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Is my AWD full-time?


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22 replies to this topic

#1 Bushwick

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Posted 21 October 2013 - 03:17 PM

Was reading another, current thread where they wanted to run a switch to force full-time in a newer model and talked about a FWD fuse? Is my auto 95' not full-time AWD? Is it FWD with the the rear getting power only on traction loss? Noticed up against the firewall, passenger side, a little black cap with "FWD" imprinted. Assumed it was a fuse holder or something and haven't messed with it. What exactly is it? If the AWD isn't full-time, I'd very much like to incorporate a switch to have that option in heavy snow, etc. Can someone explain how I'd remove voltage from the solenoid? Where is the solenoid and the wiring needed spliced to? Much appreciated. 



#2 matt167

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Posted 21 October 2013 - 03:25 PM

Yes and no. If it's an automatic, than it does have an electric clutch that regulates the power going to the rear tires.. Manuals have a viscous coupling which does the same thing.. If you get a flat tire, your supposed to put the right size tires on the front, and put the FWD fuse in, and then you have forced FWD.. There is no switch you need to flip for AWD. It's always there



#3 Fairtax4me

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Posted 21 October 2013 - 03:51 PM

It is full time, as the rear drive is never fully disengaged under normal conditions, but when needed the TCM will clamp down the transfer clutch pack and give varying amounts of power to the rear wheels. It can go from about 10% slip, to full lock depending on how much speed difference there is between the front and rear wheels.

The FWD fuse is there to fully disengage the transfer clutches for times when you need to use the spare wheel. This is used to prevent damage to the transfer clutches because of the different diameter of the temporary spare.

#4 Bushwick

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Posted 21 October 2013 - 03:52 PM

OK, so the AWD is constant but has a bias. You are saying the FWD fuse needs put in for "forced" FWD? What is the fuse rating so I have one around, just in case? Also, any ill-effects to forcing the FWD only for long periods? Thinking of running a fused switch to that socket for FWD daily driving (better MPG possibly) and AWD for wet or winter weather. Does the engine need to be OFF for the forced on/off? i.e. ECM checks for voltage at start up and adjusts accordingly? In "forced FWD", do the fronts see a true 100%, or is it still split?


Edited by Bushwick, 21 October 2013 - 03:53 PM.


#5 Fairtax4me

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Posted 21 October 2013 - 05:42 PM

Yes, basically, putting the fuse in disengages the AWD clutch pack, which limits the transmission to only FWD.
The way the trans is designed the front wheels are driven 100% of the time. So technically its always in FWD, but the rear wheels get a small amount of power under normal conditions, and increased amounts of power when throttle position or wheel slip demands more power to the rear wheels.

Fuse rating doesn't matter as its just a signal circuit from and back to the TCM.
You can rig a switch to flip back and forth on the fly.
No real gains in MPG since all of the weight of the AWD components is still there and they all still spin when the car moves.
Some people think running in FWD mode all the time will wear out the Duty C solenoid (line pressure solenoid for the transfer clutch pack) because the TCM commands it to 100% duty cycle when the fuse is in place But plenty of people have driven for years with the FWD fuse in, and their Duty C solenoids still work. Others have had different experiences. Either way the duty c solenoid is a fairly common failure, which makes it hard to say if putting the fuse in really does make it fail any sooner.

#6 MilesFox

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Posted 21 October 2013 - 07:53 PM

The fuse will be 15 amp(blue). Running in 'forced' FWD will overburden the duty c solenoid (100% duty cycle) causing it to potentially burn itself out.

 

As you mentioned the switch mod to fully disengage the duty c (thus locking in 4wd), it is possible to run as a RWD in the instance of a broken front axle to get you home. I had to resort to this. I simply cut the #11 pinout (grey wire) on the trans pigtail under the hood, and removed the inner axle from the font (leaging the outer part in the hub) and ran it as a rwd delivering pizza for a week unti i could replace the axle.

 

Nice things about these cars, is that the same car can be a FWD, 4WD, or RWD



#7 Bushwick

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Posted 22 October 2013 - 10:32 AM

@Fairtax  Yes, curb weight remains the same, but the engine has to work harder actually powering the rear axle and turning the drive shaft, along with the front. Some cars the MPG drop between FWD and identical AWD counterpart is minimal (like no change in city, 1 MPG drop on highway) whereas other cars have a more significant drop in city and highway. The .gov site says there's a 2 mpg drop in city, and 3 mpg drop on the highway between a FWD wagon vs. AWD. http://www.fuelecono...ru_Legacy.shtml   What's very interesting to me is the practically identical rating being manual trans vs. the auto, and no difference between wagon vs. sedan (which I find hard to believe given the raised roof and different amount of drag). Given the rather large highway mpg drop, I'll gladly risk a 3 mpg+ gain running forced FWD over the solenoid's longevity, especially considering how expensive gas is now (20 gallons + 3 mpg = 60 more miles). The fact they didn't include a real switch in the dash might support the argument that it could hurt the solenoid's longevity. Think a trade off will be running FWD on highway runs since city mpg is virtually identical.

 

 

@Miles When you pulled the front CV shaft from the trans, didn't it leak trans fluid? 

 

 

 

 

BTW, to get to the "C-solenoid", take it you have to unbolt the trans? If the drive shaft is dropped, can you just slide the trans back towards the firewall and unbolt it? Seems rockauto doesn't sell one, where are you guys buying yours?



#8 MilesFox

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Posted 22 October 2013 - 11:34 AM

The front axle is on a stub held on by a roll pin. The outer cv was busted. I simply popped out the pin, and pulled the axle out leaving the outer spindle with the hub. i turned the wheel sharp to pull the broken end out of the spindle. It only required a hammer and a drift punch.

 

You could take an axle apart and keep the spindle part and install that to delete a front axle if you were going RWD on purpose. How hard on the rear drivetrain 100% will be based on how you drive. I have done RWD with both the 4eat and part time 4wd ea82 manual trans.

 

Based on the posts in this forum, you remove the tail housing to the trans to get at the duty c. You can remove the trans crossmember and the pitch stopper and let the trans drop down enough to get to it without unbolting it from the motor.



#9 WoodsWagon

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Posted 22 October 2013 - 03:14 PM

The FWD fuse makes no MPG difference on a AWD equipped car. Instead of the transmission directly powering the rear driveline, it's powering it through the road. The power goes to the front wheels, moves the car, the back wheels get turned, and they then spin the rear axles, differential, and driveshaft. So all the same mechanical parts are being turned, and they are creating the same amount of drag and mpg loss.

 

The only way to see a MPG gain is by physically breaking the rear axles. You need the ends of the axles in the hubs to keep the bearings together, so you need to break apart the CV joints to remove the middle part of the axles. It makes it a lot of work to switch back to AWD if you want it again.

 

If you drive reasonably, high 20mpg's on the highway and low 20's around town are to be expected. That's not bad for a decently large wagon.



#10 Fairtax4me

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Posted 22 October 2013 - 08:27 PM

This isn't like a 4wd truck where you disengage the 4wd and the hubs and only the wheels spin, while the axles and prop shaft sit still.
The rear axles, rear diff, and rear driveshaft are still connected to the wheels, so they still turn when the car moves even though they aren't "powered".

The differences in fuel economy between an AWD and FWD Subaru are partly due to different final drive ratio (3.54 FWD, 3.90 or 4.11 AWD)
And less weight because of the lack of AWD components. The transmission weighs about 30lbs less, prop shaft, rear diff, rear axles are another 150lbs.

The other part is the decrease on driveline drag because the engine doesn't have to turn the rear axles, diff, and the prop shaft.

Just switching yours to FWD will not make a noticeable difference without removing the rear drive parts.

#11 subnz

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Posted 22 October 2013 - 11:05 PM

Was reading another, current thread where they wanted to run a switch to force full-time in a newer model and talked about a FWD fuse? Is my auto 95' not full-time AWD? Is it FWD with the the rear getting power only on traction loss? Noticed up against the firewall, passenger side, a little black cap with "FWD" imprinted. Assumed it was a fuse holder or something and haven't messed with it. What exactly is it? If the AWD isn't full-time, I'd very much like to incorporate a switch to have that option in heavy snow, etc. Can someone explain how I'd remove voltage from the solenoid? Where is the solenoid and the wiring needed spliced to? Much appreciated. 

 

l had wondered  why the differences in transmissions to how AWD works so this is explained well in this link

I think I posted this link that previous thread also.

