Due to the significant number of failures surrounding the Inter-Axle Differential (IAD) Lock, also known as the Power Divider, there is confusion and frustration with the proper use of this component. The IAD is often referred to as the “weak link” of your truck’s drive train and can be a big unexpected maintenance cost. If you catch the damage early through oil analysis tests the repair can cost $1600, but if it progresses to a catastrophic failure, it can cost up to $7000. The IAD is really undeserving of the “weak link” title, given most of the failures are due to abuse or improper use.  Read this article for tips on how to prevent damage to your IAD, which can save you thousands in repairs.
First of all, what components make up the IAD?
The IAD consists of a cross shaft, four spider gears, two drive gears, and a housing. These parts together are commonly called “The Nest.”
  • The gears: the action of the gears allows for differences in axle speeds. The greater the difference in axle speeds, the more these gears turn. Their purpose is to account for small differences in axle speeds.
  • The “Nest:” is splash lubricated by the front axle ring gear spinning through the lube oil or some axle models have a small lube pump for the IAD.
What does the IAD do and why does it get damaged so frequently?
In the unlocked position, or normal driving, the IAD allows torque to be equally applied to both the front and rear drive axles while allowing for differences in speed between the two axles. Difference in speed between the two axles is the main reason for damage to the IAD. Some of the things that create differences in axle speeds are; differences in tire diameter between the axles, and mismatched axle ratios. Having the IAD control in the unlocked position does not mean the IAD isn’t functioning. Anytime the driveline is turning, the IAD is functioning in some capacity.
How can you prevent damage to the IAD?
“Spin Out” or wheel spin creates the greatest difference in speed between the axles, and is the number one enemy of the IAD.  Spin out occurs when one or both wheels of an axle lose traction and spin while the wheels of the other axle are stationary or moving slowly. The worst case spin out is when the rear drive axle loses traction and spins while the front axle is stationary, resulting in low or no lube to the nest. Having the IAD in the locked position would prevent this worst-case scenario.
When the IAD is in the locked position, it locks the nest gears together and prevents them from turning. This creates a virtual solid shaft, which minimizes the potential for damage from spin out, but eliminates its ability to compensate for differences in axle speeds. Moving the control to the locked position merely changes the way the IAD is being used.
How do drive axles come into play?
Drive axles contain a differential assembly that is very similar in design as the IAD. Its purpose is to allow for differences in speed between the right side tires and the left side tires of that axle. These speed differences occur when turning corners, when there is a difference in tire sizes, and in a spin out condition.
What about if I have driver controlled differential lock?
Some axles have a DCDL (driver controlled differential lock). When the DCDL is engaged it locks the differential gear together, which locks the left and right side tires together. This provides better traction but eliminates the ability to compensate for differences in tire speeds.  Having the DCDL and the IAD lock engaged provides the best possible traction. Having both engaged essentially locks all four drive wheels together, which provides the best possible power transfer to the ground; but creates a severe under steer condition for the truck. Having both engaged means having eight tires turning at the same speed trying to move the truck forward and only two tires (steer tires) trying to turn the truck.
When should you use the IAD Lock and DCDL?
Due to the under steer condition, it is extremely important that the driver only use the DCDL in extreme winter driving conditions and at a speed not to exceed 25 mph.
What are the recommended procedures for using the IAD Lock and DCDL Lock?
When encountering poor road or highway conditions where maximum traction is required, perform the following:
  1. When approaching poor traction conditions, first engage the IAD switch. As conditions worsen and when vehicle travel speed is 25 mph or less, the DCDL lock can then be engaged.
  2. When the DCDL is locked, the vehicle’s turning radius will increase. The driver must use caution, good judgment and drive at low speeds when operating the vehicle with the DCDL locked.
  3. There is no speed or handling restrictions for vehicle operation with the IAD lock engaged, but for best axle performance and minimum tire wear ALWAYS unlock the IAD lock when favorable road conditions return.
  4. When the DCDL is locked, the vehicle must be operated at 25 mph or less. Lock the DCDL only when maximum traction is needed.
  5. Do not lock the DCDL when the vehicle is traveling down steep grades, or potential loss of vehicle stability could result in a jackknife of tractor and trailer.                
The main point to remember is to avoid wheel spin and you will no longer have that annoying and expensive “weak link” in your drive train.

*Click here for more information from Meritor on your IAD. 

Comments (19)

Bill McClusky

I have been in the trucking and construction equipment service industry for 23 years as a service technician, component rebuild specialist (engine, transmission, and axle), service department manager, instructor and consultant. I was a class 8 truck driver for 3 years pulling wet and dry tanks. I have been with American Truck Business Services for 4 years serving as a Business Consultant, Maintenance Consultant, and Instructor.

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Detroit Thrill Ride

April 25, 2016


Great information and great comments that help out.

November 11, 2012 11:58:29 AM

Great info, and very true, When I purchased my truck I inherited damage caused to the locking diff. and yes it was very costly to repair. I am very careful as to when I engage the system and ONLY use it when there is no way to avoid the situation.

September 27, 2012 7:14:50 AM

I wish there was some kind of indicator to remind driver just in case of distractions.

September 25, 2012 6:52:52 AM

Don'r forget when IADL is engaged you use more energy (fuel) and create a little more heat , especially in the forward axle housing. Watch those gauges and always remeber to unlock as soon as conditions for traction improve.

September 17, 2012 6:16:49 AM

Great Info that will save some$$ in costs.

September 09, 2012 16:36:24 PM

Great article.I understand the system better now. Luckily, I was using it properly but now I understand why.

August 31, 2012 13:52:23 PM

I've driven for yes and never really understood all these components. Thanks for the info!

August 30, 2012 11:59:00 AM

Great tip Roxanne!

August 29, 2012 9:58:58 AM

Great article. As I live in the great white north, I find engaging the lock while dropping or hooking to trailers works well to keep traction.I only do this once I am lined up straight and just need to get back to the trailer that is usually in the only spot that hasn't been plowed properly(or so it seems). Once I''m hoocked and moving forward I disengage the lock. So far it has worked well!

August 29, 2012 7:07:54 AM

Awesome info!

August 26, 2012 16:09:16 PM

Thanks for the insight, Bill.

August 16, 2012 9:25:30 AM

these was very helpful great information

August 11, 2012 17:33:20 PM

these was very helpful great information

August 11, 2012 17:33:06 PM

Excellent article!

August 10, 2012 12:19:27 PM

Please continue to share your infinite wisdom Bill!

August 10, 2012 7:47:24 AM

We have one drive axle with a locking Diff and have not had to use it yet... We have been able to lift our pusher axle to get out of some deep snow and then put the axle back down and we were ready to go.

Bob was a mechanic for twenty years and we saw way to many drive lines twisted off or a busted axle. When it gets to the point that we will need to lock in the differential the truck will be driven with the utmost care.

I enjoyed the article and the explanation

August 08, 2012 15:11:57 PM

Great article!

August 08, 2012 13:13:48 PM

These are great tips on this often improperly used component. Hopefully this will save some operators from having to make a unnecessary repair.

August 08, 2012 10:52:47 AM

Great information!

August 08, 2012 10:51:45 AM