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2004 Forester XT
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Discussion Starter #1
MountainBiker (I think) posted in the old subaruforester.com forums about a method by which you could flip a switch to essentially lock the center diff when needed. At the time I didn't have the service manuals and couldn't figure out how to do the mod. Now I have the manuals, and can't find the old post because the archives are apparently gone.

So, I'm hoping someone (MountainBiker, you still here?) can shed some light so I can perform this little mod. It would have been very nice to have this winter, which is almost over now, but there's always next year.

If anyone needs the relevant parts of the service manual, let me know, I have them in PDF format and would be happy to share the parts necessary to get this thing going.

Thanks!

Zhe Wiz
 

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Hopefully we'll have the old database soon. There is no way that I'll re-write all that was in that thread!

Here is a conversation from ORS that should have all the salient details.
click me

The link takes you to the page in the thread where I've actually installed the switch. By the way, I prefer the way Kevin did it with the switch in the center console rather than in the dash. The wiring runs right there near the parking brake, you just need to find the correct wire, which is no problem with the service manual. Much easier to cut the wire, solder in the connections and mount up a switch rather than laying on the floor working under the dash! Plus, a switch in the ctr console is easier to get to on the fly than on the dash.
 

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Here we go again - from MB:

Okay, it works now! I added a dummy load to the TCM when in Diff Lock mode. I used a 15 ohm, 25 watt power resistor. 10 watts should've been enough, but the local electronics supply only had the 25'er.
I may experiment a bit more with the "dummy load" resistor value. As I expected, the 15 ohm resistor gets quite hot, as it has several watts to dissipate. If I use a higher value resistor, there will be less power to dissipate (remember Ohm's Law?) However, if I go too high, the TCM may call it a malfunction. Keep in mind that it is expecting a 10-17 ohm load from the transfer clutch solenoid (according to the manual).
I found that the diagnostics section of the service manual was the most helpful in determining what to check, since it spells most of it out for you. Its real easy to get to the Transmission Control Module (TCM) under the dash without removing any panels, and no need to get near the transmission. Also, you only need a voltmeter. The expensive Subaru Select Monitor is not necessary, though that would be a cool toy!
Look in the diagnostics table labeled "Transmission Control Module (TCM) I/O Signal", which has a drawing of the TCM connectors and the pin numbering. The important line in the table is called "Transfer duty solenoid". The wiring diagram is also helpful for determining the wire color. In the wire diagram, its called the "AWD solenoid". I got some additional info from another section that I can't seem to find now, but it told me some of the additional test voltages.
Here is what you need to do:
To test the signal being sent to your transfer clutch solenoid, attach the voltmeter to ground and to connector 15 of the plug #B54 going into the TCM. This was a white wire with a green stripe. I was able to push the probe of the voltmeter into the back side of the connector, so no wire stripping was needed, nor connector removal.
With the ignition on but the engine off you can test several things, the voltages required are listed, with my results in RED:
Throttle closed in "P"= <1.0 volts (0.13) Throttle closed in "D"= 5-7 volts (5.0) FWD fuse installed = >8.5 volts (I didn't check this mode).
Throttle fully open in "1st"= <0.5 volts (0.45)
I found that I had <0.5 volts in all gears as long as the throttle is 3/4 open or greater. This indicates full application of the transfer clutch to the rear wheels under this condition, so there will be no waiting for the computer to computer to detect slip.
Notice how the voltage changes based on throttle position. Check to see if you have <0.5 when 1st or 2nd gear is selected, and the throttle is NOT depressed, since that will tell you if you have 4wd when in those gears (I don't on my 2002).
Alright, what we need for this to work is an SPDT On-On switch, preferably one that looks good, is hard to accidentally activate, and has a light when on. I've found one that meets the first two criteria, but I can't seem to locate a decent SPDT lighted rocker switch.
For details of what an SPDT ON-ON is, see this page for switch types explanation. You do NOT want a SPDT with a center OFF position. If you want to activate a buzzer or other remote reminder when the center diff is locked, then you could use a DPDT On-On switch. I may go with a DPDT, and use the additional switch circuit to power an indicator light.
Wiring details (refer to the above link for switch terminal schematic):
-COM terminal gets connected to the wire leading to the transfer clutch solenoid -Terminal A connects to the TCM wire -Terminal B connects to a 100 ohm resistor and then to ground (I don't think the exact resistor value is critical, since it is just to make it so the voltage will dissipate from the solenoid in a controlled fashion).
In Position A, everything operates normally, in Position B, you will have full pressure on the transfer clutch, providing maximum power to the rear wheels, and therefore something approximating a center diff lock.
_________
 

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More from MB:

No reason to remove the pin from the connector. I cut the White/Green wire, and then soldered wires to each end of the W/G wire to get an extension to the switch. The switch, when in the "normal" position ("A" mode on the schematic), connects the W/G wire back to itself, like the OEM configuration.

When the switch is turned to "ctr diff lock" mode ("B" mode on the Schematic), the W/G wire coming from the solenoid should be switched to connect to a small resistor to bleed off the voltage (I've always had it this way, based on internet folklore to keep the solenoid from "bouncing", so I don't know what range of resistor values will work, or if one is really needed at all). The other half of your DPDT switch will connect the TCM side of the W/G wire to the power resistor, which is then attached to ground.

The key to make this work, is the W/G wire from the TCM has to be connected to two posts on the DPDT switch, as shown in the schematic
 
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Kevin said:
More from MB:

The key to make this work, is the W/G wire from the TCM has to be connected to two posts on the DPDT switch, as shown in the schematic
Thanks for restarting this one - is there a link to the schematic somewhere?

