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Premium Member
2007 Forester XT 4EAT-VTD
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2,145 Posts
Discussion Starter #1
This mod is for a Multi Plate Transfer (MPT) to Variable Torque Distribution (VTD) swap using either just the transfer case or the entire 4EAT-VTD transmission. It removes control of the VTD clutch pack from the TCU, using a switch instead. Like an off-road 4WD vehicle, the “center-diff-lock switch” is intended only for use in low traction conditions, such as snow or mud.

Background

The original 4EAT-MPT failed in my '07 XT Ltd. at 113,000 miles. Though I have yet to dismantle it, it was a catastrophic failure under normal driving conditions that took out the entire transmission with metal shards/fragments. I believe it was the MPT transfer case that failed, but I will confirm that upon completion of the autopsy.

The 4EAT-MPT was replaced with a low-mileage 4EAT-VTD out of a '07 Forester Sports. As I have reported in my journal, I have found the VTD to be a major improvement over the MPT. The only issue is that with the MPT TCU, the FWD fuse must remain in place to avoid improper locking of the VTD clutch pack and error codes due to the VTD having a different rear shaft speed sensor. Of course, the VTD transfer case offers true full time AWD with its planetary gear set so the clutch pack is not needed to send power to the rear wheels as long as all wheels have equal traction. However, the VTD is an open differential, meaning that if traction is lost in the front, no power will be sent rearward. This is when the VTD clutch pack is needed, to lock the front and rear shafts together using the center-diff-lock switch.

Though installing a switch to remove control of the clutch pack from the TCU and control it manually is a relatively easy task, I had a few requirements;


  • I need the switch near the shifter, where my right hand spends a lot of time. The idea is to be able to engage/disengage the lock quickly and easily without having to look/reach for it.

  • I prefer a switch that is easy-to-reach yet unobtrusive aesthetically

  • I am concerned about unintended switch activation, so along with being easy to reach it also has to be out of the way of things that are normally placed on or around the console area and out of the way of normal driver/passenger activity

  • I require easy-to-see (under all conditions) visual indicators when the lock is engaged so that if it does become unintentionally activated, I will know immediately but yet won't be distracting when intentionally activated
Results

The switch worked out as expected and the VTD clutch pack locks when the switch is engaged. Photos below of the shifter-mounted switch and the 4WD LOCK indicator light, parts used and wiring/installation info in separate posts below;










Diff lock engaged, indicator light on;



Detail of wire running from switch to under boot;

► Edit - moderator's note - the images on this post are not recoverable. :frown:

Bobby...
 

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Super Moderator
2018 2.5i Premium CVT
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17,892 Posts
Note by moderator - Canonshooter recently started a thread about accessory switches, gathering information in preparation for his upcoming Diff Lock mod.




It became quite useful in its own right and is now a sticky in the Interior and Electronics Forum, gathering other contributions regarding accessory switches. But then the thread started to verge into the details of the mod. So I have moved those posts into this new thread in order that the Diff Lock mod might stand by itself in the proper forum for the benefit of all who might wish to follow this project and learn from it. As there could be a few cross-references back to the original thread, you might wish to check it out as well to get the full picture.
 

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Registered
09 Fozzy XT 4 Hungry aka 4EAT
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2,126 Posts
Perfectly well executed as well as worded. This will definitely bring smiles as well as Q & A for years to come... :grin2:
 

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Premium Member
2007 Forester XT 4EAT-VTD
Joined
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2,145 Posts
Discussion Starter #5 (Edited)
Wiring and Installation


  • The relay is installed in the console by the shifter

  • The switch is wired to the coil side of the relay to open and close it

  • The wire from the TCU to the clutch pack (terminal B54, number 5 on my TCU) was cut and the clutch pack solenoid was wired to the normally-open relay contact

  • The 4WD LOCK indicator is also wired to the normally-open relay contact, in parallel with the wire to the VTD clutch pack solenoid

  • The resistor was wired to TCU terminal B54, number 5 on one side and to ground on the other, and secured to the metal TCU mounting bracket. This is needed to prevent error codes due to cutting the wire between the TCU and the clutch pack solenoid.

  • Power for the circuit was tapped from the console 12V outlet using a 10-amp inline fuse

  • A SAE 2-pin connector was used between the shifter-mounted switch and the harness that can be separated and allow the shift knob to be unthreaded and the console shifter trim to be removed

  • All connections were soldered then covered with shrink tube and/or tape

Below is the section from the 2007 TCU pin in/out chart. The far right column is ohms, the column to the immediate left is volts;


► Edit - moderator's note - the image on this post is not recoverable. :frown:

Bobby...
 

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Premium Member
2007 Forester XT 4EAT-VTD
Joined
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2,145 Posts
Discussion Starter #6 (Edited)
Testing

The 4WD LOCK indicator illuminates when the switch is turned on and is easily seen even in daylight conditions.

To test clutch pack operation, I placed the car on a slight incline in neutral with the wheel turned and the clutch pack engaged. As the car slowly rolled forward, torque-bind was evident and the car came to a stop. Turning off the switch gave an audible "clunk" and the car began rolling forward again. All appears to be working correctly.

