What is the difference between 4wd and awd?
The Subaru All-Wheel Drive is available in both automatic and standard transmission versions. The Subaru automatic transmission with All-Wheel Drive uses a set of transfer clutches and a solenoid controlled by the transmission control module (TCM) to engage the All-Wheel Drive system. In a Subaru manual transmission with All-Wheel Drive what is found is a viscous coupler that engages the All-Wheel Drive system. Some of the key points of service and maintenance with either design is to change the transmission fluid on a regular basis, keep the tires properly inflated as well as ensuring that the tires have a similar amount of tread and tread design all the way around. Some of the things on the never do list would include towing the vehicle with the wheels on the ground, coasting the vehicle with the engine shut off, and most importantly driving the vehicle with the space saver spare tire on it for a longer distance than specified by the manufacturer.
All-Wheel Drive is a superior mode of traction in my opinion to that of a vehicle with 4-Wheel Drive. The All-Wheel Drive system is better equipped for ever changing road conditions and is a great safety feature. But not every All-Wheel Drive system is the same. The Subaru stands alone design and function. While a lot of the newer system’s being put in vehicles of every make have a lot of fancy features and names, they all seem to lack the basic design principle that makes the Subaru system superior. Regardless of how many electronic controls you through at an All-Wheel Drive system if the mechanical aspects of the system are not symmetrical the system wont be as good as one that is. One of the advantages Subaru has with the Boxer engine platform is the ability to keep the drive train centered in the vehicle and the axle lengths the same which creates better function.
4-Wheel Drive is usually engaged either by mechanical linkage or some form of actuator either vacuum or electronic. One of the problems with this design is that due to the mostly mechanical nature of the system it doesn’t really posses the ability to adapt to road conditions as quickly as an All-Wheel Drive system can. With All-Wheel Drive you have the traction you need when you need it. With 4-Wheel Drive you have to engage the system when you need it and disengage it when you don’t. Sometimes when hitting an ice patch there is much to think about and it can be hard to remember to engage the 4-Wheel Drive system. When driving through a corner in the rain, All-Wheel Drive is really the only system you want to have.
Hi there,
This is Peter from CorgiWerx in Australia. We rebuild the viscous coupling centre differentials for the Subaru manual transmissions in Australia. We support the local Subaru owners community in Australia, providing a lower cost altenative to OEM repalcement.
We find most of our business is for 5MT WRX transmissions and 6MT Spec-B transmissions. Failures usually commence from 160,000 kiliometers (100,000 miles) onwards. Either the owners of these vehicles will invest in their vehicle’s upkeep, or the extra torque available used through corners loads up the centre differentails more.
I am interested to understand the North American experiences with centre diff failure/repalcement. Cheers!
I’ve heard a lot of talk about using a full size spare instead of the compact donut. I’ve read that the tread depth difference shouldn’t be more than 2/32″ of the rest of the tires. Of course, it’s likely a full size spare is going to have a difference greater than that.
How bad is it really to have more than a 2/32″ difference? It would only be used temporarily but how far and how fast can you safely go on it without damaging the differential? Will having a different tire model matter in this case even if it’s the same tire size?
Thanks!
Al
Hi Justin – it’s a 5spd XT –
So, the way the system works, is that the right (passenger) front wheel should engage first, then all others – right? I definitely had lots of left front wheelspin with ZERO left rear wheel engagement – it was like a front wheel drive car. I have no idea what the other 2 wheels were doing.
We have lots of snow here – so I could push up against a snow bank & see which wheels turn (just by getting out after & seeing which wheels sprayed snow) – should I try this & see what’s happening?
Thanks!
David
Hey David,
So the power starts out with the right front. And then as the right front starts to slip the other wheels will engage. Based on what you are describing it sounds like maybe you have a problem, is it an auto or 5spd?
I have a similar question – I noticed that when my Forester was stuck in an icy snowbank that the drivers front wheel was spinning, but the drivers rear wheel was not (I opened the door & looked while running) – the car didn’t move an inch and both wheels were completely free & clear of snow.
Does this mean that the AWD is like a 4WD open differential system, and that diagonally opposite wheels will turn while the other two are stationary in slippery conditions? (in my case, the drivers front & I assume the passenger rear wheel were turning, while the other two were stationary)
Or does that fact that some wheels didn’t turn at all means there is a problem with the system on my car?
Thanks!
David
Thanks for the response. I just notice much more rear wheel spin in snowy weather when going around corners than before. Much like a rear wheel drive car where i have to lift to get it back under control.
The Subaru All-Wheel Drive system mechanically powers the front wheels first and the rear is engaged either electronically or mechanically based on front wheel slippage. If the front wheels were to have a lack of mechanical power to the wheels you would have a long list of symptoms. It is possible for the car to move but it wouldn’t drive well at all.
I can’t imagine the Impreza moving very far for very long without the front wheels having power to them.
Can the 4 wheel system on an impreza wrx fail and the car only drive from the rear wheels without you knowing that the system is not operating correctly?