Resource Information for Auto Repair Projects:
Variable-rate power steering is nothing new. In the 1990s, import and domestic vehicle manufacturers introduced a number of electronically controlled, variable-assist power steering systems including both rack & pinion steering and gear box power steering systems on various Ford, General Motors, Toyota, Lexus, Nissan and Infinity models.
Unlike conventional power steering systems that have a constant rate of power assist, electronic variable-assist systems vary the amount of steering effort according to vehicle speed. On the more sophisticated applications, steering effort may be further modified according to steering inputs and vehicle dynamics.
As a rule, most variable-assist power steering systems provide maximum assist at low speed for easy parking maneuvers, and reduce assist at higher speeds to improve steering stability and road feel.
With conventional power steering, the amount of power assist is fixed and is determined by the calibration (stiffness) of the torsion bar that connects the steering input shaft to the pinion gear. By using differently sized torsion bars, engineers can fine tune the steering feel to best suit a particular vehicle or type of driver. For a large, heavy luxury car, a lighter torsion would typically be used to provide maximum steering assist. A small sports car, by comparison, would likely use a stiffer torsion bar to reduce assist and improve steering stability and feel. But regardless of how the steering is tuned, fixed-rate power steering is always a compromise between steering effort and road feel.
Many variable-assist power steering systems use a solenoid valve or a small stepper motor on the steering gear inlet port or pump outlet port to reduce hydraulic pressure once the vehicle exceeds a certain speed. This is called "Electronic Variable Orifice" (EVO) variable-assist steering. The operation of the orifice valve or stepper motor is controlled by the PCM or a separate module that takes its cues from the vehicle speed sensor. Some EVO systems simply open or close the EVO valve at a preset speed. Others use a duty cycle to gradually reduce power assist as speed increases.
A DIFFERENT APPROACH: MAGNASTEER
General motors has also used EVO variable-assist steering on a number of models over the years. But in 1996, a more sophisticated type of variable-assist power steering, developed by Delphi Saginaw Steering Systems, called "Magnasteer" was introduced on: Buick Electra, Le Sabre, Park Avenue and Reatta; Cadillac Eldorado, Seville STS and DeVille Concours; Chevrolet Corvette; Oldsmobile Aurora; and Pontiac Bonneville. In 1997, Magnasteer became standard on all Cadillacs and has since been added to a wide range of other GM makes and models.
Unlike EVO variable-assist power steering systems, Magnasteer does not use an orifice valve to reduce pressure to the steering gear. Thereís no solenoid or stepper motor involved. Instead, Magnasteer uses magnets to assist or resist steering input.
The Magnasteer control valve assembly is mounted on the steering rack in the same location as a regular control valve assembly on an ordinary power rack. Inside the base of the Magnasteer unit is a large electromagnetic coil. Just above the coil is an inner and outer pole assembly with a permanent magnet in the center. Steering feel is varied by changing the strength and polarity of the magnets, which in turn, is controlled by changing the pulse width and direction of the current to the coil.
On 1996 and 1997 applications, Magnasteer has its own separate control module. But on the 1998 and newer applications, the Electronic Brake Control Module (EBCM), which also oversees the operation of the anti-lock brake system, handles the Magnasteer control function.
HOW IT WORKS
The only input the control electronics uses for Magnasteer on most applications is the vehicle speed sensor. At zero mph, a negative current of approximately two to three amps flows to the Magnasteer coil. This causes the magnets to repel each other, which in turn allows more deflection in the torsion bar inside the spool valve assembly. This increases fluid flow through the spool valve to the steering gear for maximum power assist when it is needed most.
As vehicle speed increases, current flow to the Magnasteer coil is gradually reduced. At about 45 mph, current flow to the coil hits zero and Magnasteer has no effect on the amount of power assist (which is determined only by the torsion bar and pump flow).
Above 45 mph, the direction of current to the coil is reversed and continues to gradually increase up to a maximum of about three amps at 75 to 85 mph. Reversing the polarity of the coil causes the magnets to attract each other, which has the effect of stiffening the torsion bar. This reduces the amount of deflection in the torsion bar that normally occurs when the wheels are steered and causes an increase in steering effort for better road feel and high speed steering stability.
On some Magnasteer systems, such at that used in the Corvette, the Electronic Brake Traction Control Module (EBTCM) also looks at input from a steering wheel position sensor to determine if more or less steering assist is needed under certain driving conditions. The main purpose here is to reduce oversteer by reducing assist when excessive lateral G-forces are detected.
Compared to conventional power steering systems, Magnasteer gives the driver a much broader range of power assist with a smooth transition from low to high speed. Whatís more, the system can be recalibrated to modify steering feel using a Tech 2 or equivalent aftermarket scan tool. There are three settings: factory, more firm and less firm. Just follow the scan tool prompts and thatís all there is to it.
In 1998, a second generation "Magnasteer MAGe" system was introduced on some GM vehicles. This version of the system does not use a permanent magnet, but uses a redesigned electromagnet that has one pole mounted on the input shaft and the other on the pinion shaft.
At low speed, there is no current flow through the electromagnetic coil. The level of steering assist depends solely on the calibration (stiffness) of the torsion bar. As speed increases, current flow to the coil is gradually increased up to a maximum of several amps to decrease assist and increase steering effort. Magnasteer MAGe uses vehicle speed as its only input, so there are no changes in steering effort during sudden steering maneuvers.
