2002 light truck service report

Motor, Jan 2002 by Weissler, Paul

Consumers are asking for more and more utility in their trucks and SUVs. And vehicle manufacuturersare more than willing to make the technology investments necessary to make them happen.

Trucks and SUVs have taken over nearly half of the U.S. vehicle market, so they also get much of the investment in new technology. The reasons are simple: People are willing to pay more for trucks and SUVs that do more-- whether it's carrying lots of cargo, toeing a boat or trailor or maintaining surefootedness in mud or snow and on off-road trails. However, there also many of the luxury and comfort features and ride quality of comparable priced car.

To show that the customer is always right, the 2002 model year includes GM full-size C/K pickup trucks that use rear-wheel steering to produce the handling of a small sedan. GM also introduces the Avalanche, a four-- door, five-passenger extended-cab pickup that allows the rear seats and rear barrier/window to fold down forward and flat. That extends the cargo bed into the rear seat area-an ingenious design called the "midgate."

DaimlerChrysler's newest Jeep, the Liberty, not only can go anywhere a Wrangler can, but has the sedanlike ride of independent front suspension. DC's Dodge Ram, meanwhile, has a totally new design for '02, although it continues with the bold front-end appearance that brought it out of obscurity back in '94. However, when you lift the hood and don't see the familiar 3.9 V6 or 5.2 V8 anymore, you'll know it was given a lot more than a face lift.

And to prove that a truck can be truly transformed, Ford has come out with the new Lincoln Blackwood, a luxury vehicle built off the F-series truck platform.

The increasing technological complexity of today's trucks means a big change in the way these vehicles are serviced. Since most of what's new for the 2002 model year is in domestic nameplates, we'll focus on them. And there's no better example than the new C/K trucks' rear-steer, so we'll lead off with that.

General Motors

Rear steering was a flop on Mazda and Honda cars because any improvement in handling was beyond what the average driver could appreciate. That's not true with the Delphi-- built setup on the 2002 GM C/K pickups. We drove them with long trailers attached and were able to wiggle through tight turns, because the rear wheels pivoted in the opposite direction, shortening the turn circle diameter from more than 46 feet to just over 37 feet. To put that in perspective, that's the same turning circle as a Saturn coupe or a Jeep Grand Cherokee, which has to make tight turns to pass an engineering requirement for off-road use.

Backing up or going forward through tight turns with a trailer attached also is far easier, because the opposite steer of the rear wheels occurs at low speeds. So the trailer more precisely follows the intended path. At higher road speeds, the rear wheels turn in the same direction as the fronts, improving highway handling and stability.

The rear-steer system is available now on the GMC Sierra Denali, and will be offered soon on the Chevy Silverado. The main component is a rack & pinion design rear axle (with tripod-type constant-velocity joints in the axle shafts). The steering gear is powered by an electric motor that's controlled by a computer above the spare tire in back (to minimize the voltage drop to the motor). The module makes its decisions primarily based on signals from steering wheel and vehicle speed sensors, with feedback from an axle steering position sensor. The sensors and the motor are hard-wired to the module to minimize voltage drops, and the harnesses are well-protected.

Rear-steer C/K trucks get a 5-inch wider track (to 71 inches), driver-selectable shock controls and a higher-- capacity rear axle.

The basic computer algorithm limits rear-wheel steer to 12 deg (in the opposite direction and at low speed). If the towing mode is selected, the limit is 9.5 deg, to prevent jackknifing. The system shifts to straight-ahead wheels at 40 mph (25 mph in tow mode). The rear wheels start to turn in the same direction as the fronts (gradually up to about 5 deg) at speeds over 50 mph.

The system has been designed for fail-safe operation and exhaustive diagnostics. The tie-rod ends have a capture design so they don't separate completely from the knuckle in case of breakage. And each knuckle assembly has a cast-in stop to prevent it from going past 15 deg.

But the real sophistication of the rear-steer system is in the case of an electrical failure. The computer runs 63 diagnostic routines-some during startup and some that repeat every 4 milliseconds. If the system is shut down, the dashboard message center turns on a warning lamp and the system reverts back to two-wheel steering, if necessary, with the aid of a powerful Belleville spring system to push the rear wheels back to straight ahead. The trouble codes are standard GM Class 2 data (J1850), and come through the OBD II diagnostic connector.

There are a few fail-safe strategies for specific situations where the motor still works. For example: