One of GM's newest rack designs is the "Magnasteer" variable-assist system made by Delphi Saginaw Steering Systems. Magnasteer was introduced in 1995 on Buick Riviera and Oldsmobile Aurora. In 1996, it was added to the Cadillac Seville and DeVille, Oldsmobile 88 and Pontiac Bonneville. In 1997, it was added to the Cadillac Eldorado, Chevrolet Corvette, Buick Park Avenue and Regal, Pontiac Grand Prix and Oldsmobile Intrigue.
Magnasteer is a variable-effort system that uses vehicle speed input to modify steering assist. It does so by using magnets inside the rack's valve assembly to vary the rate of the valve assembly torsion bar. The control module varies the current to an inner and outer magnetic coil inside the valve assembly according to input from the vehicle speed sensor. The two coils are separated by a permanent magnet. At low speed, the current is routed through the coils to create magnetic repulsion. This aids column rotation when the steering wheel is turned for maximum power assist. As speed increases, the current to the coils is gradually reduced to increase steering feel and reduce assist. At about 45 mph, the current is reversed to change the magnetic polarity. At this point, magnetic effects are balanced and steering effort is "normal." At 60 mph, more current is supplied to the coils to increase magnetic attraction, increasing the amount of steering effort until the maximum is reached at about 85 mph.
At a recent Automotive Parts Rebuilders Association (APRA) clinic, Rich Jester of Delphi Saginaw Steering Systems discussed some of the service aspects of the Magnasteer system (Saginaw currently has no remanufacturing program of its own for the Magnasteer system).
Jester said the permanent magnet in the Magnasteer valve assembly is made from a proprietary high-strength magnetic material. The magnet is brittle, so the rack should be handled with care. The magnet and retainer are attached to the spool shaft, and the pole piece is attached to the valve body. There is currently no way to disassemble and repin a Magnasteer valve assembly because the position of the magnet with respect to the pole piece is set by an injection molding process.
"The original magnetic balance is too sensitive to be maintained after repinning," Jester said. This means aftermarket rack rebuilders are going to have a hard time remanufacturing this rack.
Jester cautioned against handling the gear housing by its pigtail electrical connector because it can be easily damaged. Care should also be used when inserting test probes into the connector body when testing the coil circuit or when test powering the gear. Use only a well-regulated 12v DC power supply with a maximum of plus or minus three amps to prevent damage to the coil. If the electrical connector must be replaced, it is critical to maintain the correct polarity. If the leads are reversed, the gear will perform exactly the opposite of the way it was designed.
According to Jester, the coil's resistance is normally only about two ohms, so an accurate ohmmeter is needed to check resistance. An infinite or open reading indicates a bad coil and requires replacing the rack. When checking for short circuits, the coil itself may measure correctly, but the coil may not be isolated from the housing. Jester recommends checking for shorts between both sides of the coil and ground. If the resistance measures anything less than infinite, the circuit is shorted and the rack needs to be replaced.
DIAGNOSIS & REPAIR
Any rack that is leaking fluid past the rack or pinion shaft seals should be replaced. The same goes for any rack that has center wear, is cracked, has wear in the spool valve housing or is not providing normal power assist. If the power steering pump is working normally, but there is little or no steering assist, the spool valve or piston seals within the rack are probably leaking and the rack needs to be replaced.
Stiff steering, memory steer and poor steering return can also be caused by bad upper bearing plates on MacPherson struts. Rarely do the lower ball joints cause such problems because they carry no weight in front-wheel-drive applications. So make sure these other possibilities have been ruled out before condemning the rack.
Replacing a worn or leaky steering rack is usually no big deal. All you do is remove anything that's in the way, disconnect the steering coupling and tie rod ends, unhook the fluid lines, undo the rack mounting bolts and pull it out. Simple as that. Then slip in the replacement rack, tighten the mounting bolts, flush the pump and lines, reconnect the lines to the rack, reconnect the steering coupling and tie rod ends, replace anything that had to be removed to extract the rack, top off the power steering pump with fluid, bleed all the air out of the system, make sure the steering wheel is centered, then adjust toe Ñ and you're done.
Flushing is one step you don't want to skip because any contaminant that is in the old power steering fluid will find its way into the replacement rack. First drain the pump reservoir. Then inspect the pump lines or hoses to make sure they are not blocked, deteriorating internally or leaking. If the hoses are OK, connect the lines to the rack, but disconnect the return line from the pump and place it in a container. Fill the pump reservoir with fluid that meets GM specifications, then disable the ignition and crank the engine. Do not start the engine or crank for more than 15 seconds. Or, remove the belt and turn the pump by hand. Continue to add fluid until the fluid coming out of the return hose is clear. Then reconnect the return hose to the pump, start the engine and cycle the steering from side-toside to purge air from the system. Avoid hitting the steering stops during this process. No bubbles mean no troubles.
If you're replacing a rack on an air-bag equipped vehicle, don't rotate the steering wheel while the rack is disconnected. Also, make sure the replacement rack is centered before reconnecting the steering column coupling. These precautions are necessary to prevent damage to the clockspring mechanism that's located under the steering wheel. The clockspring forms the electrical connection between the air bag module in the steering wheel and the rest of the air bag system. If the steering is not centered when the rack is installed, it's possible to overextend and break the clockspring when the steering is turned all the way to one side or the other. A broken clockspring would prevent the air bag from deploying in an accident.
Something else that should be checked when replacing a rack is the condition and tension of the power steering pump belt. Check for glazing, cracks, fraying or oil contamination. If the belt isn't in good condition, replace it. If the belt is more than four or five years old, recommend a new one for preventive maintenance
