ADD - Maintenance/ troubleshooting
Article: Ron Photos: Ron



There have been a lot of questions about the ADD (Automatic disconnecting differential) system on the Surf/4Runner. Hopefully this article will go some way to explain it. Assuming vehicle is right hand drive (Australian) and right means drivers side

The front differential is equipped with an Automatic Disconnecting Differential (ADD). The ADD can be shifted from 2WD to
4WD with the vehicle moving (manuals). The ADD controls engagement of intermediate shaft on front differential to provide 4WD operation. 4WD is selected by a switch on the transfer case gear lever. There are variations on this with manual and automatic gearboxes, fixed and manual hubs, and transfer cases with H4-N-L4 with a switch or H2-H4-N-L4 with no user operated switch. The basic operation is still the same.

During 2WD operation the front axle is not connected. The left axle is not connected to the right axle and the transfer case is not engaged. This can be checked by first rotating the short front drive shaft. It should be free. Then by jacking up the front of the car to lift both front wheels of the ground and rotating the front tires. When you rotate either tire, nothing else will move past the CV boots. This is because when you rotate the right wheel, it only rotates side and pinion gears in the diff. When you rotate the left wheel, it only rotates the left axle which has been disconnected from any gears by a split shaft.
When selecting 4WD either by pressing the switch or moving the transfer case lever, the ADD is activated.

Electrical Diagram

When 4wd is selected, the control relay is earthed completing the circuit and the VSV for 4WD is selected. This can only happen when the key is in ON or START for the positive side of the circuit. The VSV (Vacuum switching valve) supplies the vacuum to operate the yoke to connect the left axle to the right.

The Vacuum comes through a filter to a tank (see pic) under the right wheel arch and back to the VSV solenoids (see pic). Depending on which solenoid is active will determine whether the front axle is in 2WD or 4WD

Schematic of Front Diff Vacuum

Outside Axle
As the picture shows is the ADD component on the front axle. It is simply vacuum operating a yoke sliding a sleeve along splines. If vacuum is applied to the right port then 4WD is engaged. If vacuum is applied to the outside left port then the axle is separated

Inside axle
The picture right shows the yoke of the ADD actuator in the position of 2WD

The picture above and on the left shows the collar to which the yoke is attached. Again this is in the 2WD position. The picture on the right shows the collar pushed over (past normal 4WD position) to show the splines under the collar. The join in the shafts is in line with the body between the two gasket surfaces.

Trouble Shooting
This is what I do to check my system and it may vary. I have a manual with the 4WD button on the transfer gear stick.
When it is in 2WD, I can twist the front drive shaft and when I jack up the front axle both wheels will turn freely by hand.
To change from 2WD to 4WD or back, vacuum is required so therefore the engine has to be turned on. The tank under the right wheel will allow me to switch back and forth about 2-3 times without replenishing the vacuum but the key needs to be in the ON ( or START) position When the 4WD button is pressed, I can feel the electric motor engage the transfer case. When the transfer case is engaged it sends a signal to engage the VSV. If the sensor is disconnected, the VSV’s will not click.

Checking the vacuum is just a matter of pulling of the hoses one at a time. I have easy access to the ADD valve on the front diff and can check that I have vacuum on the correct hose. If you have access to an Ohmmeter, the VSV’s have a resistance of about 40ohm and can be switched by manually applying 12volts. When the yoke on the ADD valve is moved into the 4WD position, it is then that the 4WD light will illuminate on the dash.
When 4WD is engaged, the –ve signal used to illuminate the signal on the dash is also used to notify the ECU that things have changed.
If the electrics don’t work, and the VSV are not switching then check the 4WD fuse (20A) but if the VSV are switching and the vacuums are correct and the indicator light isn’t working then it is the GUAGE fuse (15A) but this would be evident that none of the other indicator lights aren’t working.
If you jack up the front of the car and 4WD is selected then the front drive shaft should turn at the same rate as the rear drive shaft (if the rear wheels are on the ground then you will not be able to turn the front drive shaft). And when you turn any front wheel, then either the front drive shaft or the other wheel should turn.

I believe that the automatics need to be in PARK or Neutral before switching to 4WD and the indicator AT/P has to be illuminated on the dash.

At the back of the front diff is another sensor which is a temperature or an oil pressure sensor. Connected or disconnected it doesn’t seem to make any difference to mine but if your 4WD light flashes or doesn’t engage, it might be worth while checking if you have oil in your diff in the first place

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This article is to be used as a guide only. No responsibility taken for incorrect information. I must acknowledge the information stolen from sources such as Haynes manuals, Max Ellery’s manuals and Toyota and the numerous car yards in the area that allowed me to crawl under their vehicles to take pictures J © 23 Aug 03 Revision 1