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Old 09-16-2008, 09:26 PM   #1
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Airstream Isuzu Diesel THM475 + Overdrive

First thanks for all the comments on the THM475 vs 700R4 transmissions. The reason for the question was the bell housing is cracked on the tranny and I 'm having it pulled and fixed. I found a local transmission shop that will put my parts in a used case and rebuild the tranny as they do it. I asked about and overdrive unit. They said no problem and could add it as they rebuild the tranny.

What is going to be involed in putting the tranny back in????

The truck service that is pulling the tranny rebuilds and shortens drive shafts every day, so that in not a problem. What is involved with the mounts???

At 2800 RPM I'm doing 55. What will I get with the overdrive unit? Is it worth the expense?? Or should I just drive in the slow lane. Those of you that have them how do you like them? what else should I know???

Thanks

Gary Greenwood
Leslie, MI
1982 310 Isuzu Diesel
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Old 09-16-2008, 09:30 PM   #2
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Gary, the overdrive units typically reduce rpm by about 20%. So, you'll probably be running around 2200 rpm at 55 with an overdrive unit.
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Old 09-19-2008, 07:39 AM   #3
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Gary, we have the same engine+tranny and had the same problem with cracking. After two "fixes" that didn't work, he problem turned out to be the alignment of the adapter ring between the two. Be sure the rebuilders observe the changes AS made when they converted the chassis from gas to diesel - different torque springs, etc. inside the tranny. That is a high-rpm low-torque engine, designed and built that way. No problem with adding the overdrive, but evaluate the cost vs the benefit ... we decided not to do it. Jamie
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Old 09-19-2008, 08:26 AM   #4
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We have a U.S. Gear overdrive and are very happy with it, the instructions for the
the driveshaft modifications are very complete. Since the trans was re-built by someone
who knew what Allison parts could be used, it's smooth as silk. With the overdrive, we
get 65 mph @ 3K.
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Old 09-19-2008, 08:30 AM   #5
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Not exactly sure what transmission you have in your units. My 1985 has an Allison 4-spd and at 2800 rpm I am doing over 60 mph. At 3250 I am doing over 70 mph. If you don not have an Allison you might consider it. They are a very popular transmission and should install right onto an Isuzu.
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Old 09-19-2008, 08:34 AM   #6
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I did some research on installing a Allison in ours, the overdrive was WAYYYY cheaper.
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Old 09-19-2008, 11:17 PM   #7
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I've been off line for a while so I'm late to this thread. Have a 700R in my 87 345 w/fresh 454, performer intake, stock quadrajet and coated headers. To make it interesting, I installed a Gear Vendors OD on the back of the 700. The sweet spot is 700 in direct, GV in OD, torque converter unlocked. There are times is good to lock the converter and there are a few times when the 700 30% OD is useful, but not often. Double OD is never useful. The 4L80 is a great piece but the torque converter weighs 80#s and you do not want to get one with the split stator. Watch those wrecking yard units. My shop uses the TCI stand alone control and it works acceptably.
I have successfully towed over 100k miles using a 700 w/454 and later a stroked truck block 520 c.i. monster that really moved the 25,000 #s I normally hauled. The vehicle was a high cube Chevy van, streched wheel base and we wore out a couple Pace trailers. Now before the flames start, I never tried to be first up the mountain, rarely got into the secondaries of the carb. Never on the side of the road due to trans problems. 700s have always worked for me, very well in fact.
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Old 09-20-2008, 09:06 AM   #8
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No flames from me, I'm always the last up the hill.
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Old 09-21-2008, 07:49 AM   #9
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Well I'm still here but not often.

Airstream One is now out of business. That's to say I'm not using it to work from anymore.

I'm not certain of the future but know now is not the right time to sell the beast.

I did finally get the heart transplant donor brought down from MN. The donor is an 86 'Burb with a fresh engine / tranny with a B&M kit and a GearVendor. Bought for the price of the GV.

The bad news is that I don't yet have a local mechanic who can do the swap. It's way above my pay grade!

For now AS One is in storage waiting for the right surgeon to appear.
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Old 09-21-2008, 08:11 AM   #10
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Quote:
Originally Posted by M Hohnstein View Post
I've been off line for a while so I'm late to this thread. Have a 700R in my 87 345 w/fresh 454, performer intake, stock quadrajet and coated headers. To make it interesting, I installed a Gear Vendors OD on the back of the 700. The sweet spot is 700 in direct, GV in OD, torque converter unlocked. There are times is good to lock the converter and there are a few times when the 700 30% OD is useful, but not often. Double OD is never useful. The 4L80 is a great piece but the torque converter weighs 80#s and you do not want to get one with the split stator. Watch those wrecking yard units. My shop uses the TCI stand alone control and it works acceptably.
I have successfully towed over 100k miles using a 700 w/454 and later a stroked truck block 520 c.i. monster that really moved the 25,000 #s I normally hauled. The vehicle was a high cube Chevy van, streched wheel base and we wore out a couple Pace trailers. Now before the flames start, I never tried to be first up the mountain, rarely got into the secondaries of the carb. Never on the side of the road due to trans problems. 700s have always worked for me, very well in fact.
MH
Did you do any modification to the 700? We had pretty good success with modifying the torque converter (slightly wider clutch, better than stock stator). If I had a dollar for every one that has come into my shop with a bunch of broken parts in the pan, I could treat my family to a steak dinner...
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Old 07-15-2009, 06:08 PM   #11
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Getting the 81 Excella 280 TD next Friday

