Did you know that the transfer case on your 2001 - 2019 4WD Allison transmission weighs in at a stout 80 pounds? Did you also know that this crucial component is hanging off the back of the transmission with no additional support?

Yes, when it comes right down to it, the rear extension housing on 4WD Allison transmissions is a problematic design. Even if you’re not particularly harsh on your truck, the high horsepower / high torque produced by the 2001 - 2019 Duramax engine puts considerable strain on the rear housing even while performing normal, day-to-day tasks like towing or heavy acceleration. This driveline strain is further amplified by failing components that create excessive vibration like U joints or bent driveshafts.

This means the rear transfer case housing (extension housing) can suffer from hard-to-find cracks and once the transfer case housing cracks, it can weep fluid and eventually lead to an expensive repair or replacement.

Yikes!

Sure, your truck is designed to operate at the extreme ends of the spectrum (after all, that’s why you bought a Duramax/Allison combo right?) but even great factory engineering could use an assist every now and again. So what can you do to ensure that the rear housing on your 2001 - 2019 Allison transmission doesn’t suffer damage from day-to-day usage or from vibration due to failing components?

We’re going to cover everything you need to know about this common and frustrating problem and tell you the best way to avoid expensive and time-consuming repairs down the road:

• What does a rear housing do on the 2001 - 2019 Allison transmission?
• Why do rear transfer case housings crack on the 2001 - 2019 Allison transmission?
• How Can You Prevent Cracks In Your Transfer Case Housing?
• How Does A Rear Housing Support Prevent Cracks and Expensive Repairs?

Let’s get into the nitty-gritty and give you the low down on this common and pervasive problem!

What Does A Rear Housing Do On The 2001 - 2019 Allison Transmission?

You may already know this but let’s cover the main components of your 2001 - 2019 Allison 4WD transmission so you can better understand why this design flaw matters in the bigger picture.

The rear housing on the 2001 - 2019 Allison transmission is attached to the rear of the Allison 1000 transmission where the output shaft connects to the driveshaft. As noted above, this component is attached only to the transmission itself by a series of bolts. It is kept from leaking by a series of gaskets as the output shaft exits the housing and connects with the driveshaft. This component is also known as an extension housing since it serves as an adapter to allow the transfer case to bolt to the transmission.

As the output shaft exits the rear transfer case, it connects to the driveshaft via a U joint. The driveshaft then turns a differential at the rear of the truck via a transfer case (remember, 4WD) which then, in turn, rotates the wheels. Each of these components is tied together but is designed with tolerances to allow all components to move along with the axle (more on that later).

If everything works together perfectly, system operation is smooth and linear. Then again, let’s not kid ourselves here, when is anything that perfectly smooth and linear?

Why do rear transfer case housings crack on 2001 - 2019 Allison transmissions?

There are a variety of reasons why a rear housing may crack on the 2001 - 2019 Allison transmissions but every single reason has to do with extreme amounts of vibration and driveline flex that are transferred directly to the housing, which then causes cracks in the metal. Once the metal is cracked, even if it’s only a hairline crack, fluid can escape the housing and lead to massive issues for you and your truck.

The extreme transfer of vibrations is due to the design on the Allison 1000 transmission where the rear housing is bolted only to the transmission itself with no other support and is, therefore, more susceptible to damage due to vibrations from driveline component failure found on high-torque transmissions like the Allison 1000. Beyond the high propensity for vibration transfer, the potential damage is enhanced by the 80-pound transfer case hanging off the back of the transmission itself.

There are several possible sources of excessive vibration that can be found within the driveline:

Failing U - Joint

In order to transfer power from the output shaft to the driveshaft, transmissions like the Allison utilize U joints or Universal joints. These simple pieces of engineering mastery allow some tolerance in the movements between the transmission, driveshaft, and differential. This is necessary because these three components are rarely lined up exactly, especially as trucks move over uneven terrain. A rigid connection would lead to stress on the driveline components and a quick failure so engineers built-in joints that could move in sync.

If the U-joint on your Allison-equipped truck is failing, it can cause the driveshaft to create vibrations/harmonics with the rest of the driveline and put stress on the housing by moving outside of tolerance limits. Since the housing is not supported, it will move with the driveline and cause stress on the housing. Just like that, you can have a crack.

