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Old 06-24-2014, 01:23 PM   #1
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1990 Mobile Traveler

Hello my name is Redds and this is my first post. I am looking to buy an RV. It's a 1990 MT. It has a 7.3 engine installed and the guy said it is only getting 7 mpg. He also told me the last owner replaced the gas for the diesel engine. Being a retired diesel mechanic my thoughts went right to the transmission as the cause. If the old trans was used then it's holding the engine back. He said it takes some time for the RV to get to 65 on the highway so I think it needs another trans to take care of this problem. I was in class 8 semi's so I know little of this class of what will work. From what I read John Deere did this unit. I know the 7.3 IH engine are good but it needs help. Any help would be great as to what trans it will need.
Thank you

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Old 06-24-2014, 02:44 PM   #2
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I don't know what a MT is. I'm assuming a Class A. An engine swap would be, what?, $20,000 to $30,000? Doesn't seem likely that someone would do that. If it's a rear engine Class A, then it undoubtedly came with a diesel...and the current tranni.

Many Class A's, depending on length and weight, only get 7.5MPG to 9.5MPG with the average being 8.5MPG. As far as the previous owner only getting 7MPG, maybe he's a lead foot. You're pushing a large box down the road, and if you treat it like it's a hot rod, you'll probably only get 7MPG.

Maybe you should drive it like you drive your trucks for the greatest economy...I'll bet you get better then that 7MPG. 55MPH uses a lot less fuel then 65MPH.

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Old 06-24-2014, 08:39 PM   #3
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Diesel run at lower RPM then gas, might need overdrive. What RPM is it turning at 60 MPH ?
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Old 06-25-2014, 05:30 AM   #4
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Thanks. The RV is a 1990 Mobile Traveler that came with a 454 Chevy engine was by John Deere and the last year for this company. I was just hoping someone knew of a trans that would go on that engine.
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Old 06-25-2014, 01:57 PM   #5
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I'm pretty sure a 1990 (or 1989) John Deere gasser chassis would have originally come with a Ford 460 engine and the Ford 4-speed OD trans. If it originally had a 454 Chev then it is a Chev chassis and the '89 & '90 models would have still had the old TH400 non-OD trans which would definitely lower the MPG.

Peek under the front end of the coach. If it has a solid front axle, then it will be a JD/Ford chassis. If it has independent suspension with A-arms, then it is a Chevy chassis.

I'm thinking if it is a Ford OD trans and the gas engine got switched to diesel then computer controls are probably not all hooked up so the tranny will not know to shift into OD and/or lock up the converter. That would have the diesel engine spinning way too fast at highway speeds.
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Old 06-25-2014, 02:24 PM   #6
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I agree with Nick-B. The John Deere chassis typically had a 460 Ford. I don't recall if the Chevy 454 was an option, but Nick identified the transmission that was used with it in heavier rigs.

You say it has a 7.3L Ford diesel now? If so, it was likely a Ford gas engine originally. Ford used the same tranny with the gas engine and 7.3 diesel, so that part may be ok. Not positive if the trannys had the exact same gearing, though.

What about the rear axle ratio? I think the diesel used a different rear end.recaRear axle ration
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Old 06-26-2014, 05:19 AM   #7
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What a world of thanks on what to look for, I hope to see it Friday or Sat. But the guy told me it had a 7.3L International Harvester Diesel engine in it. He also said it was by John Deere and was gas converted to diesel. As for the trans it was my first thought on the slow power problem, I never thought of the electronic part of it I never was a car nut always Class 8 trucks. I was also trying to research this RV and seam to see parts are very hard to come by. Guess might need to install an Allison Trans. I have been looking for other conversions but most used the Cummins engine with the AT452 trans.

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