From a mechanical standpoint, Magnasteer is vulnerable to the same kinds of problems as any other power steering rack. It can develop leaks, center wear or morning sickness at high mileages. So if a Magnasteer rack is leaking fluid, has excessive play or feels sluggish when the vehicle is first started on a cold morning, it needs to be replaced the same as any other steering gear.
If something goes wrong with the Magnasteer control system, there will be a noticeable change in the amount of steering effort at all speeds. The steering may feel stiffer than normal at low speed, and lighter than normal at high speed. Or, the amount of steering effort may be erratic at various speeds.
The Magnasteer system does have self-diagnostic capability, but there is only one fault code: C1241 (Magnasteer circuit malfunction). The code is set if the module detects an open or a short in the coil circuit. If this code is present, the Magnasteer system is disabled and will not vary the steering effort as vehicle speed changes.
The C1241 body code can be read with a Tech 2 or equivalent scan tool. On models with a driver message center, a warning will appear if the code is present.
The Tech 2 tool can also be used to perform a Magnasteer function test. The test varies the current to the coil so you can check for a change in steering effort when turning the steering wheel. A simple test drive can also be used to confirm any problems with the system. At low speed, the steering should require minimal effort and feel the same in both directions.
As speed increases, steering effort should gradually increase. If the steering feels lighter than normal at high speed and/or unusually stiff at low speed, Magnasteer isnít working. But if the steering feels stiff at all speeds, the problem may be hydraulic. Noise would indicate air in the system or a bad pump valve.
If an electronic problem is suspected, the first thing to check would be the electrical connection to the coil on the Magnasteer unit. Coil resistance between terminals A and B can be checked with an ohmmeter, and should read about two ohms (the actual GM spec is 1.6 to 3.1 ohms). An infinite (open) reading indicates a bad coil (requires replacing the rack since the coil is not serviceable). Checking for shorts between both sides of the coil assembly and rack housing is also recommended.
If the coil is defective, the entire Magnasteer unit must be replaced because there are no serviceable parts inside. New units are available through General Motors, and remanufactured units are available from several aftermarket suppliers.
If the Magnasteer coil checks out OK, the problem may be in the wiring between the coil and control module, or at the control module itself. Check the wiring continuity for grounds, shorts or opens, and check the control module for voltage when the key is on (terminal F on the EBCM or EBTCM). If thereís no voltage at the module, check the module fuse and wiring.
The only other fault that could affect the operation of the Magnasteer system would be a fault in the vehicle speed sensor circuit. A problem here should set a separate fault code and turn on the Check Engine light. Generic OBD2 codes for a problem in the VSS circuit include P0500, P0501, P0502 and P0503. GMís enhanced codes for the VSS circuit include P1501 (vehicle speed sensor intermittent).
As for hydraulic problems, a Magnasteer rack can be tested the same as any other power rack using a high pressure gauge to check pump output and the operation of the steering gear. Most systems operate at a maximum of about 1,500 psi with a flow rate of 2-1/2 gallons per minute. A weak pump, bad pump valve or a restriction could all have an adverse effect on steering effort.
When inspecting the rack, check the bellows on both ends for the presence of fluid. Large amounts of fluid in either bellow means the seals are leaking and the rack needs to be replaced. Leaks around the input shaft would also call for replacement.
If a Magnasteer rack has to be replaced, be sure to change the power steering fluid, too. Old fluid contains abrasive particles that can damage new steering gears and pumps. The last thing you want to do is contaminate a new rack with old, dirty fluid. Installing an in-line fluid filter can also help protect new components and prolong their service life.
Pay close attention to the condition of the power steering hose. Old hose can become brittle and leak. They can also deteriorate internally, releasing small flakes of rubber into the fluid that can cause problems in the pump and control valve. Replacing high mileage hose is recommended to minimize the risk of future problems. The pump drive belt should also be inspected and replaced as needed.
When installing a new rack, make sure the pump and hose have been completely drained before reconnecting the lines to the rack. Then refill the system with power steering fluid that meets GM service specifications #9985010 (GM part number 1050017 or equivalent).
Once the system has been filled, youíll also have to bleed it to get rid of any trapped air inside. Bleeding may also be necessary if the fluid level in the pump reservoir has gotten low and allowed air to enter the lines. Air can cause noisy operation as well as reduced power assist.
One way to tell if thereís air in the system is to observe the appearance of the fluid. If the fluid is discolored or has a foamy appearance, itís full of air bubbles.
After filling the pump reservoir with the proper fluid, raise the wheels off the ground and slowly turn the steering from lock to lock with the engine off. When you reach the stop at one side, wait about five seconds before cranking the wheels the other way. Continue cranking the wheels back and forth 10 to 12 times while keeping an eye on the fluid reservoir and adding fluid as needed to keep it full. This should get rid of most of the air in the system while giving you a good upper body workout.
On some applications, you may have to use a power steering pump air evacuator to purge all of the air from the system. The tool consists of a rubber stopper that fits into the filler opening of the pump reservoir. Attached to it is a vacuum hose connected to a vacuum pump or siphon. The tool applies vacuum to the reservoir while the engine is idling to help draw out dissolved air bubbles.
Cycle the steering from lock to lock every 30 seconds for approximately five minutes while maintaining 15 inches Hg of vacuum. Do not hold the steering wheel at the stops while cycling because this increases pressure. If one session doesnít completely purge all the air, you may have to repeat the vacuum procedure several times.
Finally, give the vehicle a short test drive to make sure the Magnasteer system is changing the amount of steering effort required as vehicle speed changes, and that the system is operating quietly and no fault codes reappear.
Auto Parts &