If all goes right... Anyhow the current owner's good buddy and former traveling companion is quoting standard running speed for the coach at 50 Mph (many questions on that next week). So I'm already thinking of putting a Gear Vendors OD on it. Maybe get it up to 60 or even 65 and do a bit better on the MPGs. From what I'm hearing this is not too radical (other then the cost). I know the seller has updated the torque converter to an Allison unit and the engine is a new replacement Isuzu (with an Isuzu factory turbo), so I think the drive train is solid. Any ideas and recommendations like reputable installers?

Thanks,
Ken
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Old 07-16-2009, 06:01 AM   #12
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Ken, the folks at GearVendors will be able to steer you in the right direction. They are very helpful.

My project still lingers on as they are trying to get the vibration out of the new shortened driveshaft.

If you are limited in top speed with that new setup it is going to likely be the rear end ratio. Changing the pinion and diff. gears is only for the trained technician with all the right tools and experience.

Worthy of yet another question/thread.
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Old 07-16-2009, 11:31 AM   #13
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Thanks Glen.

After checking out the GV site, I guess actually knowing what tranny is in this 81 Excella is my first requirement. I was assuming a 4 spd but after consulting Wikipedia (using Gary's model numbers) I see it is a 3 spd. Wow!
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Old 09-16-2009, 03:02 PM   #14
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I am finally getting it done. Our 345 diesel doesn't like this new low sulfur diesel very much. It has less power, it runs warmer and the mileage went down. Okay, in order to run 70mph, I have to turn over 3000 rpms. That could be a major reason for it running warmer, along with having to burn more fuel to sustain that speed.

So, I am finally getting that problem fixed. First of all I looked ate GV and saw the parts alone were over $3000.00 and they didn't have a model specifically for the 345 diesel. Bummer.

Back to my old solution. Yesterday we removed the rear drive axle. Yep, the complete banjo, and took it to a rear end shop to have the gear ratio changed. I should be able to get the complete job done for around thirteen hundred dollars including labor.

Their calculations say that it should lower the rpms at 70 mpg to around 2500 to 2600. That is huge! The power range is still lower than that, but I don't want to lose the power in the lower gears, so its a trade off. However, I think it will be a big help.
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Old 09-16-2009, 04:38 PM   #15
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Quote:
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Yesterday we removed the rear drive axle. Yep, the complete banjo, and took it to a rear end shop to have the gear ratio changed.
As long as you cruise the flats, you're fine. Don't try to climb west of the Rockies, though. Been there with our first diesel mo/ho. Hard to believe U.S. Gear does not make a o.d. for your coach.
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Old 09-16-2009, 05:36 PM   #16
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Use the link below and you can calculate exactly what your RPM:Speed is or will be
* * Automotive Programs by Bowling & Grippo * *

As far as the mount that is a very simple fix, the overdrive will usually add no more then a couple inches and new mounts are easily fabricated

Good Luck
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Old 09-17-2009, 08:17 AM   #17
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Quote:
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As long as you cruise the flats, you're fine. Don't try to climb west of the Rockies, though. Been there with our first diesel mo/ho. Hard to believe U.S. Gear does not make a o.d. for your coach.
Not worried about it. The 86 model has a much more powerful engine than the 83.
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Old 09-17-2009, 08:53 AM   #18
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Not worried about it. The 86 model has a much more powerful engine than the 83.
Lucky you!
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Old 09-19-2009, 09:29 AM   #19
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The numbers on our chasis verified that the motorhome has the factory turbo and not the after market. Airstream did not start putting those engines in until late 84. This engine is superior to all previous years. It has Cylinder oiling, a heavier crankshaft, stronger connecting rods, and stronger pistons.
In addition the cylinder head has better engineered valves.

You can get this information directly from Isuzu's Industrial Division web site.

The engine also has about 30 more horsepower than earlier models.
Since the transmission and drive train swallow a lot of the horsepower produced by the engine, this is an additional 30 hp with no additional resistance. It applies directly to the wheels.

Now while I realize that your last post was just short of sarcasm, I am providing this information to explain my lack of concern. Also, I have climbed quite a few mountains already and had no problem at all. The only time I ever had the unit shift down to low was on a 12 percent grade towing a car on a very hot day and I had to take it easy not to overheat.

OK?
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Old 09-19-2009, 10:38 AM   #20
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I meant "lucky you" seriously! I was not aware Izuzu made a bigger engine. Is that a Allison trans?
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