"Manual" control for the AWD

[Skip]

Although this subject has been covered.

I thought I'd throw my method into the mix.

Comments are encouraged.

 

 

 

While wiring my transmission shift pattern display

(see http://www.ultimatesubaru.org/forum/showthread.php?t=54985 )

I had the opportunity to connect a control system

so I could switch the rear drive “full on” at my discretion.

 

 

Note: this work is performed in the vehicle

working under the dash.

Removing the TCU makes life much easier.

It is held in with two 10 mm nuts and

is very easy to access.

(Please see above article for a photo of the location.)

 

I wanted to do this in a fashion that

1)would have convenient switch

2)would have an indicator

3)would not cause the TCU to get it’s panties in a bunch

 

Looking at the schematic for the TCU,

I noticed the “Manual” switch on the gear selector lever

was wired to provide a ground to a pin on the TCU.

This ground also illuminates the dash indicator lamp.

 

This afforded me a simple solution as I never use the “Manual” switch.

 

Next, using a multimeter I measured the pure resistance of the duty solenoid C's coil.

It measured around 15 ohms.

 

Wired as shown,

1) the resistor keeps the ECU happy when locked in AWD,

2) the “Manual” indicator on the dash illuminates to tell me I am in manual control AWD,

3) the switch is very easy to reach and is pre-installed.

 

Note: Duty solenoid C controls (opens) a hydraulic pressure drain valve for the rear drive clutch pack.

When DsC is not energized the drain is closed

and the clutch pack locks the rear drive to the engine output.

 

 

Simply opening the circuit to duty solenoid C (causing the drain valve to close)

will not cause TCU problems unless you leave it open and stop and start the engine.

 

 

The TCU does a diagnostic check each time the engine is switched on.

It will detect the open circuit and flash the “Power” light indicating a control error.

Should you leave this system in the manual mode,

the resistor will prevent the TCU from “seeing” an open circuit.

 

If you have any questions about this circuit, please

click on my board name and email me.

[nipper]

Simply opening the circuit to duty solenoid C (causing the drain valve to close)

will not cause TCU problems unless you leave it open and stop and start the engine.

 

 

The TCU does a diagnostic check each time the engine is switched on.

It will detect the open circuit and flash the “Power” light indicating a control error.

Should you leave this system in the manual mode,

the resistor will prevent the TCU from “seeing” an open circuit.

 

 

You could get around the mistake of leaving the the car in the AWD On mode by using a latching relay. This method when releasing the switch or shutting the car off, the relay would unlatch. Another possability is to modify the switch so that it doesnt operate as on/off and only as a momentary switch (this too would work with a latching relay, press once for on, then once for off). This would require you to latch the relay again manually on startup. Since you have a visual indicator, you dont need to know what position the switch is in to tell if the AWD is fully engaged or not.

 

nipper

[Skip]

Thanks for the reply Nipper

but

if one incorporates a "latching relay"

They would have to modify the manual switch to be

momentary.

 

The latching relay would have to be a flip flop style

first pulse - on

second pulse - off

If it were not a flip flop, there would be no way of turning the system off

aside from turning off the ignition switch

or killing the power to the relay

(via another switch)

 

The SPDT relay is much easier to obtain

Some of the ice cube styles are configured this way.

 

I tend toward the KISS method when at all possible.

[nipper]

Thanks for the reply Nipper

but

if one incorporates a "latching relay"

They would have to modify the manual switch to be

momentary.

 

The latching relay would have to be a flip flop style

first pulse - on

second pulse - off

If it were not a flip flop, there would be no way of turning the system off

aside from turning off the ignition switch

or killing the power to the relay

(via another switch)

 

The SPDT relay is much easier to obtain

Some of the ice cube styles are configured this way.

 

I tend toward the KISS method when at all possible.

 

There are many configurations of relay mechanical and electrnoic to make it do this, and it is still a KISS system. Too many years working as a manufacturing technologist (fancy gov't term) i like things are idiot proof

 

nipper

 

PS cool tranny display

[Skip]

thanks for the props on the display unit Nipper

It does work rather well.

 

On the manual AWD control.

The resistor and the "MANUAL" light on the dash

go a pretty long way to making it idiot proof, in my opinion.

 

I have not removed my manual switch as possibly you have

so I am just guessing here.

But I think modifying the

switch on the selector lever so that it is momentary

may be more difficult than simply

cutting and splicing some wires going to the TCU.

 

Obviously one could add a momentary push button but I like the convenience of

the switch on the gear lever.

 

Thanks again for your replies and your thoughts.

[johnceggleston]

does my 95 leg or 97 obw have a manual switch?? if so i've never seen it or operated it. must be a 90 - 94 thing??

 

 

 

thanks for the props on the display unit Nipper

It does work rather well.

 

On the manual AWD control.

The resistor and the "MANUAL" light on the dash

go a pretty long way to making it idiot proof, in my opinion.

 

I have not removed my manual switch as possibly you have

so I am just guessing here.

But I think modifying the

switch on the selector lever so that it is momentary

may be more difficult than simply

cutting and splicing some wires going to the TCU.

 

Obviously one could add a momentary push button but I like the convenience of

the switch on the gear lever.

 

Thanks again for your replies and your thoughts.

[nipper]

does my 95 leg or 97 obw have a manual switch?? if so i've never seen it or operated it. must be a 90 - 94 thing??

 

No we dont have them, they stoped at some point. i was going to put mine in the sonsole someplace.

 

 

nipper

[pamike]

if i put a switch in the power wire going to the solonoid and left it on so the car could still control it for normal driving. would there be any problems with added resistance.(would it still be the same until the switch is flipped)

[Skip]

Mike, I read you mean the resistor would be in series with DsC when the switch is flipped?

I don't know what that would do to the control duty cycle signal.

But

you could wire a SPDT switch

with the common pole going to the TCU pin

one of the switch poles going to the resistor then to ground

the other switched pole going to DsC

 

You would not know it was in four wheel drive untill the binding occured.

Use a DPDT and wire a light to the other contacts??

 

Sorry about my lack of knowledge in reference to the newer cars. All my Legacys are 91 -> 94.