Tech: ARB Air Locker for Toyota IFS Short Cuts

| Toyota 4Runner | Toyota Tech | Toyota Section |

By: Lars Dennert - 05/03.

Ultimate trail traction - dual ARB air lockers for Toyota IFS!

For those of you looking for some increased traction for a Toyota beyond a rear locking differential, ARB has recently introduced the RD90. ARB's latest creation is a newly designed Air Locker fitting Toyota 4x4 IFS vehicles. The beauty of the Air Locker is that it functions like a normal open differential but can be spooled at the press of a button to provide 100% traction to both front wheels.

The RD90 fits both the '89-'95 torsion bar suspensions (which include the mini-truck, 4Runner, and T100), as well as the '95.5+ strut suspensions (which include the Tacoma, 4Runner, Tundra, and Sequoia). Since the IFS utilitizes Toyota's 7.5" ring gear, the RD90 will also fit the rear axles of certain 2WD trucks, vans and Celicas. The RD90 does not fit the IFS Land Cruiser / Lexus LX470 which use a special 8" IFS (RD92); and it does not fit the 2003 4Runner / Lexus GX470; nor does it fit all-wheel-drive Toyotas without a two speed transfer case.

The new, improved Air Locker
The new, improved ARB RD90 Toyota IFS air locker.
Image provided courtesy of ARB.

This ARB unit is their latest in design. It is a new timed-gear Air Locker which has its side-gears and spider gears clocked (timed) from the factory to provide a perfect distribution of the load over all teeth when the Air Locker is locked. The resulting strength increase in some applications has approached 300%. In basing the RD90 on their timed-gear design, changes were also made to the way the air is delivered. Traditional Air Lockers receive their air at the bearing opposite the ring-gear. The timed-gear Air Lockers receive their air at the bearing on the ring-gear side. This means that on the IFS Toyota the air line enters on the small side of the split case. In addition to this change, the timed-gear Air Locker also benefits from a two-piece case instead of the traditional three-piece design and they have eliminated the need for the annular piston used to transmit the air's force to the locking clutch-ring. In a nutshell, the RD90 is stronger, simpler in design and easier to install than a traditional Air Locker.

The RD90 is also the same unit being touted by the Jeepers as the Super 30. A modified version of the RD90 is used with Dana 44 or Dana 60 30 spline axles to provide a much improved Dana 30.

Installation of the RD90 is considerably more complex than installation of a rear Air Locker like the RD23 or RD89. Toyota IFS has many parts but the job can be performed by anyone skilled in setting up differentials. The key is to be prepared and avoid some of the possible pitfalls. In this article I will describe the process of installation into a late model high pinion clamshell differential. Installation into an early IFS is somewhat less difficult. This article is not meant to replace ARB's comprehensive 50 pages of installation instructions, instead it is intended to provide a detailed overview along with some tips allowing the reader to become familiar with the process.

Removing the axles and differential
Extracting the stub axles.

Work begins with the usual vehicle preparation of hoisting the vehicle, removing the wheels and skid-plating and then draining the differential. Next, the axles and differential must be removed from the vehicle. Begin by disconnecting the upper A-arm balljoints and then extracting the stub shafts from the differential. The stubs are held in by a snap ring of sorts which is a one time use part and usually breaks when the stubs are pulled out. Make sure to have spares available - they are a special order item from the dealer. Now disconnect the driveshaft from the pinion as well as the electrical and vacuum lines to the ADD. Disconnect the three differential mounts and remove the assembly. The breather will unplug itself from the differential.

Differential disassembly
Opening up the Toyota IFS clamshell case.

Before disassembling the differential, measure and record the current backlash setting. On the clamshell differential this involves blocking the carrier gears and inserting a long dial caliper into the drain plug to access the ring gear. Now remove the ADD section and floating shaft from the differential. Remove the bolts surrounding the case and gently pry it open. The differential center can now be removed and the existing bearing caps and shims pressed out of the housing. Keep track of which side each set of shims and caps belong.

Remove the ring gear and bearings from the center, also keeping track of each bearing's proper side. It is likely that one or more bearings will become damaged and it is a good idea to have spares.

Ring and pinion setup
The OEM carrier is pictured left, and the stout new ARB carrier is pictured right.

From here, both carriers will need to be measured in order to reproduce similar preload and backlash. A measurement is performed on the existing carrier from the ring gear mounting surface to the bearing shoulder on the opposite side of the carrier. Add to this the thickness of the existing shim pack on the far side of the ring gear to get the current preload distance. Now measure the same dimension on the ARB carrier and the difference between that and the current preload distance is how thick your new shim pack should be. Hopefully it is the same as the existing pack or a new pack will need to be obtained. This new pack and the matching bearing cup can now be pressed into the deep side of the housing.

Pressing the carrier bearings on the ARB carrier.

Install the ring gear on the new carrier by heating the gear in boiling water, applying a thin coat of grease to the shoulder and gently tapping it into place with a soft mallet. Be sure the holes are lined up. Torque the bolts down in a cross over pattern. Now press the original bearing opposite the ring onto the carrier. Press ARB's bearing cone onto the ring gear side and then the bearing as well. Final measurements are now required to calculate the bearing preload.

Install the assembled differential into the housing.

