Subaru Forester R160 Torq Locker with VTD Center Differential Controller Demonstration

There are two types of center differentials used with 4EATs: VTD (Variable Torque Distribution), which is a mechanically based center differential, and the clutch-based MPT (multiplate transfer clutch) center differential. The clutch-based MPT is most common in older Foresters. The VTD can be found in most newer 4EATs, WRXs, etc. The main difference in operation is that the VTD needs 0V to lock up, whereas the clutch-based differential needs 12V to lock up. We have found the VTD to be more reliable when locking and staying locked because it is mechanical, whereas the other system relies on clutch plates and fluid pressure.

Our shop Forester is a 2003 into which we have swapped a VTD center differential. If you are doing this swap, note that the TCU in the Forester does not correctly communicate with the VTD differential because the two systems operate opposite from each other.

There are two options for wiring in a center differential lock:

Option #1 (Best for a Forester with a VTD Swap)

We don’t want the TCU controlling the center differential in this case. To fool the TCU, we need to make sure it reads the correct resistance; otherwise, the transmission light will flash because it thinks something is wrong.

From there, we want to wire in switched 12V power (key off = 0V). This will ensure you don’t forget to turn the switch off and kill your battery. Next is a standard 12V switch or button. These can vary in how they are wired, and most will come with a diagram showing the correct wiring.

Next is a dimmer switch. This isn’t required, but I recommend it for controlling the center differential and keeping the vehicle from being front-wheel drive. It is the best option for fully controlling how your center differential operates around town and off-road. You will need a 0–12V dimmer, which can be found here: DIMMER SWITCH.

Also optional is a live readout of your center differential voltage. You can use this or something similar: 0–12V Readout.

Last but not least, the wire from the readout will lead back to the center differential. This is where you cut the wire at the TCU that leads farther into the wiring harness.

On the TCU side, you need to fool the TCU into thinking it is still connected. You can find the exact resistance requirements in your vehicle’s factory service manual. In our case, we used a 25-watt, 15-ohm resistor wired directly to ground. It still gets a little hot, so a higher-wattage resistor could help, but it works fine and the transmission light does not flash.

Option #2

This option is recommended for those still running the factory center differential. A Double-Pole, Double-Throw Switch is needed. This allows the TCU to continue controlling the center differential when you don’t want it locked or don’t want to control it manually—most likely during normal road use.

When you activate the switch, it will switch the TCU circuit to the resistor while sending 12V to the center differential. You can also wire in the dimmer and voltage readout with this setup if desired.

Remember, the VTD needs 0V to lock up, so it requires a ground instead of 12V. We grounded ours at the dimmer switch. The exact wiring will vary depending on the application and which wire needs power or ground, as there are three wires on the back of the dimmer. If you have a VTD and don’t want to use the dimmer, the 12V power supply described above will be replaced with a ground.

If you have any questions about this modification, let us know!