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Toyota Tech: Defeating the ADD


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Toyota introduced the Automatic Differential Disconnect (ADD) 4-wheel drive system on its trucks in 1989, and on the 4Runner with the 1990 redesign. This system is composed of a vacuum-activated shift fork in the front differential driver's side axle tube: when the transfer case lever is moved to the 4Hi position, vacuum is applied to the fork which in turn moves a sleeve to lock the inner stub axle shaft from the carrier to another shaft in the tube. This axle assembly powers the driver's side wheel, while the passenger side is a single shaft from the carrier to the inner CV joint. While in 2WD, the driver's side axle is disconnected within the tube. The ADD trucks also have hub flanges at the wheels instead of conventional hubs; a series of vacuum switching valves (VSV's) in the engine bay to control the actuator; and a transfer case position indictor switch, which controls the dash light.


While the ADD system is convenient and seems reliable, I wanted to eliminate mine as a potential source of failure - vacuum, electrical, or sleeve failures are possible in this critical system. The system also requires a vacuum hose from in the engine bay (especially with an automatic transmission), which makes field troubleshooting a chore. On trucks with automatic transmissions, additional electronics are added to monitor transfer case lever position. I found that by removing the ADD vacuum and electrical subsystems, I'd wind up with a simplified 4WD system, and therefore reduce possible failures.


First, remove the front skidplate. You should either drain the front differential of its oil, or wait until you remove the actuator assembly from the differential to let it drain (this will preserve about 1/2 of your existing gear oil).

Actuator with shift fork.

The actuator assembly is on the driver's side of the differential. Remove the 4WD dash indicator plug (white plug) from the assembly, then remove the vacuum lines from the assembly. You can also remove the vacuum line brackets and hard lines from the differential housing.

There are four 12mm bolts holding the actuator in place. Remove these bolts and lightly tap the assembly with a hammer handle to jar it loose. If you haven't drained your differential oil, it will pour out at this point. Remove the actuator assembly.


Next, clean the old gasket material from the differential housing and the actuator assembly with a plastic putty knife.


Place the clamp around the fork rod.

The shift fork rides on a rod within the assembly. When the fork is to the passenger side, the shafts are locked. Slide the fork to the passenger side, then cap the vacuum ports on the assembly. The key to guaranteeing the fork will never move is to place a hose clamp or CV boot clamp around the rod the fork slides on - and be sure to trim any excess clamp length. This will eliminate any chance of the fork moving from the locked position.



Within the opening in the housing, you can manipulate the locking sleeve. Moving it to the passenger side locks the shafts together -- but if you slide it too far, you'll expose the gear and NOT lock the shafts (don't worry, the sleeve will not slide out of reach). The recessed section of the sleeve should be visible when it is in the proper position. When the actuator is placed on the housing, be sure to test engagement of the sleeve by turning the front driveshaft - the driver's side CV/axle should also turn.

Disengaged sleeve position. Sleeve too far to passenger side. Sleeve in proper position.

When you are sure it is in the locked position, test fit the fork assembly to the housing and test for lock again. It's very easy to move the sleeve to the unlocked position while fitting the fork. I found it easiest to mount the assembly on my V6 truck by going up between the front lower control arm crossmember and IFS, positioning the fork at an angle to the sleeve, then sliding it in. This was much easier than going at it from the front, where the oil pan interferes.

When you're ready to finally fit the assembly, apply new gasket-maker to the assembly and seat it. Test it again to make certain the shafts are locked together! Then tighten it down, refill your differential and get ready to rip...........


Vacuum Switch Valves (VSV's) in engine bay.

Trace the two vacuum lines you removed from the front of the differential up through the engine bay. They terminate in the lower ports of VSV's behind the fuse box on the passenger side. Remove the lines. From the VSV's there is a single vacuum line to the engine vacuum source which can be removed, and the port capped. At this point, you can remove the electricla sensors from the VSV's, and completely remove the VSV assembly.

Removing vacuum parts - this is just a start!

You can also remove the vacuum tank in the wheel well, along with the lines to and from it. One vacuum line goes up between the fenders to the VSV's (through a grommet in the inner fender), and can be removed by pulling the grommet and line through the opening.


The 4WD dash indicator sensor (white plug) on the front differential can be removed as well. Seal up the fitting for the indicator, and trace the wires back to the firewall. Snip them, strip some insulation, wind the leads together, then wrap them well - now when you select 4WD via your transfer case lever, the indicator in the dash will work properly. If you don't feel like tracing this, simply put the white plug back into the actuator and your dash indicator will work fine. Initially, I left the sensor plug in the assembly, but have since gone back and cut the wires in the engine bay.

Eliminating the ADD system works with either the ADD hub flanges or manual hubs. With either hub type, you retain shift-on-the-fly capability: however, with manual hubs you eliminate any front drivetrain "drag" with the hubs unlocked and in 2WD. Upon completion, you have a greatly simplified and bulletproof 4WD activation system that works the same as stock!

Assembly remounted to differential.

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