The difference between four-wheel and all-wheel drive

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This article was published 03/08/2012 (5108 days ago), so information in it may no longer be current.

QUESTION: I have a couple questions. What is the difference between an all-wheel and a four-wheel drive? Also, what is the difference between these two – safety-wise? Thanks.

ANSWER: While both all-wheel drive (AWD) and four-wheel drive (4WD) can power all four tires, there are some important differences. All-wheel drive is designed with a transfer case that can split the power between the two axles and allows the axles to turn at different speeds, which happens whenever the vehicle is turning a corner. This allows all tires to maintain their maximum grip under all driving conditions.

4WD vehicles typically have a transfer case that can be shifted between two-wheel drive and four-wheel drive modes. Some also have automatic modes and four-wheel low capability. If you are slogging through tough off-road terrain, the 4wd low capability is important, but whenever these transfer cases are in a 4WD mode, the front and rear axles are locked together so they turn at the same speed. While this isn’t a problem on loose terrain such as sand, snow, gravel and mud, it does cause problems on hard-packed surfaces or pavement.

Because the axles are locked together, whenever you turn a corner, one or more of the tires have to slip on the road surface to allow for the different distances the tires have to travel. On hard road surfaces, this places a high load on drivetrain components and can quickly damage transfer cases and differentials. If you are driving on ice, the tires slip and it is easy for the vehicle to “spin” out on a turn.

4WD systems with Automatic mode actually operate in 2wd mode unless there is a difference in speed between the axles, such as when a tire spins on ice or gravel. Then the computer engages the transfer case automatically into 4wd mode to provide traction and as soon as the axles are turning at the same speed, the system returns to 2wd mode. These automatic mode systems offer many of the advantages of all-wheel drive systems but aren’t quite as good in the handling safety areas, where all-wheel drive excels.

Regardless of the system, it is important that all tires on these vehicles be of the same size and tread wear or damage to the drivetrain will occur. Rotating tires frequently will help them to wear evenly.

QUESTION: I own a 1997 Mazda SE B4000 truck with 118,000 kilometres. Last year the dreaded check engine light came on. My code reader indicated codes P0171 and P0174, both banks running lean. In addition after the vehicle reaches operating temperature, it begins a rapid surge at idle. Also while surging, if I place it in drive, the surging seems to stop. However, as soon as I place it back in neutral or park, the surging resumes.

I replaced the MAF sensor and installed three new O2 sensors. No change. I replaced the catalytic converter, fuel filter, PCV valve, spark plugs and wires. I replaced the upper intake manifold gasket, fuel rail gasket and throttle body gasket. I’ve checked the EGR as suggested by a mechanic, whereby I would release propane gas under the EGR and see if idling smoothes. No difference. I’ve also checked for vacuum leaks. I sure would appreciate all the help I can get before I empty the dealer’s parts shelves.

ANSWER: A faulty Mass airflow sensor or leaking upper intake manifold gasket are the most common causes of a surging problem on this engine but you have already changed these with no improvement. One area you haven’t checked is fuel pressure.

Fuel pressure that is slightly low can cause lean operating conditions. There may not be a driveability problem under load when the computer richens the fuel mixture, but it can create a surge at hot idle. If fuel pressure is low, look for poor electrical connections to the pump. The electric fuel pump itself may need to be changed.

Also, look at the electrical connections in the engine compartment to the injectors. If there isn’t enough current flow due to a poor electrical connection, fuel isn’t sprayed crisply into the engine and doesn’t burn completely. It acts like a lean condition and surging can result.

Jim Kerr is an experienced mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists’ Association of Canada.

kerr.jim@sasktel.net