A.D.D stands for Automatic Disconnect Differential.
You may see on certain product descriptions, mainly front differential, the term “ADD”, but what does this actually mean or do?
The short explanation is that A.D.D is a 4WD system that connects/disconnects the differential to output shaft (effectively to the wheel) usually by a touch of a button from the drivers seat.
On some models, the front differential is equipped with an Automatic Disconnecting Differential (A.D.D). When shifting from 2WD to 4WD the front drive shaft is engaged by means of a sliding hub/collar on the intermediate shaft, effectively locking the splines between the shaft and differential.
The actuator is usually vacuum or electronically operated to provide 4WD operation and 4WD is usually selected by a switch on the transfer case gear lever.
Why do I need to check if I have A.D.D?
On a few of our Front Differential Bearing Rebuild Kits, it states that the kits are only compatible with vehicles with ADD Locker, these include DIFFKIT004, DIFFKIT007, DIFFKIT011, DIFFKIT012, DIFFKIT033.
A.D.D differentials use alternative bearings, crush spacers or seals etc to cope with extra loadings, therefore the correct kit must be selected depending on the specification of your model.
How do I know if I have A.D.D?
The best way, rather than looking for certain switches etc, is to pop your head under the front differential area and look for the differential actuator – these are clearly visible from those models that are not equipped with the setup.
Here are a couple of Toyota diagrams showing the different setups of vehicles with and without A.D.D:
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| An example of a Front Differential Vacuum Actuator (A.D.D) DIFF019:
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