 

http://www.awdwiki.com/en/subaru/



#12 Bushwick

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Posted 22 October 2013 - 11:58 PM

The weight of AWD wagon shouldn't really matter at highway speeds unless constantly climbing hills, and these wagons are actually pretty light compared to other cars. My 99' 9-3 is a 4 door hatch FWD and had a 3000 pound curb weight before I pulled 300 from it. My old Lincoln Mark VIII with DOHC V8, auto, and roughly 3800 pound curb weight got 28 on the highway. I was able to get that to 32 highway with full synthetics (trans, engine, rear) and freer flowing turbo mufflers. Adding 200 pounds of amp, subs, etc. saw a change from 32 to 33 on the highway (real time gauge on the dash) with cruise on and flat roads. Also lowered 2" with sensor bracket mod to lower the bags helped get closer to 32.

 

I still might install a switch for convenience reasons and will do some testing later between forced FWD and leaving it alone to see if there is indeed any difference, but I'm guessing the switch to steeper gearing is why the drop is most pronounced. But gotta say if this has 4.11's and can get 26-27 on the highway, it's still fairly impressive.



#13 MilesFox

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Posted 23 October 2013 - 08:14 AM

If you are into modding, you can tap into manual mode to start off in a selected gear. The TCU has it although there is no button on the shifter. worth looking at if you are getting into such a project.



#14 Bushwick

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Posted 09 November 2013 - 07:33 PM

If you are into modding, you can tap into manual mode to start off in a selected gear. The TCU has it although there is no button on the shifter. worth looking at if you are getting into such a project.

 

What are you referring to? Are you talking about manually holding a gear 1, 2, 3, etc.? Or are you saying there's a connector you splice into for a different effect? Can you elaborate?



#15 MilesFox

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Posted 09 November 2013 - 10:44 PM

The first gen legacy had the manu mode button on the shifter. They did away with it in the 2nd gen, but the TCU still has it in its circuitry. Also, some overseas spec 2nd gens had the feature. Yes, this feature allos you to lock in a gear and start off in 2nd gear. I have not oem the mod myself, but i could figure it out by comparing schematics. What i am saying is the TCU has the feature if you tap the pinouts to use it. there should be articles in the archives as i am mentioning the idea having read about it on the forum. As far as where to tap in, i would need to see a schematic and poke at harnesses to figure it out.



#16 Bushwick

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Posted 09 November 2013 - 11:12 PM

I haven't tried downshifting into 1st or 2nd while moving (if car was turbo'd then that'd be a different story), couple times I've forced 3, but what's the difference between what you are saying and dropping the lever into 2nd? Wouldn't it start in 2nd and hold 2nd til redline?



#17 MilesFox

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Posted 10 November 2013 - 12:36 AM

It would start in 1 and shift into 2 and stay there. It would probably shift if you forced it long enough. this is an electronically shifted trans with no vacuum modulation. Therefore it can be electrically manipulated.

 

There is also a 'power mode' that works something like being in manual mode and a signal from the kickdown switch will alter the shift map and hold the rpms out longer.



#18 Bushwick

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Posted 11 November 2013 - 12:29 AM

A power mode would be nice. Haven't seen that in a car for some time. I take it the "Power Mode" on/off could be manipulated with a simple rocker switch? Or do you think you need to create jumpers, etc. internally plus a switch?



#19 eulogious

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Posted 12 November 2013 - 02:10 AM

For power mode all you need to do is ground out a pin on the tcu to activate, if your tcu has that "option". The power mode really is not that impressive though, but I can help like right before you pass a car.

The awd "lock" is a great mod to do as well. I had a switch wired into a dummy resistor pack so that the tcu wouldn't throw a fit when I disconnected the duty c from the tcu. Came in handy during a ice storm a couple of years back. Great to have that switch for those type of situations.

#20 Bushwick

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Posted 12 November 2013 - 05:29 AM

For power mode all you need to do is ground out a pin on the tcu to activate, if your tcu has that "option". The power mode really is not that impressive though, but I can help like right before you pass a car.

The awd "lock" is a great mod to do as well. I had a switch wired into a dummy resistor pack so that the tcu wouldn't throw a fit when I disconnected the duty c from the tcu. Came in handy during a ice storm a couple of years back. Great to have that switch for those type of situations.

 

Well, I'm guessing it'll increase line pressure while raising the rpm shift point which can be nice at times as it transmits power more efficiently. Especially when you don't have the extra hp from a turbo'd set up or maybe a six cylinder for hills or passing a 400hp minivan :mellow: I know the old AODE Fords had a screw under the pan cover that would let you increase line pressure, which would wake the tires up :) 

 

Any idea where this pin to ground is? Where is the TCU? Also, if possible I'd like to repurpose a dash switch to use on the left side of the column cluster. My right side has rear defrost and cruise button. Left side has 2 dummy covers that'd be a perfect spot for this "power" mode switch as well as a FWD switch for the FWD fuse slot. If at all possible, I'd like to try and keep the switch's appearance "correct". The years where power mode option was used, any idea what the front of the switch looked like, or was that on the shifter stalk? Since the FWD has a dash light, not worried there as it's easy to cover up a defrost switch and use that. But w/o an indicator light for power mode, it'd be nice to use a correct switch with an insignia, even if it has to be pirated from another car and glued in place of say the insignia for a cruise control sticker. Any ideas for donors? 



#21 eulogious

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Posted 15 November 2013 - 05:34 PM

Well, I'm guessing it'll increase line pressure while raising the rpm shift point which can be nice at times as it transmits power more efficiently. Especially when you don't have the extra hp from a turbo'd set up or maybe a six cylinder for hills or passing a 400hp minivan :mellow: I know the old AODE Fords had a screw under the pan cover that would let you increase line pressure, which would wake the tires up :)

 

Any idea where this pin to ground is? Where is the TCU? Also, if possible I'd like to repurpose a dash switch to use on the left side of the column cluster. My right side has rear defrost and cruise button. Left side has 2 dummy covers that'd be a perfect spot for this "power" mode switch as well as a FWD switch for the FWD fuse slot. If at all possible, I'd like to try and keep the switch's appearance "correct". The years where power mode option was used, any idea what the front of the switch looked like, or was that on the shifter stalk? Since the FWD has a dash light, not worried there as it's easy to cover up a defrost switch and use that. But w/o an indicator light for power mode, it'd be nice to use a correct switch with an insignia, even if it has to be pirated from another car and glued in place of say the insignia for a cruise control sticker. Any ideas for donors? 

Power mode really just changes the shift mapping.  It might change pressure, but I don't think it does.

 

A power mode button was only offered in the Japanese market and maybe the European market, and only for a couple of years, and it was on the shifter stalk as well, IIRC.  I think the American market got the manual button, and the others got the power button.  But I could be wrong on that, its been a couple of years since I was looking into this.

 

I wouldn't install a switch for the FWD, I would install a switch to "lock up" the AWD.  Much more useful.  And yes, you should just be able to find some stock switches and wire them up the way you want.  I did this in my old loyale and it worked great.  As for a power mode switch, I have no idea, lol.  Maybe grab a traction control sticker off the traction control button on a newer car?  That might be a cool insignia for it.

 

As for the location of the TCU, it should be up under the dash.  My 1990 leggy is located under the dash, in the middle, but it is different than a 1996.  And the pin, I have no idea off the top of my head.  I have a 1996 FSM in pdf format around here somewhere, I will try to look up the info for you this weekend and post back if you haven't figured it out by then...


Edited by eulogious, 15 November 2013 - 05:35 PM.


#22 eulogious

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Posted 15 November 2013 - 06:01 PM

Found a much better explanation for ya on this site non-the-less:

 

http://www.ultimates...5-99-4eat-tcus/

 

That should explain it all for you...



#23 Bushwick

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Posted 16 November 2013 - 04:20 AM

Thanks for the link! Sounds pretty easy to get at. I know some 89' Ford Probe had this, but finding one to pirate the insignia from the cluster would be too difficult. One of those SVX's would be easier (the cluster probably still has the correct cover, just no light). Thinking of using a dummy light slot in the cluster (plenty of blanks to choose from or pop out the air ride cover) and swapping insignias while running a simple bulb holder on pigtail.

 

The FWD switch would be more of a convenience (like a low air rear tire which I seem to keep getting) or to mess around in the snow, etc. Would just be nicer to do from the driver's seat as opposed to stopping, raising hood, fumble in dark, etc.


Edited by Bushwick, 16 November 2013 - 04:22 AM.





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