cummins in the Limited Chassis.
It was a lot like building a 1200lb iron ship inside a bottle.
And I did the conversion all by myself. Absolutely no help!
Here is a shot of the EFI 460. I wore it our at 75K miles of pulling my toys to the mountains. We did go across the Nation 5 times in it over the years.
Here’s the 12 Valve Cummins in “The Big Girl”.
Here’s a shot of the Spicer 5831 I have been talking about.
It’s a heavy mother!
Having not actually worked with one makes this makes this sounds rather simple.
I'm sure it isn't that easy.
Can't be any harder than fitting the Cummins in the Limited Chassis.
It was a lot like building a 1200lb iron ship inside a bottle.
And I did the conversion all by myself. Absolutely no help!
Here is a shot of the EFI 460. I wore it our at 75K miles of pulling my toys to the mountains. We did go across the Nation 5 times in it over the years.
Here’s the 12 Valve Cummins in “The Big Girl”.
Here’s a shot of the Spicer 5831 I have been talking about.
It’s a heavy mother!
Curtis in Texas
Adventurer
I don't want to hijack this thread with my build, but I will give you a link to my Photo Ablumn for you to look at. Since the OP did want some info on a gas to diesel swap I think it would be proper.
http://s31.photobucket.com/albums/c369/Curtis-J/Motorhome Rebuild/
I did a Build Thread on a Cummins Forum about 10 years ago, but I'm not a member there anymore so I can't link you to it. They wouldn't let you see it anyway without a paid membership.
I'll look through my files and see if I can find a copy of the write up.
Maybe I can post it up again in the proper section of this Forum later.
It took me 4 weeks to do and cost $10K with the Hot Rod Cummins build.
We're still running it. Only not so much now that Diesel Fuel cost has gotten nuts. It cost a lot to fill up a 120 gallon fuel tank.:Wow1: