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READING CODES
Modes will not switch while the engine is running. The ECM automatically switches to MODE I when the ignition is switchedOFF.
1984-90 Vehicles With Dual LED System
The self-diagnostic system has 5 modes. To switch modes, switch the ignition ON and use a screwdriver to turn the mode select switch through the access port on the ECM. The inspection lights will begin to flash. When the desired mode has been entered, turn the screwdriver fully counterclockwise. The red LED and green LED perform different functions in different modes.
Mode I-
This is the normal vehicle operating mode. The green LED will indicate loop status. If the green LED is not blinking when the engine is running, the vehicle is in open loop, or the HO 2 S, O 2 S, or sensor circuit has a malfunction. If the green LED is blinking, the vehicle is in closed loop. If a malfunction takes place, the red LED and the MIL will glow, indicating a malfunction has occurred.
Mode II-
Both the red LED and green LED are used to monitor mixture ratio feedback control. The green LED will function as described in MODE I. If vehicle is in open loop, the red LED and green LED will remain on or off. In order for results to be valid, vehicle must be in closed loop. In closed loop, the red LED remains off if the system is running rich, comes on if the system is running lean, and blinks at the same time with the green LED if system is within 5% of the exact air/fuel ratio.
Mode III
-When this mode is accessed, the codes stored in the ECM's memory will be flashed by the green LED and red LED on the side of the ECM. The red LED will light first. This is the first digit of the trouble code; the green LED will flash next, showing the second digit of the code. For example: 2 flashes of the red LED followed by 1 flash of the green LED corresponds to Code 21.
Mode IV-
This mode is used for checking the throttle plate (TP) switch, starter, and vehicle speed switch status. When the TP switch or starter switch is activated, the red LED will briefly flash as the status changes. For VSS status, the green LED will remain off when vehicle speed is less than 12 mph (19 km/h) and come on when vehicle speed is greater than 12 mph (19 km/h).
Mode V-
This mode represents a real-time diagnostic test of the crank angle sensor, ignition signal and the mass airflow signal. The malfunction code will be displayed only when the malfunction occurs and will not be stored in memory. The output code will appear as a single or short string of pulses. Count the number of pulses for the malfunction code.
1 pulse-CMP sensor signal lost or noise in signal
2 pulses-MAF Sensor signal abnormal
3 pulses-Fuel pump circuit shorted or open
4 pulses-Ignition signal lost
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