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Old 11-25-2017, 10:20 AM   #21
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Thanks Dan. I was told by GV's rep that the inline 6 Isuzu TD problem with marrying it up with the GV's OD is the Power pulse of the engine. The rep told me it is very rough on the GV OD. All of this terminology is new to me. All my injectors and pump were completely rebuilt and dialed up a bit, so the Turbo is outputting more than 10 lbs of boost. I didn't know about the frequency of the oil change in the OD unit, and that I should not use the OD on downhills. I will abide by GV's suggestions of its use or go broke.
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Old 11-25-2017, 11:37 AM   #22
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Ive read close to 14lb boost on my '82 TD. Could be a bad gauge I suppose but I'll see well over 10lbs on uphill grades consistently. Or the PO had adjustments made that I'm not aware of. My fuel mileage seems consistent with other TDs.
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Old 11-25-2017, 11:45 AM   #23
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It's hard to know just what your mechanic has done but I would have some questions. The GV control module is a fancy unit to engage and disengage at set speeds. It is connected to the speedometer and engages OD at about 39-40 mph and disengage at 10-15 mph. You can also engage or disengage manually with a foot switch or dash mounted switch.

In reality the GV has a 12 volt solenoid that allows or prevents oil pressure to engage the OD. You can bypass all the fancy controls by temporarily installing a simple on/off switch and wire providing 12volts to the solenoid, this will tell you if the problem is in the control or the GV itself. To check the GV, check the oil pressure and the solenoid. If both are good the problem is internal and an overhaul is in order.

Another thing that stands out is 6 hours to change the unit. To change the unit, you drop the drive shaft, disconnect the wire loom and speedometer cable and remove 6 nuts. Reverse the above and add oil. I changed mine without even jacking the moho up. Certainly not 6 hours.

Another thing that bothers me is GV sells these units for Ford, Chevy and Dodge diesel pickups as well as many motorhomes. The Isuzu was only used in a very small number of Airstreams and the number of those with GV is probably in the single digit range. To state the Isuzu is a problem kind of bothers me, especially since it is probably the lowest power unit in their catalog.

One other item, I hope they sold you the deep sump oil pan for your GV. It add oil capacity and runs a little cooler.
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Old 11-25-2017, 11:51 AM   #24
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We had an electric OD in a diesel pickup for 25 years. Towed heavy with it.

But the OD unit in a MH does not make as much sense to me. In the pickup it lets you run a high gear with no load and to run the lower gear when towing a load. But in the motorhome you are running the heavy load all the time and there is no low load time. So it would seem like with the MH just gear it the best you can once and live with it.
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Old 11-25-2017, 12:04 PM   #25
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Ive read close to 14lb boost on my '82 TD. Could be a bad gauge I suppose but I'll see well over 10lbs on uphill grades consistently. Or the PO had adjustments made that I'm not aware of. My fuel mileage seems consistent with other TDs.
14lbs! WHAT? I'm lucky if I see 8lbs of boost.

Jeez Mike, now I have boost envy, not just Allison envy.

Cheers
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Old 11-25-2017, 12:06 PM   #26
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Ive read close to 14lb boost on my '82 TD. Could be a bad gauge I suppose but I'll see well over 10lbs on uphill grades consistently. Or the PO had adjustments made that I'm not aware of. My fuel mileage seems consistent with other TDs.
The lads at Hamilton told me the 10 psi. number. My pyrometer died a few years ago so I bought a new Isspro unit. It read at least 250 degrees higher than the old gauge so I sent it back thinking it was bad. They sent me a new gauge that read the same. I took it out and bought an Autometer gauge. It read the same so I have to believe the old BAE gauge was "optimistic."
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Old 11-25-2017, 01:36 PM   #27
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Ahhh the turbo... the main reason that one really want it is when you start driving at higher elevations...without it the diesel drops off radically as elevation increases... which then with it... the turbo spools up and supplies air at the 30 inches needed to maintain the PH and torque as if it were at sea level... ..

One doesn't need to be a engineer... to figure out that By slowing down the drive shaft... one really puts more torque on it... to get the same speed out of the wheels... thus the pulses are in spacing.. are spaced out further... thus to get the same amount of torque through it.. the pressure pulses (fire pulse) must be logically higher than if the drive shaft was allowed to spin at a higher rpm and the torque is developed at the rear end gears.

However, the after market OD units attached to the back of the trans... most of the time auto types... They then took the RPM of the engine ... lowered it by gears... and speed up the driveshaft...

three things then happened ...
One is that the OD gears ended up with more torque and pressure on them... when in OD... thus they got a lot hotter... most needed a oil cooler attached to the trans or put out in line with the radiator...

The second thing that happens.. is by slowing down the rpm of the engine.. caused the increase in fireing cyc pressure to have to go up... to meet the same or increased load as one increases the speed of the vheicle... so what goes south is ...one also starts to approch the converter 'stall speed' .(most are around 1800-2200 rpm... if you have a tac on your car.. Put your car in 2 or select starting in second gear... from a stop light.. start accelerating.. and you will see that the RPM will increase to a point... before the trans/converter locks up... at low rpm the torque converter actually is why the engine can idle at the stop light and not stall the engine)
What happens then when in OD it causes the converter to lock and unlock as the engine fires... at or just above the stall speed of the converter... and because its oil coupled... causes the oil to boil at the torque converter vanes due to the increase in pressure required in OD...

And finally... while the the diesel engine is good for low rpm.. one can over load it.. thus lugging it... which cause all kinds of bad things to happen... one being the black carbon trail coming out of the exhaust...

