Jim Grant's Tech Tips
‘99 Subaru Forester AWD, Binding Transmission
I own a ‘99 Subaru Forester AWD with a 2.5L engine and manual transmission. After the engine is hot and the car has been run about 20 minutes over the road, when I make a hard left there is an intermittent binding feeling that comes through the steering wheel and then a burning smell. You can also hear a binding noise. It's like a thump, thump, thump type of thing. It seems to come from the front wheels. My local car repair guy thinks it might be in the transmission clutches that transfer power to the front wheels. We have already replaced one half shaft on the right front, it didn't help. There is also a lesser problem when turning sharp right. It doesn't happen when cold and only happens on hard turns, not normal street driving. Any clues?
- Rig Mullin
A: The first item to check on your Subaru is the tires, they must all be the same size and make of tire. In addition, all of the tires must be of the same mileage and wear. In all wheel drive vehicles you cannot mix new with old tires and you should never mix brands and sizes. In most All Wheel Drive (AWD) vehicles there is a device called a viscous coupler. The viscous coupler works in conjunction with the center differerential, which is located inside of the transaxle/transmission. The center differential has the job of distributing engine power to the front and rear wheels on an equal basis, along with allowing for a difference of wheel speed when a vehicle is turning. The viscous coupler takes the differential action a step further by limiting the differential’s ability to allow for a difference of wheel speed. How does this all work?. If you’re in snow or slippery driving and the front wheels start spinning, the viscous coupler will react by causing the engine torque that is spinning the front wheels to go to the rear wheels. What is really cool about a viscous coupler is its reaction time and the smoothness in which it performs this task. It reacts in a fraction of a second (faster than you can blink your eye) and is so smooth you just don’t feel it happen. All you know is you have one heck of a vehicle that rips through the snow like a monster. But, viscous couplers can fail or be damaged. When that occurs the silky smooth reaction time of the viscous coupler will become anything but smooth. A difference in tires causes a battle to begin between the action of the differential and reaction of the viscous coupler. This battle, by design, can go on for a while, but not as a daily part of life for these two components. In the end battle the viscous coupler will overheat (yes, can cause a hot burning smell) and the differential will be victorious. But, in its death/failing the viscous coupler can deal a fatal blow to the check book by rendering an AWD vehicle nearly impossible to drive in any direction but straight to the repair facility. If diagnosis finds that the viscous coupler has failed a replacement will be needed to allow your vehicle to turn smoothly. Viscous couplers do fail, but many times it is an outside influence that is the cause. Remember the tires? Be sure to double check that they’re right. You don’t want the battle to begin again.