Full time FWD?

medic481

I just bought a 2010 Forester. While thumbing thru the owner's manual, I saw the section addressing using the spare tire on vehicles with automatic transmissions. The instructions are to disable the AWD, by placing a jumper (fuse) in the fusebox. This got me wondering- could you leave the AWD disabled, and just use FWD? I live in the city, and don't need AWD, I just liked the car and features and the fact Consumer's Report loved it.

xwesx

It will not be good for the center differential over the long term plus you will most likely not see any fuel savings as a result of doing that. I suspect the rationale for inserting the fuse would be for increasing fuel economy?

medic481

Not as much fuel savings, but I always understood that using four wheel drive on regular roads, was not good for the differential. All other 4x4's, like Jeep, tell you you that four wheel driving is best when used on snow, mud, etc, where the extra traction is required, and that once on asphalt, the vehicle should be returned to 2WD mode, as driving on asphalt while in 4 wheel mode will damage the front differential. Most of my "off roading" is going to consist of pulling into a store parking lot. Thought this might save on the wear and tear.

xwesx

No, not for this type of AWD system. Over the practical life of the car in normal use, you will never have a problem with the differentials.

If you wanted to run FWD only in this car, it would be a far better option to remove the rear driveshaft so as not to have the electronic override of the center differential engaged.

rsholland

That only applies to vehicles with old-fashioned "part-time 4WD," like most (not all, but most) trucks.

For vehicles with "full-time 4WD," like Subarus (AWD = 4WD), this is a non-issue. They're designed to operate all the time in 4WD/AWD.

Bob

aatherton

Subaru has a permanent AWD system, as opposed to a 2WD system that can be switched into 4WD like a Jeep.

As I understand it, a spare fuse is provided to energize a circuit to a solenoid that is part of the automatic transmission's clutch pack that apportions a varying percent of power to the rear wheels. The fuse gives the solenoid continuous power which disengages the rear drive train from power, turning AWD into FWD.

This feature is only for an automatic transmission when a compact spare is used on the rear (the compact spare should never be used on the front). The purpose of using FWD is to save the AWD drive train from damage from using the smaller size compact spare on the rear.

The solenoid is not designed for continuous duty, but will keep the car in FWD long enough to get the spare replaced. Something like 50 miles under 50 mph. I don't know what happens when the solenoid burns out from being continuously energized to keep the car in FWD, but it must not be good or many people with automatic transmissions would be using the FWD.

I have heard there is no improvement in mileage while in FWD as the unpowered rear wheels are still dragging the rear axles, differential, drive shaft and transmission.

medic481

Thanks- I was curious after I started reading the manual. Not going to mess with it.

aatherton

Here is a post by someone who rewired the transfer clutch solenoid to provide 50/50 lockup, like 4WD:
http://www.subaruforester.org/vbulletin/f89/4wd-demand-4eat-3091/#post66302

aatherton

"... vehicles with "full-time 4WD," like Subarus (AWD = 4WD)... They're designed to operate all the time in 4WD/AWD."

In 4WD vehicles there is a solid geared connection between the transmission and the wheels.

Subaru's AWD is different from 4WD in that there is no direct geared connection from the transmission to the wheels. Depending on the transmission, power goes through things like clutch packs and viscous couplings and multi-plate transfer clutch limited-slip centre differentials.

There is a good explanation of Subaru's AWD here:
http://www.subaruspecialists.com/mainframe/AllWheelDrive.html

rsholland

4WD? AWD? It's just marketing. In other markets Subaru has in the past called their AWD, 4WD. They don't do it any more mainly because they want to present one consistent message worldwide.

Also, Honda's "Real-Time 4WD" is nothing but AWD, same with the so-called "4WD" on the Toyota RAV4 and Highlander.

In all those cases all 4 wheels are driven, hence 4WD. Like I said: It's just marketing, nothing more. I do agree with you that there are many different types of 4WD/AWD out there (part-time, full-time, plus low-range, on-demand, selectable, permanent, etc.), but in the end, if all the wheels are powered, it's still 4WD.

Bob

aatherton

"4WD? AWD? It's just marketing... In all those cases all 4 wheels are driven, hence 4WD. Like I said: It's just marketing, nothing more... in the end, if all the wheels are powered, it's still 4WD."

In 4WD all the wheels are directly connected to the transmission by gears.
In AWD they are not.
There is a big difference in how the two work and affect the driving experience.

"... I do agree with you that there are many different types of 4WD/AWD out there (part-time, full-time, plus low-range, on-demand, selectable, permanent, etc.)..."

I do not use the combined term 4WD/AWD. Some of those terms apply to 4WD and others apply to AWD.
4WD is never full time, but is always selectable, to save the drive train.
Some AWDs are on-demand, meaning they only work after rear wheel spin begins.

If you have a Subaru, you should appreciate the unique advantages of its AWD, not only over 4WD, but over most of the other AWD systems.

rsholland

"4WD is never full time, but is always selectable, to save the drive train."

Not true. The Toyota Land Cruiser/Lexus GX-LX, Land Rover LR4/Range Rovers, Jeep Grand Cherokee with QuadraDrive/QuadraTrac (and others too) all have permanent 4WD (or AWD if you prefer). All wheels are driven all the time, and there is no 2WD mode available.

"If you have a Subaru, you should appreciate the unique advantages of its AWD, not only over 4WD, but over most of the other AWD systems."

I'm on my 5th Subaru, so yes, I understand and appreciate their AWD systems.

Bob

aatherton

"The Toyota Land Cruiser/Lexus GX-LX, Land Rover LR4/Range Rovers, Jeep Grand Cherokee with QuadraDrive/QuadraTrac (and others too) all have permanent 4WD (or AWD if you prefer). All wheels are driven all the time, and there is no 2WD mode available."

Those are all AWD, not 4WD.
I think you are defining AWD as permanent 4WD.

rsholland

All four wheels are permanently engaged all the time, they all have a low range gear in the transfer box, and are all excellent in off-road situations. Sounds like 4WD to me, but call it what you want.

You can also include the Mercedes G-wagon in that group as well; all premier off-roaders with permanent 4WD (or AWD if you prefer).

Again, I say this AWD/4WD debate is all about marketing. They're all four-wheel drive vehicles. It just depends on how the brand wants to present itself; AWD or 4WD. The RAV4, Highlander, Pilot, CRV and Grand Cherokee (maybe others too?) all have clutch pack center difs, yet those brands specifically call their systems "4WD." Other models with similar clutch-pack units market their drive system as AWD.

Now some differ in that they're reactive "on-demand" units, engaging either the front or rear axles only when tire slippage occurs (none of the ones I mentioned are that way); while others are proactive "permanent" units, in which some power is always sent to both the front and rear axles.

Bob