What the engineers do at GM or others.. is find the sweet spot for operating the engine at the optimal economic speed... (normally about 60% of the max torque/hp point on the curve... but, diesels are strange... they can be lugged at lower rpm...hp/torque... remaining constant... and they can be lugged at higher rpms.. at max effort... unlike gas engines..

As to the fireing of the charge in the cyc... gas engines are more like steam engines... where the burn rate is controlled by the retardants added to the gas... Diesels on the other hand are fired by compression.. which is the next best thing to a explosion in the cyc.... thus the fireing of the gas engine is much smoother... than a diesel... and that becomes a problem where the rubber meets the road.. kinda thing..

With all that in mind... I found on of the two speed rear axles.... made for diesel trucks.... i.e they actually had two sets of gears in it... that were on the pinion side of the rear end... and you electrically then could change it while you unloaded the drive... and shifted... This worked better than the OD unit and helped keep the amount of heat down at the OD box...

But, I don't think they have these around anymore for low ton'ge vehicles...

However, even that doesn't help the auto trans out... because it does the same thing.. in slowing down the diesel engine... and making it come up with lower RPM... for the given speed down the road...

One other thing one wants to look at is the towing ability of the engine /trans package . Here one can see what the manufactures are saying the max load which a utility vehicle they make can tow/pull... and is the best way to get a ideal powerplant/trans for the replacement... but, be advised.. your in the experimental area.. and on your own if something happens... insurance normally won't cover mods made to the vehicle... including adding OD aftermarket units... so I found also...

Good luck
I'm sorry but I have to disagreed with some of these thoughts.

Transmissions, overdrive units and differentials are all gear sets. Whether you change ratios in one or the other they all transmit power and generate heat. The only units that require external cooling are automatic transmissions with torque converters. Manual transmissions, over drives and rear ends do not generally use external cooling and they last a very long time.

We can talk forever about engine speed vs driveline speed vs gear ratio and where the gear loading winds up however this thread is about the durability and strength of the GV overdrive. Until the unit has been disassembled and diagnosed we don't have an answer.

We can also talk about diesel pulse. In this situation Airstream used a TH400/475 transmission. There is no torque converter lock up. Torque converter lock up did not come about until the overdrive electronic transmissions were introduced. With the TH400 you can see 300 to 400 rpm difference at the same speed on a grade. There is a lot of slip in the torque converter and that will minimize the effect of diesel impulse the drivetrain sees. Witness the fact that those of us with Isuzu's are not seeing flex plate, u joint or other driveline issues. Therefore I do not see this as a significant problem for a GV.

Two speed axles were originally built for gasoline engines. I have two of them on my Model T Fords. They are made by Ruckstell and they use a planetary gear just like the GV overdrive. I have a 1938 Ford truck with two speed Spicer rear axle with a mechanical shift and in the 40's they used a vacuum shift and later an electric shift. The Borg Warner T11 overdrive unit came in the middle 50's and were used behind the transmissions of many cars and trucks, Ford, Chevy, Dodge, Studebaker and many others. The also were a planetary gear set up shifted by an electric solenoid that mechanically slid the sun gear in and out. It was about this time that Laycock DeNormanville developed their unit using oil pressure to shift and a cone clutch. This technology has been around for over 100 years.

The goal here is to set up a drive train so that the engine is operating at a happy rpm and load to get the vehicle down the road at a reasonable rate of speed. GM designed the P-30 to do that at a time the speed limit was 55 mph. The lads at Airstream put an Isuzu in a few of their mohos which some of us now own. The problem is the Isuzu operates at very different speed and power range than the 454. Airstream changed the engine but nothing else. Some of us are now trying to accommodate the differences with what is available to us in the market place today. Considering cost, ease of installation, and overall drivability the Gear Vendor is a solution. It is certainly not the only solution and it may or may not be the best but it is a solution and for me it works.

I see you live in Calif. and I left that state 20 years ago so I am not current on all the laws. My understanding back then was that smog involved the engine and what came out the exhaust pipe. Gears were not part of the equation. You were free to install a newer, larger engine as long as you met the smog requirements of that newer engine. Again I left years ago but as I drive through the state the hot rod industry seems as healthy as ever so I would guess the hot-rodder's have figured out a way to keep up with the laws.

I have been around and have worked in the trucking industry for many years and have been playing with cars since I was big enough to hold a wrench and I suspect we disagree on many things but I guess we will just have to agree to disagree. For now lets go camping.
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Old 11-25-2017, 01:51 PM   #28
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14lbs! WHAT? I'm lucky if I see 8lbs of boost.

Jeez Mike, now I have boost envy, not just Allison envy.

Cheers
Tony
Yup Tony. It approaches 14. Even on the grades out in NY, there were some long pulls where my boost measured over 12. 13+ is the highest Ive seen, out in Colorado. Ive read here and there that the add on Turbo maxed out at 10 so I was pretty surprised. Maybe the gauge is wrong, who knows.
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Old 11-25-2017, 02:10 PM   #29
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Yup Tony. It approaches 14. Even on the grades out in NY, there were some long pulls where my boost measured over 12. 13+ is the highest Ive seen, out in Colorado. Ive read here and there that the add on Turbo maxed out at 10 so I was pretty surprised. Maybe the gauge is wrong, who knows.
I would question your gauge but whatever it reads doesn't really mean much. The thing to watch is your exhaust temp. That tells you how hard the old girl is working and when she gets too hot, things start to melt.
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Old 11-25-2017, 05:17 PM   #30
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I would question your gauge but whatever it reads doesn't really mean much. The thing to watch is your exhaust temp. That tells you how hard the old girl is working and when she gets too hot, things start to melt.
Yup I do. Especially while climbing up long pulls.
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