Positions
A/C Compressor Switch
The scan switch displays the on and off status of the ECM A/C signal line. On condition indicates ECM command to energize compressor clutch.
Air Control Switch (On/Off)
Displays status of ECM signals to air cleaner divert and air switching solenoids. With air cleaner divert signal in off condition, air pump output should be routed to atmosphere. In the on position, air pump output should be routed to the air switching solenoid. When the air divert solenoid is on and the air switching solenoid is on, air should be delivered to the exhaust ports. When the divert solenoid is on and the switching solenoid is off, air should be delivered to the catalytic converter.
Air Switch-AIR Control Solenoid
This switch can be used to check for proper operation of the AIR system by directing air to the exhaust ports to test effect on readings such as the O 2 sensor voltage or MC solenoid dwell. When activated, the solenoid remains energized for up to 20 seconds. After 20 seconds, a timer energizes the solenoid to prevent heat damage to the catalytic converter.
Baro-Barometric Pressure
In this position Baro sensor output voltage is monitored.
Batt
On some systems this position is used to monitor battery voltage.
Loop Status (O/L-C/L)
This position indicates whether the engine control system is operating in open (O/L) or closed (C/L) mode. Most systems are designed to enter closed loop after a certain amount of run time; when the coolant temperature exceeds a specified value and the oxygen sensor begins to function. This mode can be helpful in detecting intermittent opens in the O 2 sensor circuit, as the SCAN tool will indicate open loop operation long before the appropriate trouble code will be set in the ECM Memory.
Coolant Temp.
This position displays engine coolant temperature in degrees Centigrade. Can be used to detect malfunctions in the coolant temperature circuit if readings are not consistent with engine operation.
Ign/Crank-Ignition Switch Status
The crank mode (on) position engages the starter. The SCAN displays the on/off status of the ignition switch as it is transmitted to the ECM.
Crossover Counts (Numeric Display)
The O2 sensor voltage fluctuates around a threshold voltage, the value of which is stored in the PROM. A counter within the ECM ``counts'' the number of times that actual oxygen sensor voltage crosses this threshold to indicate how fast the sensor is oscillating. The crossover count is the a value representing the number of times that the sensor value crosses this threshold in 1.25 seconds. A higher crossover count number indicates a more closely air/fuel mixture ratio.
M/C Dwell-Mixture Control Solenoid Dwell
The M/C solenoid regulates fuel flow through the carburetor to control air/fuel mixtures based on operating signals from the ECM. When the ECM completes the solenoid ground circuit, the solenoid is energized and fuel flow is reduced, providing leaner air/fuel mixtures. When the ECM opens the ground circuit, the solenoid is de-energized and more fuel is allowed to flow through the carburetor providing richer air/fuel mixtures.
The solenoid is cycled on and off 10 times per second. The amount of on time versus off time (duty cycle) for these 10 pulses determines the air fuel mixtures, and the duty cycle is measured in degrees of dwell. A low dwell reading indicates more solenoid off per cycle, or a rich mixture command, while a higher dwell indicates more solenoid on time or a lean mixture command. With the ignition on, engine stopped and the SCAN tool connected, most ECM's will output a median command of 30 degrees dwell.
EFE Solenoid
The SCAN tool displays the on/off status of the EFE system. When the ignition switch is on and coolant temperature is low, the ECM completes the EFE solenoid ground path, energizing the solenoid, which in turn energizes the heater grid. When temperature increases, the ECM opens the solenoid ground path and the EFE heater is turned off.
EGR Solenoid
The SCAN tool monitors only the ECM drive command to the EGR solenoid. When the engine is cold, the ECM energizes the solenoid and vacuum signals to the EGR valve are blocked. When the engine is warm, the ECM switches the solenoid off and vacuum is allowed to reach the EGR valve.
Knock Retard
On engines equipped with ESC, this mode will indicate if the ECM is recognizing the retard signal from the ESC module and taking appropriate action. If knock is being detected, the numerical display on the tool will count up to 255, then start over.
MAP/VAC Sensor
Voltage generated by the Manifold Absolute Pressure (MAP) or vacuum sensors will be displayed. Sensors generate low voltage (approx. 1 volt) when pressure is low or manifold vacuum is high, and high voltage (approx. 4.5 volts) when pressure is high or vacuum is low.
MPH
Displays vehicle speed by sensing pulses transmitted to the ECM from the vehicle speed sensor.
ISC/Nose-Idle Speed Control Nose Switch
This position indicates the on or off condition of the ISC nose switch. The switch, mounted in the ISC housing is closed (on) whenever the ISC plunger is in contact with the throttle lever.
O2 Sensor Voltage
Oxygen sensor voltage is displayed in millivolts. If displayed voltage is consistently below 350 mv, the ECM is receiving a lean signal; if voltage is consistently above 550 mv, ECM is receiving a rich mixture signal. If reading remains between 350 mv and 550 mv for a specified amount of time, trouble code 13 will set in ECM.
RPM (Engine Speed)
Displays engine RPM as transmitted to ECM from reference sensor. Can be used to detect extra reference pulses caused by electrical interference in reference circuit.
TPS-Throttle Position Sensor Voltage
Values displayed will be voltage values through the TPS as sensed by the ECM. Value at idle should be as specified in adjustment procedure, while value at wide open throttle should be approximately 5 volts.
WOT-Wide Open Throttle Switch
SCAN displays on/off status of WOT switch as sensed by ECM. Switch should be on when throttle is in wide open position.