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4EAT Subaru Center Diff Lock

Subaru Forester R160 Torq Locker with VTD Center differential Controller demonstration

There are 2 types of center differential on the 4EAT's. VTD (variable torque distribution) which is a mechanical based center differential and the Clutch based Center differential. The clutch MPT (multiplate transfer clutch) based is most common in the older forester. The VTD can be found in most newer 4eat's WRX's ect. The main difference in opperation is the VTD needs 0v to lock up, where as the Clutch base needs 12v to lock up. We have found the VTD to be more reliable in its locking up and staying locked because it is mechanical where the other is replying on clutch plates and fluid pressure.

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

There are 2 options to wiring in a center diff lock, 

Option #1 (best for Forester swapped VTD) 

We dont want the TCU to control the center differential in this case. So to fool the TCU we need to make sure its reading the correct ohms (or the Transmission light will flash) as it thinks something is wrong. Front there we want to wire is a switched 12v power (key off = 0v) This will insure you dont forget to run the switch off and kill your battery. Next is a standard 12v switch of button, these can very on how they are wired and most will come with diagrams ect to show how they should be wired in. Next is a dimmer switch, this doesn't have to be done but to control the center diff and keeping it from being front wheel drive i recommend this as its the best option to fully control how your center differential acts around town and off road. This will be a 0-12v, can be found here DIMMER SWITCH, Also not needed but to see your live read out of your center diff voltage use this or something similar. 0-12V Read Out  Last but not least after the readout it will lead right back to the Center differential. This is where you cut the wire at the TCU leading further into the harness. 

On the TCU side you need to fool the TCU to think its still connected. In your vehicles FSM you can find the exact requirements for this. In our case we ran a 25Watt 15 Ohm resistor and wires it right to the ground. It still gets a little hot, could go larger to help this but works fine. No Trans light flashing. 

 

Option #2

This one is recommended for those who are still running the factory center diff. A Dual Pull Dual Throw switch is needed. This allows you to keep the TCU controlling the center differential when you don't want it to be locked (of you don't want to be controlling it) Most likely for normal road use. When you hit the switch the dual pull will switch to the resistor, and switch 12v leading to the center diff. You can still wire in the simmer and read out if wanted on this set up as well.

But remember, the VTD will need 0v to lock up. So this will require a grounding instead of 12v. We grounded our at the dimmer switch which will very on application on which wire needs power and which needs grounding as there are 3 wires on the back on the dimmer. If you don't want to run the dimmer, and have the VTD the 12V power Supply will be a ground Instead.   

 If you have any question about this modification let us know! 

 


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