Although, thinking out loud here Kevin, in a previous post I mentioned that I got stuck with diagonally oposite wheels off the ground when crossing a ditch on an angle when we were talking about LSD or lack there of. If I had the centre diff locked and still no front or rear LSD's, wouldnt this have just made both the front and rear free floating wheels spin at the same speed, still with no power to the remaining wheels on the ground?
 
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Hi anyone interested. I have spent most of the afternoon scanning a few other sites and I must say hats off to MountainBiker for all the testing and thought he has put into this, and to Kevin for digging it all up.

After looking everywhere though, I was not able to find a diagram, so put my thinking cap on and have come up with my variation.

Cheers, Dave.

:chair:
EDIT: PLEASE USE THIS INFORMATION AT YOUR OWN RISK AND NOTE THAT THIS DIAGRAM IS ONLY BASED ON THE INFORMATION I HAVE GLEANED FROM SEVERAL FORUMS WRITTEN BY OTHER PEOPLE. I HAVE NOT VERIFIED THIS ON MY OWN VEHICLE AS YET
(mainly because I haven't found the the TCM unit....Kevin - might you be able to enlighten me as to where it is hiding one the Aussie models? Thought I had found it just toward the center of the vehicle from the brake pedal brackets, but that appears to the be the engine control unit, then on the passenger side I found the cruise control module - not having any luck)
 

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The TCM on a RHD is to the left of the steering column, the plug is near the steering column as I recall - it's "non-coloured" (white). There's a bunch (3 I think) of plugs together the plug colour and wiring colour may help decide which is which.

Also, I used 2 x 27 Ohm resistors (10 watts I think) in parallel to help dissipate the heat.

The diagram looks good to me!
 

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SeaComms said:
Although, thinking out loud here Kevin, in a previous post I mentioned that I got stuck with diagonally oposite wheels off the ground when crossing a ditch on an angle when we were talking about LSD or lack there of. If I had the centre diff locked and still no front or rear LSD's, wouldnt this have just made both the front and rear free floating wheels spin at the same speed, still with no power to the remaining wheels on the ground?
This wont stop a diagonal wheel spin (and the rear lsd wont do much either)
To get around that problem you would probably need locking diffs front and rear as well (such as air lockers, ones that can be forced into lock position by pressurised air)
 

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At first glance, SeaComms' schematic looks right. I'll dig up my old schematic, which is on my home computer.

BTW, if I did this again, I would probably capture the wire as it comes into the body under the center console. It is easier to work here (compared to under the dash), and a switch near the shift levermakes more sense then on the dash.
 

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SeaComms, just a couple comments about your schematic:
-Nice! Looks a lot better than my version!
-You don't need a DPDT switch, since you are using a DPDT relay.
-I wired mine a little different, but yours should work too. My way uses one less wire! But my schematic looks like :censored: !

BTW, the TCU is located in the same place on LHD cars, just to the left of the steering column! At least I think so, it's been over a year since I've taken a look at it.

Kevin: are we ever going to see the old database ressurected?
 

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MountainBiker said:
BTW, if I did this again, I would probably capture the wire as it comes into the body under the center console. It is easier to work here (compared to under the dash), and a switch near the shift levermakes more sense then on the dash.
Hey MB, I looked at the wiring running across the transmission hump a couple of inches toward the rear of the shift lever and couldnt see any white wire there. So I crawled under and had a look at the wiring come from the tranny and found 2 looms, one either side, both coming right up into the engine bay then joining into another single quite thick loom, that pokes back through the firewall to under the dash up to the location you describe (silver box on left of steering column with 3 connectors on it - thought he TCM had 2 connectors?? at least thats what I was looking for!).

Anyway, will have a better look when I get a chance. Oh, and thanks for the compliment on the schematic - got bored and was having troule picturing it so had to comit it to paper.... I decided to use a relay to keep majority of wiring close together and allow for a nicer and easier to get illuminated switch - found a nice black oval rocker switch at local auto accessories place with red LED built in (from SupaCheap Auto and also available with green or yellow LED if anyone interested).
Dave.
 

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I had heard from others that you could get to the wiring under the center console, but hadn't checked it out. Perhaps they caught the wire at the transmission, then pulled the wires into the interior? Still a better location for the switch than on the dash.
 
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I like that one MB, although, looks more like it should be for the hidden rocket launchers....
 

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sheepish said:
Armed and dangerous!

But I thought you weren't going to use a relay, and do I understand correctly that the centre diff locked is the default position for the solenoid?
Yes, you are correct. According to the manual, in the failed condition (TCM failure, Solenoid failure or wiring failure) the default position for the solenoid actually engages the clutch fully, acheiving 50/50 drive. I do find this unusual, especially with the comments made by others of the "binding" feeling on hard surface when in this mode.

One can only assume that Subaru is comfortable with the strength of the drivetrain to not fail totally under these circumstances. Obviously, there will be a lovely flashing light on the dash, although some of the backward cap wearing fraternity may mistake this for disco lights.....
 

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MountainBiker said:
By the way, I prefer the way Kevin did it with the switch in the center console rather than in the dash. The wiring runs right there near the parking brake, you just need to find the correct wire, which is no problem with the service manual.
I'm beginning to think I'm missing out on a lot without a service manual. Is there an offiical Subaru one or (to the Aussies) should I look for a Hanes or Gregories workshop manual?

FWIW I had a glance at SOA's online manual thing but couldn't be bothered seeing if I could register (being very much an out-of-towner). And "Sane" is the only thing I can ever think of to put for my State. :shrug:
 
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