Bent or Damaged Driveshaft

Much like a U joint, a driveshaft that has been bent or damaged by debris can present a vibration in the driveline that can have the same effect as a malfunctioning U joint. Remember, the driveshaft moves round thousands of times per minute and must be carefully balanced to avoid the elliptical movements characteristic of vibrations. This vibration is transmitted right to the housing extension and you guessed it, can cause cracks that will lead to fluid loss.

Extreme Usage

When you dump hundreds of pound-feet through a driveline that has resistance from a heavy object or extreme acceleration; the driveline flexes in response. Yes, your truck is designed to do this on a regular basis but constantly taxing the driveline will put force on the poorly designed housing, and over thousands of miles, a crack can form in the housing from even normal levels of usage. This effect is amplified if you’re consistently pulling extreme loads or performing extreme acceleration runs and these actions can accelerate failure.

How Can You Prevent Cracks In Your Transfer Case Housing?

So, what can you do if you own a 2001 - 2019 Allison 4WD model and have concerns about cracking?

To help prevent potential damage via driveline vibration, you’ll need to pick up a rear housing support that’s specifically made for your particular truck, like this unit from Merchant Automotive. A properly designed, heavily constructed, transfer case support will reinforce the weight of the housing, while also providing a higher level of resistance to vibrations in your driveline. Rather than wait for failure, hooking up a rear housing support prevents the issue from occurring in the first place.

This custom engineered piece of equipment utilizes high-quality 3/16-inch plate steel that is cut to an exact fit on the transmission and transfer case of specific trucks. The housing and transmission are linked together with 7/8-inch 0.120-inch wall thickness DOM mild steel tubing to provide support for your rear housing, even under the most extreme of conditions, including vibration from driveline component failure.

How Does A Rear Housing Support Prevent Cracks and Expensive Repairs?

The rear housing support is a heavy-duty, precision machined piece of equipment that is designed to take on vibration and driveline flex and move it away from the housing and onto the support structure of the component.

It does this task in two ways:

1. By building a bridge and taking the stress off the transmission/housing bolts that support the 80-pound weight of the housing.
2. If issues like failing U joints, or bent drive shafts rear their ugly head, the support brace will take the vibrations these conditions cause instead of the housing itself, thus preventing the metal from the housing itself from cracking and losing fluid.

Once the support is installed, which can be done in your driveway with basic hand tools, the overall effect is reduced driveline flex, reduced vibrations to the housing, and reduced repair bills down the line. Why not prevent the issue from happening instead of leaving it up to chance? A busted transfer car can leave you in an incredibly bad situation, especially if you’re towing or on vacation!

What Years and Engines Does The Support Work With?

The rear housing support from Merchant Automotive works with 2001-2019 General Motors Duramax equipped 2500HD, 3500HD trucks from both Chevrolet and GMC. It’s designed to work specifically with the 6.6 V8 Diesel paired specifically with Allison 4WD Transmissions.

Why You NEED a Rear Transfer Case Support

Do you like paying money for unnecessary repairs? Have an extra $1000.00 lying around? We didn’t think so and that’s exactly why you need to get yourself a rear transfer case support from the experts at Merchant Automotive.

We know that you love your truck and overall, the Duramax / Allison combination is hard to beat for overall performance and righteous durability but even the best designed combinations have limitations in their design. Bolting up this incredibly beefy housing support that our Allison 4WD is ready for whatever you can throw at it, and can withstand the types of driveline failures that come with age and the usage these trucks were designed to tackle.

At Merchant Automotive we custom engineer our components in-house to fit your exact truck. This approach ensures an effective, quality product that we can comfortably back up with our Merchant Automotive industry-standard limited lifetime warranty. Yes, that’s right, we’re so confident in our product that we back it up with a limited lifetime warranty.

So, what exactly are you waiting for? Let us hook you up with the crucial component today! You’ll thank us down the road when you’re traveling worry-free knowing that your rear transfer case is supported by the Duramax / Allison experts at Merchant Automotive.