IDENTIFIX Helps Tech Diagnose Steering Wheel Problem on TahoePosted 4/14/2005
By Bill Petersen
The vehicle: a 1999 Chevrolet Tahoe 4x4, 5.7L with automatic transmission.
The complaint: Jerking feel from the steering wheel, much like a broken belt in a tire. The problem happens while driving between 35 and 55 mph straight ahead before the vehicle and tires have warmed up. It sometimes disappears for a few moments and then reappears during the drive.
When the tech called the IDENTIFIX hotline, he told me he had found no problem with the tires, steering or suspension. He confirmed the vehicle has variable effort steering (VES), also referred to as electronic variable orifice steering (EVO). After disconnecting the steering assist control solenoid, a test drive confirmed the problem was gone. The EVO system was causing or contributing to the problem.
The next step was to use a scan tool to test for EVO system codes. If there are codes, see if they are related to the symptom. Then, clear the codes and again, starting from cold with everything reconnected, take another road test to see if the codes reappear.
If there were no codes when first testing with the scan tool, then from the chassis menu, select electronic variable orifice data display. Monitor the steering wheel rotation sensor voltage, the steering assist control solenoid amp flow and the vehicle speed sensor value.
The steering wheel rotation signal is from the hand wheel speed sensor (HWSS). The HWSS is located at the bottom of the steering column tube, clipped to the lower tube bearing. The three-wire sensor has a gray 5-volt reference wire, an orange/black sensor return (ground) wire, and a light blue steering wheel position signal wire. The sensor has four resistors wired in series to form a box effect.
The sensor has two points 180 degrees apart, connected to 5 volts; and another two terminals 90 degrees from the 5-volt terminals, which are each connected to return (ground). The sensor produces 0.2 to 4.9 volts when the steering wheel is rotated 90 degrees. Then the sensor produces 4.9 to 0.2 volts for the next 90 degrees of steering wheel rotation in the same direction. When the steering wheel has been rotated 360 degrees, the voltage will have gone 0.2 to 4.9, 4.9 to 0.2, 0.2 to 4.9, 4.9-0.2 volts in a smooth and consistent manner.
The control solenoid is located in the rear of the power steering pump in an adapter to the pressure line fitting. The solenoid has two electrical terminals - one is battery voltage, the other is a pulse width modulated (PWM) ground. The PWM controls current from zero to 850 milliamps. There is an internal valve in the solenoid. When the PWM is high, the current is high and the internal valve is moved into an orifice to reduce fluid flow, reducing power steering assist. When the current is low, the valve is moved further out of the orifice, the fluid flow increases and pressure increases. If the solenoid is disconnected, the valve defaults to max flow/max assist.
The EVO/passlock module is mounted behind the dash panel, snapped into the underside of the dash carrier, below the radio. The vehicle speed sensor (VSS) input is from the vehicle control module (VCM) on gas engines. The EVO/passlock module, which is the control for this system, sends a voltage to the VCM on a single wire. The VCM toggles the voltage to ground at a pulse rate of 4,000 pulses per mile. The EVO/passlock module uses the pulsed signal as vehicle speed, and vehicle speed is used to control steering assist more precisely. At 0 mph you get full assist for easy steering. At speeds above zero mph, assist is based on HWSS and VSS inputs for better road feel.
The EVO/passlock module also has an ignition power supply, a battery continuous power supply, and a ground from the engine. The remaining circuit for EVO is a class two serial data circuit for communications to the diagnostic link connector and other modules. The EVO/passlock module also contains the passlock function of reading the passlock sensor and sending the password to the VCM through the class two serial data line. If the EVO/passlock module is replaced, a passlock relearn procedure has to be completed.
To diagnose this problem, the technician needed to scan the HWSS, VSS, and the steering control valve amps very carefully when the problem happened, making sure all the values were stable during the problem. If the scanner is not fast enough to display the problem, it may be necessary to test voltages and amperage at the EVO/passlock module connector terminals. An oscilloscope/labscope may be required for extremely fast glitches. An intermittent input or output could change the assist enough to cause a steering wheel tremor.
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