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Car Doctor: What's the difference between 4WD and AWD?

Posted by John Paul  February 1, 2011 10:55 AM

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Reader feedback: Regarding the question and my answer concerning windshield wiper chatter yielded positive results for many readers. The repair involved cleaning the windshield, the wipers and adjusting the wiper blade to windshield angle.  

Q. Can you tell me the different between four-wheel-drive and all-wheel-drive? I sometimes see the terms used interchangeably, but I suspect they are different types of systems. With the snow we have received so far this year I think it is time to buy a four-wheel-drive vehicle.

A. There are actually several types of drive systems but essentially they all describe when engine power is transmitted to the four wheels. Many all-wheel-drive units power either the front or rear wheels most of the time. When those wheels start to slip, engine power is transmitted to the other axle. In traditional four-wheel-drive, the front and rear axles are locked together using a shift lever, button, or switch. These systems are designed for slippery roads or off-road use.

Permanent all-wheel-drive is a 50/50 split of torque between the front and rear wheels. In addition, there are variations of these systems that use a combination of electronic, traction, and brake control systems to improve traction and handling. Keep in mind that while all-wheel-drive will improve traction, it will do little to improve stopping. Many times over the winter I see the driver of an all-wheel-drive vehicle driving too fast for the road conditions only to lose control and get into a crash. 

Q. We reside in cold Minnesota and recently bought our 16-year-old a used convertible. Every morning the windows are completely frosted on the inside, requiring scraping. This even happens if the outside isn't frosted over. How can this be fixed?

A. There is frost on the inside of the windows due to accumulated dampness inside the car. This could be caused by a leak from the top or body seam. It could also be a result of a previous owner who got caught in the rain with the top down (the moisture can stay trapped for months). At this point you need to completely dry out the car. Moisture can be trapped under the carpets, in the trunk, and even in the doors. At this point I would vacuum the interior with a "wet" vacuum. If the car has air conditioning, run the air conditioner to dehumidify and dry out the interior. You could also try using "dry-packs"; these pint sized containers contain calcium chloride that dries out moisture.  

Q. My car's battery light came on then later my car completely cut off while I was driving. I waited a few minutes, it started up, but then ran about a mile and died again. I got snowed in and didn't drive for about a week. It cranked right up and I drove about five miles before it died again. What could it be?

A. This is a classic example of a poorly performing alternator. The alternator recharges the battery and the engine needs electricity to run. Basically what is happening is your car is running out of electricity. Have the battery and charging system checked by a qualified technician. 

Q. I purchased a 2007 Toyota Camry four years ago. For three years I drove this car with no problems. When Toyota had their gas pedal recall they also did a software update that I was unaware of. Since that time, my car developed a shutter in the gas pedal and slight vibration in the steering wheel. After several trips to the dealership, and various attempts to correct problem, which everyone agreed existed, I contacted Toyota and I set up a meeting with a technician at the car dealership. They diagnosed the problem as a torque convertor shutter in the transmission from the software update. They also informed me there was nothing they could do about it. Needless to say, this did not sit well with me.

So without going into all that happened, they finally tried flushing out all the transmission fluid and felt this solved the problem. I don't agree, the problem is still there. Have you ever heard of this problem? I am a 30-year Toyota owner and I am very disgusted with them.

A. What you are describing is a classic example of torque converter "shudder." Of the hundreds of thousands of updates performed by Toyota as part of the recall, this is the first that I have heard of any problems. Although I'm not a huge believer in additives, one product, Lubegard, has a good track record of eliminating torque convertor shudder.

Q. Recently the battery in my car died and roadside assistance came. Everything is awesome, but now the speedometer in not working. Any ideas or information on how to fix this problem?

A. I have seen this problem before and providing nothing was damaged when the battery was jumpstarted it can be fixed quite easily. Similar to a computer being rebooted, the car's problems can be remedied in the same fashion. Have someone disconnected the battery cables from the battery and hold them together for about a minute, then reconnect the battery. This procedure will sometimes exorcise the electrical gremlins. The other issue is sometimes the speedometer needle will spin completely around and get stuck. Removing the speedometer and resetting the needle generally fixes the problem.

John Paul is the public affairs manager for AAA Southern New England. He can be reached at or on Twitter @johnfpaul.

This blog is not written or edited by or the Boston Globe.
The author is solely responsible for the content.

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Clifford Atiyeh is an automotive writer and car enthusiast . He has spent his entire life driving cars he doesn't own.
In the garage: 1995 21-speed Iron Horse, 2002 Jeep Wrangler X (by association)
Bill Griffith is a veteran Boston Globe reporter, having reviewed cars for more than 10 years and serving as assistant sports editor for 25 years. He was also the paper's sports media columnist.
In the garage: 2006 Subaru Baja
AAA's Car Doctor, John Paul John Paul is public affairs manager for AAA Southern New England, a certified mechanic, and a Globe columnist. He hosts a weekly radio show on WROL.
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Craig Fitzgerald has been writing about cars, motorcycles, and the automotive industry since 1999. He is the former editor of Hemmings Sports & Exotic Car.
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