The Automatic Disconnecting Differential System

09-10-2009, 02:59 PM
I assume my 1999 4Runner has the Automatic Disconnecting Differential and I did search this forum and other websites trying to learn how it works. Unsuccessfully.
Can anyone explain it in simple words?

Brian R.
09-11-2009, 09:33 AM
It uses vacuum to power an actuator on the front differential. The actuator engages and disengages the front drive shaft from the differential. The actuator is vacuum engaged (actuated) and spring pressure disengaged. The front wheels are always coupled to the half-shafts. Placing the 4WD lever in the 4WD mode, both sends vacuum to the differential actuator (ADD) and couples the front differential to the transfer case. When you are in 2WD mode, the front drive shaft is freely rotating and you can turn it by hand.

09-11-2009, 11:14 AM
Thanks Brian.
The Manual advises to drive in four-wheel drive for at least 10 miles each month to ensure that front drive components are lubricate. I have bought my 1999 4Runner from the guy who has never used 4WD and I wonder if damage has been done to the drive.
On the other hand if half-shafts are coupled to the front wheels all the time meaning that the front differential is always spinning I am not sure what else gets lubrication when you engage 4WD.

Brian R.
09-11-2009, 04:04 PM
If it engages, then you are good to go.
Change the gear oil as required.
Operating in 4WD periodically means the front drive shaft u-joints are lubed, as well the ADD actuator is kept free. There are valves in the ADD system that actuate also.

09-16-2009, 11:59 AM
The manual may also state that when you do engage the 4wd, that it be done under conditions that there is some slippage such as wet pavement, dirt, etc. If this is not feasible, then a long length of straight road is usually sufficient to at least circulate the fluids. This is so that any difference in rotational speeds between the left and right side tires (notably in turns) is allowed by letting the tires slip slightly while contacting the pavement.

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