Insert the carrier into the deep housing where you already reinstalled one bearing cup. Install all remaining shims and any extras into the shallow half of the housing along with the other cup. Carefully insert the shallow housing until it bottoms over the carrier. You will notice that the housing doesn't close all the way due to all of the extra shims. Measure this distance assuring that you keep an even gap around the circumferance of the case. This is called the Preload Gap. Subtract this Preload Gap from the thickness of the top shim pack and you get the end-float in the differential. The service manual will give you a figure for what is called (desired) preload as expressed in gap. This value is likely to be .25mm. Preload plus end-float equals the actual top shim pack thickness you need to properly preload the differential. Remove the top bearing cup and shims so that you can install the properly calculated shim pack instead.

Air line routing
Installation of the air line.

The bulkhead port must be drilled in the most shallow portion of the small housing to allow clearance. It is important to also observe the outside of the housing for obstructions, such as the oil pan, when choosing a spot for the 1/4" NPT bulkhead fitting. Seal the threads against leaks and lubricate the delicate seal O-rings. The correct preload shim pack, bearing cup and seal housing can then be pressed into the shallow half of the clamshell. Make sure no dirt is contaminating the seals and that they are not twisted in their seats.


Reassemble the housing and measure the backlash to make sure it is to spec and/or matches your original setting. If not, disassembly will be required to change the thickness of the shim packs. To decrease the backlash, increase the thickness of the ring gear side pack, simultaneously removing an equal amount from the opposing pack. To increase backlash, the opposite is true. Though the instructions do not indicate a check of the mesh pattern you can integrate that into your reassembly at this point should you desire. Since you are retaining the original ring and pinion, a change in pattern is less likely.

You may want to install new stub axle seals while you have everything apart. Bench test the locker for leaks using 90-100psi of air. Reinstallation consists of mounting the axle back into the vehicle making sure to reattach the breather hose in the process.

Using new axle seals is recommended for reinstallation. Note the position of the breather fitting - and be careful when refilling with gear oil! New retaining clips may be needed on the stub shafts.

The photo above shows the breather on top of the housing. You may notice that this breather is located very low on the housing and that it is just above the oil level. The position of the breather is a compromise at best. Toyota had to compromise in its placement due to tight quarters and consequently many people, including myself, have had problems with it. Refilling the housing by simply adding oil up to the fill hole will cause the differential to blow oil out of the breather! This is a common problem and is amplified by the installation of the larger ARB differential. Fill the oil level to between 1/2" and 1" below the fill hole when you have completed installation. It is very important that there be no dips in the breather line to catch and hold oil. Check your breathers for proper operation. Toyota makes two kinds: one has a dust cap but flows air in both directions, the other has a dust cap and a one way internal valve. Many people have changed leaking axle seals or torn apart lockers thinking that they were bad when, in reality, the problem often was with proper breather function.

Install new retaining clips on the stub axles before reinserting the tulip joints.

Supplied air line, solenoid, and fittings.

As with all Air Lockers, the ARB comes with an air line, fittings, solenoid and switch that mates with the ARB compressor. I found that the ARB compressor will just squeeze up under the hood next to the ABS if you fabricate a small bracket. The disadvantage is that the compressor receives heat soak from the engine when mounted high under the hood. This can be problematic if you work the compressor hard filling up multiple sets of tires. It is otherwise not an issue.

The compressor and solenoid installed underhood.

Having owned an ARB locker for several years in the rear of my 4Runner, I am truly impressed with the quality and operation of it. The addition of the front locker has been an equal boost in ability in comparison to the addition of the rear. Obstacles that once caused me to struggle are now almost effortless as the vehicle glides up them choosing traction from whichever of the four wheels offers the best. Instead of the rear having to push the front up low traction situations, the front now fully pulls and in many instances does all the pulling. The beauty of being able to select the locking operation means that it is completely transparent under most situations. When engaged, however, the front tires grab like mad at anything they can reach. Steering radius, even on loose sand, is drastically increased when both front tires have traction. The vehicle simply doesn't want to turn. In those situations when you legitamately need the locker your turning radius is unaffected since you are using it because of one wheel having no traction. In that case, the tires aren't fighting each other for differentiation. Also, I only use the lockers for brief moments and then usually in stages. I first use the rear and then add the front if needed.

The wiring harness supplied with the ARB compressor is designed to defeat the use of only the front locker without the rear being engaged. This is to reduce stress on the weaker front axle: however, this feature can easily be defeated with some simple wiring changes should someone have the need.

As with any selectable locking differential, care must be taken when engaging it. Though they can be engaged at any speed, it is hazardous to do so if the two wheels on that axle are spinning at different speeds. For instance if one wheel is stuck and one wheel is clawing for traction, or if you are turning a corner rapidly, or if one tire is flat or sized differently, engaging the locker will crash the gears together possibly causing damage to the locker and drivetrain. I am generally not moving or I am moving very slowly when I engage my locking differentials. The RD90 also needs such care taken. Moreover, due to the design of Toyota's ADD (Automatic Differential Disconnect), one axle shaft leading into the differential is always spinning; even in 2WD. Accidently engaging the front locker while moving at any significant speed in 2WD will cause a tremendous shock to the drivetrain. This has been a serious hazard for people having automatic hubs and automatic lockers such as the Detroit Locker. Once engaged, you may travel at any (safe) vehicle speed though.

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