Powertrain Diagnosis

Diagnostic Chart:

Wiring Diagram:

The Electronic Variable Orifice (EVO) signal is pulse width modulated to control the power steering pressure/flow from the power steering pump. The PCM controls the ground and source voltage. When the vehicle speed is zero, current flow is zero milliamps. When vehicle speed reaches 16 km/h (10 mph), current starts to flow and steering effort begins to increase.

DTC PARAMETERS
DTC P1555 will set if the PCM desired current does not match feedback current when:
^ The engine has been running for 2.5 seconds.
^ EVO is commanded (VSS above 16 km/h (10mph).

IMPORTANT: The EVO circuit is continuously checked with engine running and EVO commanded (16km/h [10 mph]).

DIAGNOSTIC AIDS
^ DTC P1555 may be set by loss of ignition when the vehicle is still moving. This is a normal condition.
^ Review malfunction history data to verify failure was feedback low/high. Feedback high indicates a short to B+ on circuit 1294. EVO low indicates an open or short to ground in the circuit.
^ If no DTC is set and customer complaint is steering effort refer to the "Steering" section of "Chassis Service Manual."
^ In the event a shorted solenoid is detected or a short between circuits 1294 and 1295, the PCM will not provide output voltage on 1294.
^ The Scan tool displays EVO Output and feedback values to assist in EVO diagnosis. The output is displayed in duty cycle and the feedback in voltage. At idle, when full power steering assist is required, no duty cycle is commanded. As the vehicle speed increases (above 16 km/h [10 mph]) the output and feedback increase to reduce power steering assist. An increase in both duty cycle (output) and voltage in (feedback) as vehicle speed increases indicates an operational EVO system.

IMPORTANT: The ignition must be cycled after a repair or DTC P1555 will reset.