ABS Light: Description and Operation

Antilock Light Circuit:

The 4WAL ECU uses an amber-colored "Antilock" warning light to alert the driver of a malfunction it has detected. The ECU completes the "Antilock" light circuit through the 10-way connector terminals B and J.

The ECU completes the circuit for the "Antilock" light by providing a ground. It operates the light by making it do one of two things:
^ Light steadily during an initial test at the beginning of an ignition cycle, when a system malfunction exists.
^ Flash ON and OFF (to present trouble code information) when a service technician grounds the ALDL diagnostic request terminal H to A.

An input monitor circuit within the ECU allows it to detect an open or shorted circuit for the "Antilock" light during the initial test for each ignition cycle:
^ When the "Antilock" light is unlit, the voltage at the monitoring point within the ECU will be high.
^ The voltage at the monitor point will be low when the ECU grounds the "Antilock" light circuit.

If the ECU monitors low voltage at terminal B before it attempts to ground the light circuit, it determines that a short or open circuit exists. If the ECU monitors high voltage after it attempts to ground the "Antilock" light circuit, it determines that a short to voltage condition exists.

In the case of either malfunction, the ECU will take diagnostic action by:
^ Completing the "Brake" light to alert the driver
^ Storing a trouble Code 86 (short)