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Old 11-27-2017, 10:29 AM   #1
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2wd Underdrive Unit Options

Hey fellow Vanners,

My van is a Ď99 E350 7.3L 2wd with WeldTec suspension lift and an OX full locking diff in the Dana 60 rear axle with 4:10 gear set. It is fully cable of going a lot of places. After 4 months on the road, and off the road, I would say the only downfall to my 2wd setup is not having a low range. Long, slow, steep, rocky uphills in 1st with a heavy, fully built out camper van toting around 30 gallons of water and a motorcycle just donít make the tranny happy... And the van does have a tranny cooler.

Does anyone have any experience with an underdrive unit that can be added to the 4R100 tranny? As I understand, an underdrive unit is a gear reduction that functions like low range in a transfer case which would provide the torque Iím looking for to get over the next rock ledge or mountain.

Basically, Iím looking for a relatively inexpensive and easy way to add low range without going to a 4x4 conversion and keeping the current axle/ gear ratio.

Has anyone used an underdrive unit before and what was your experience with it?
What other options may there be?

Thanks!
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Old 11-27-2017, 12:22 PM   #2
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I'm paying close attention here too...same situation....never had any problems with traction yet (2WD, OX locker like you've got), but the van definitely is working hard on the steeps to keep going sometimes. (Momentum is your friend....)

Interested to learn more about what the "under-drive" setups are that you're referencing....was thinking the only option was to install a 4WD low-range transfer case and have it hooked up only to the rear wheels.
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Old 11-27-2017, 12:52 PM   #3
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What are the issues?
Temps?
Hunting for gears?
Be specific.
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Old 11-27-2017, 12:57 PM   #4
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Gear Vendors is the only one I've ever heard about. No personal experience with them.

Gear Vendors under/overdrive transmissions the most awarded auxiliary transmissions.


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Old 11-27-2017, 01:36 PM   #5
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Nobody makes an underdrive anymore. US Gears did, but they eventually got bought out by somebody bigger and their products pared down.

The GearVendors unit is just an overdrive. It won't help in the slightest off-road, only for towing and fuel economy.

Best bet is just installing a transfer case. The stock 271/273 handles 2-low just fine (normally done by keeping the front hubs unlocked). Beware that this adds lots of torque to the rear axle. I'm not sure I'd feel comfortable doing this on a semi-float.
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Old 11-27-2017, 06:49 PM   #6
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Quote:
Originally Posted by Bbasso View Post
What are the issues?
Temps?
Hunting for gears?
Be specific.
Issues:

1. Overheating. I had to abort crawling (1st gear walking speed) a steep, rocky trail in BC when the tranny got hot enough to set off a code with the blinking overdrive button light.

2. Lack of torque. Just the other day I was going up a steep, snowed in trail at about 8500ft elevation and got to a point where the tires were stuck in the snow and the olí 7.3 belched smoke but couldnít turn the tires anymore. Shoveling ensued and with just the slightest amount of movement I was able to get going again.

Both scenarios could be addressed by:

1. Not going down these trails. But how could I provide adventure for my copilot?

2. Having the ability to select low range for these situations.

Otherwise, I have no issues with the tranny or how the van performs. Just need that little bit extra...
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Old 11-27-2017, 06:57 PM   #7
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Quote:
Originally Posted by carringb View Post
Nobody makes an underdrive anymore. US Gears did, but they eventually got bought out by somebody bigger and their products pared down.

The GearVendors unit is just an overdrive. It won't help in the slightest off-road, only for towing and fuel economy.

Best bet is just installing a transfer case. The stock 271/273 handles 2-low just fine (normally done by keeping the front hubs unlocked). Beware that this adds lots of torque to the rear axle. I'm not sure I'd feel comfortable doing this on a semi-float.
Yeah, unfortunately with the bit of searching I did I couldn’t find a US Gear underdrive.

The Dana 60 is a full float with upgraded 35 spline axles

It does seem like a transfer case would be the most economical doesn’t it?

Could a transfer case be clocked so that it does not interfere with the tank?
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Old 11-27-2017, 07:18 PM   #8
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You'll need to modify the tank with UJOR kit or the full tank they sell (Both are proven.) to fit the 271 under the van.

Honestly with the deep snow, weight, over heating, and not trying to kill the van I think going 4X4 might be in your best interest.
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Old 11-27-2017, 10:16 PM   #9
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Quote:
Originally Posted by B350 View Post
Issues:

1. Overheating. I had to abort crawling (1st gear walking speed) a steep, rocky trail in BC when the tranny got hot enough to set off a code with the blinking overdrive button light.

2. Lack of torque. Just the other day I was going up a steep, snowed in trail at about 8500ft elevation and got to a point where the tires were stuck in the snow and the olí 7.3 belched smoke but couldnít turn the tires anymore. Shoveling ensued and with just the slightest amount of movement I was able to get going again.
Unfortunately your symptoms, combined with having a Powerstroke and 4.10 gears, don't bode well.

Crawling at slow speed certainly can make things warm, but even dragging my trailer up steep draws at least 35%, sometimes wheels off the dirt, I've never gotten mine that hot.

And more importantly, I can't imaging situation where I'd stall out. I stuck the right side of my trailer in a muddy tree well this weekend, up to the axles, and had zero issue spinning my tires trying to yank my trailer's 11,000 pounds out of that hole.

I agree that a crawling would help, but I don't think you should have those issues to start with.

That said.... overheated fluid foams and loses viscosity, which can cause symptom 2.

#1 thing I'd check first is that your cooler is actually getting oil flow. Sometimes the 4R100 end up with a stuck pressure relieve valve, which bypasses the cooler loop (which are in series, so both cooler in reality). Aside from rust and old age, they can also properly bypass if the cooler is plugged with debris. But if there's debris in there, your tranny probably doesn't have much life left anyways.

Now, if you can verify oil flow (stick the oil cooler outlet in a bucket and start the van, it should flow a quart per 15 seconds minimum at idle when warm), you might just need a larger oil-to-air cooler. But once you'e gotten your fluid hot enough to stall out or flash the O/D light, you should change it anyways...

That all said... One workaround for a lack of off-the-shelf under-drives, is to go ahead with the Gear Vendors Overdrive, combined with much lower gears. You'll get lower overall ratio needed for off-road driving, but still retain a reasonable top final-driving ratio for highway cruising, AND gain much needed "in-between" ratios. Only real downside is $$$. You don't even have to swap the fuel tank for this. But you do have to drop the tranny to swap the tail shaft.
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Old 11-27-2017, 11:02 PM   #10
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Quote:
Originally Posted by carringb View Post
Nobody makes an underdrive anymore.
The GearVendors unit is just an overdrive. It won't help in the slightest off-road, only for towing and fuel economy.

Best bet is just installing a transfer case. The stock 271/273 handles 2-low just fine (normally done by keeping the front hubs unlocked). Beware that this adds lots of torque to the rear axle. I'm not sure I'd feel comfortable doing this on a semi-float.
I could be wrong, but Gearvendors calls thier units underdrive/overdrive. Perhaps the term is misleading when they talk about gear splitting plan though. First OD would be lower than second non OD, but thats no help when crawling. Gear Vendors Under/Overdrive Ford Truck-Car-Van 2-Wheel Drive 4-Speed and 5-Speed Automatics.

As for your concerns about using a transfer case with a semi-float, is that because its a diesel? Plenty of conversions are done with semi's, perhaps the vast majority are.
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