SERVICE MANUAL
  Applies to: Excel 1994
GROUP
Fuel SystemMFI Control System

IDLE SPEED CONTROL (ISC) CIRCUIT: CODE 15
CIRCUIT DESCRIPTION

The idle speed control circuit consists of the idle speed control (ISC) motor, the ISC motor position sensor, the idle switch and the engine control module (ECM). The ECM maintains engine idle speed by adjusting the position of the ISC motor. When the ECM detects that the idle switch is closed, it looks at the output voltage from the ISC motor position sensor and then adjusts engine idle speed by extending or retracting the ISC motor screw mechanism. When the idle switch is open, the ECM monitors throttle position sensor voltage and adjusts ISC motor position. Generally, in this mode, the ECM retracts the ISC motor screw mechanism to allow for rapid deceleration. Other parameters that affect ISC motor position are engine coolant temperature, air conditioning on or off, gear position and power steering load.

The ECM changes the direction of ISC motor rotation by controlling the polarity of voltage applied to the motor. The ISC motor position sensor is a potentiometer (variable resistor). As ISC motor position changes, sensor resistance changes. The ECM monitors the voltage drop across the sensor to determine ISC motor position.

FAILURE CONDITIONS

Diagnostic trouble code 15 will set when the ECM detects that the idle switch is closed with ISC motor position sensor voltage above 4.8 volts or below 0.2 volts for at least 4 seconds. Code 15 can also be set if the ECM senses that the ISC motor does not reach the commanded position.

SYSTEM RESPONSE

Engine stalling will occur when the accelerator pedal is released with diagnostic trouble code 15 set. When the ECM sets code 15, the CHECK ENGINE indicator will turn on and the code will be recorded in memory. If the ISC motor circuit starts operating correctly, the CHECK ENGINE indicator will turn off, but the code will remain in memory.

FAILURE CAUSES

Failures that will cause diagnostic trouble code 15 to set:

  • Open or short in the ISC motor position sensor or wiring.
  • Open or short in the ISC motor or wiring.
  • Shorted idle switch or idle switch wiring.
  • Misadjusted throttle position sensor or throttle cable.
MODEL DIFFERENCES
  • 1994 Excel vehicles, idle switch is built into the ISC motor position sensor.
  • 1994 Elantra vehicles use a different type of idle speed control mechanism and have a stand-alone idle switch. Code 15 is not a valid code for these vehicles.
Schematics
EXCEL
NOTES ON INTERMITTENTS

Most likely causes for code 15 setting due to an intermittent failure are:

  • A poor splice crimp or connection in the ISC motor position sensor ground circuit (see TSB 021 for 1990 Excel models and TSB 91#36#003 for 1991 Sonata models equipped with the 2.4L engine). This splice also serves several other components and other codes may set in conjunction with code 15.
  • Poor or corroded terminal contacts at the ISC motor or ISC motor position sensor connectors.
TEST DESCRIPTIONS
  1. CHECK FOR CORRECT THROTTLE POSITION SENSOR SIGNAL
  1. CHECK FOR CORRECT ISC MOTOR POSITION SENSOR SIGNAL
  1. CHECK FOR CORRECT IDLE SWITCH OPERATION
  1. CHECK FOR INOPERATIVE ISC MOTOR
  1. CHECK FOR REFERENCE VOLTAGE AT IDLE SWITCH
  1. CHECK FOR SHORT TO GROUND IN WIRING BETWEEN ECM AND ISC MOTOR
  1. CHECK FOR OPEN CIRCUIT IN WIRING BETWEEN ECM AND ISC MOTOR
  1. CHECK FOR REFERENCE VOLTAGE AT ISC MOTOR POSITION SENSOR
  1. CHECK FOR GROUND AT ISC MOTOR POSITION SENSOR
  1. CHECK FOR OPEN ISC MOTOR POSITION SENSOR SIGNAL CIRCUIT
  1. CHECK FOR SHORT TO GROUND IN ISC MOTOR POSITION SENSOR SIGNAL CIRCUIT
TESTS
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