The 50-watt 2.7 ohm resistor gets quite warm when the car is driven, but there is less then one volt going through it with the FWD fuse inserted. I will keep my eye on it and will probably wire a second identical resistor in parallel with this one to better dissipate the load.

With the FWD fuse inserted, the AWD dash light remains on. Terminal B55, number 13 on the TCU is for the AWD light and when I disconnected it, the light went out. I will keep it connected for now until I can figure out a better solution.

The mod worked out just as I envisioned: I can manually lock the center diff when needed and allow the normal 45/55 F/R split through the VTD planetary gear set operate at all other times without using the clutch pack. Though the real test will be this winter in the snow, I have no doubt it will perform admirably.

If you have any questions about this mod, please post them below or feel free to PM me.
 

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Premium Member
2007 Forester XT 4EAT-VTD
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2,145 Posts
Discussion Starter #7 (Edited)
AWD Warning Light Solution

According to the Owner's Manual, the AWD warning light in the instrument cluster indicates the following;


  • Steady on indicates the FWD fuse is inserted
  • Blinking indicates a detected speed difference in the front and rear shafts due to using different size tires
Since the FWD fuse must remain inserted with this mod and detecting a difference in shaft speed is a moot point since the use of the FWD fuse makes the TCU ignore those readings, the AWD light is of no practical use left as is. Therefore, my goal is to have the AWD light illuminate with the 4WD LOCK indicator whenever the diff lock switch is engaged.

Below is the section from the 2007 TCU pin in/out chart showing the connection to the AWD indicator;




Under normal circumstances, the AWD light illuminates when the ignition is turned on but without the engine running: it goes out when the engine is started. I figured that disconnecting the wire from the TCU and applying voltage to the lead to the light would make it go out but in fact, once the lead was disconnected from the TCU, the light went out. As far as I can tell from driving the car with it disconnected, there are no other unintended consequences.

The wiring plan to use the AWD light as a second indicator that the diff lock is engaged is to install a relay that will keep the light disconnected from the TCU (and off) when the diff lock switch is off and reconnect it to the TCU through the relay to illuminate it when the switch is turned on. As it turns out, this will be a very simple task so I have another relay on order.

My last finishing touch to this mod is actually an important one - to install a more robust resistor to the TCU clutch pack solenoid lead. With the diff lock mod for the MPT transfer case, the resistors are only needed when the switch is engaged - with this mod, they are needed all the time.

As you can see in the chart below, with the FWD fuse inserted the TCU supplies less than one volt and expects to see between 2.0 and 4.5 ohms of resistance;




The 50 watt, 2.7 ohm resistor I am using is actually adequate for this but it does get quite warm. To ensure longevity and avoid failure of the resistor, I am replacing this resistor with two 100 watt, 3.3 ohm resistors wired in parallel and mounting them on a square, 1-inch aluminum tube for greater heat dissipation.

The new resistors/heat sink and additional relay for the AWD light will be secured to the metal TCU mounting bracket. I will update this thread with photos as soon as it is complete.
► Edit - moderator's note - the images on this post are not recoverable. :frown:

Bobby...
 

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Premium Member
2007 Forester XT 4EAT-VTD
Joined
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2,145 Posts
Discussion Starter #8
Diff Lock Switch Install Complete

I completed the switch 100% today, which included controlling the AWD dash light with the diff lock switch and installing bigger resistors to absorb the continuous TCU output from pin B54-5. It's nice having the AWD light off, and it's an easily-noticed indicator when the center differential is locked. When the diff lock switch is engaged, both the 4WD LOCK indicator I installed and the AWD indicator in the dash come on together.

To control the AWD light, a second relay was installed in series with the first relay: when the primary relay (controlled by the switch) is turned on, 12 volts is sent to the transfer duty solenoid to lock the clutch pack, so it was an easy matter of tapping into that normally open lead from the primary relay to power the coil of the second relay. When the lead to the AWD light is disconnected from TCU pin B55-13 (as in cut), the light goes out so it's simply a matter of wiring that lead through the normally open contacts of the second relay.

Though less than one volt is delivered from pin B54-5 to the transfer duty solenoid with the FWD fuse installed, the load is present when ever the engine is turned on. Because of this, I decided to go total overkill on the resistive load to keep heat to a minimum and maximize the life of the resistors. The single 50 watt, 2.7 ohm power resistor was replaced with a pair of 100 watt, 3.3 ohm power resistors wired in parallel. The resistors were attached to a short piece of 1" aluminum angle stock, which in turn was fastened to the metal TCU bracket. The secondary relay was also secured to the TCU bracket. Here are the resistors and second relay attached to the TCU bracket before bolting it in place;





The TCU reinstalled, a shot straight up into the back of the dash;




The resistors get just barely warm to the touch, which will ensure long life and reliability. The mod is complete and working as expected!
► Edit - moderator's note - the images on this post are not recoverable. :frown:

Bobby...
 

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Registered
2004 Forester X Auto
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94 Posts
CanonShooter, AFAICT, all of your pictures have disappeared. I believe that I'm in search of MPT posts on this subject so it doesn't matter too much to me but I wanted to ping the thread to let you know that this important forum thread appears to have lost the pics.
 

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Super Moderator
2007 Forester Sports XT 4EAT
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