Join Date: Jan 2006
Location: Sydney, Australia
Car Year: MY03
Car Model: XS Premium / Luxury
Transmission: 4EAT w/diff lock
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.