General Description

Fig.3 ECU Location -- Under Rear Seat:

Fig. 4 Electronic Control Unit (ECU):

ELECTRONIC CONTROL UNIT (ECU)
The ECU is attached to a bracket located under the rear seat (fig. 3), and is used to monitor and control the entire anti-lock system (fig. 4). The ECU is connected to the wiring harness through two connectors, one is a 32 pin and the other is a 24 pin. The power up source for the ECU is through the ignition switch in the "RUN" or "ON" position.

The ECU receives information from the wheel speed sensors, modulator solenoid voltage, and the pressure and brake switches. These are routed through signal conditioners, that convert the AC signals to square wave signals; voltage comparators, that sense under voltage conditions, and input buffers, that isolate the microprocessor from the switches. The microprocessor receives these inputs and makes decisions based on them and then controls the various output drivers.

AUTOMATIC RESET
The ECU has several hardware controls that help ensure proper operation. The ECU has internal circuitry to monitor itself. The self-test circuitry will cause the anti-lock brake ECU to reset when the following conditions are seen:

^ Solenoid voltage less than 6.0 volts.
^ Two decay solenoids active for greater than 1-2 seconds
^ A build and decay solenoid on the same control channel active at the same time.

The ECU contains a self-diagnostic program which triggers the indicator lights when a system circuit fault is detected. Faults are stored in a diagnostic program memory. There are 14 different faults that can be detected by the ECU. However, the ECU will only store each fault one time. Therefore, if a vehicle has a problem that has occurred more than one time the ECU will only record that fault once. These faults remain in memory until cleared by disconnecting the battery from the ECU.

CIRCUITS THAT COULD FAULT FAULT CODES
The circuits that could indicate a problem to the ECU and be stored as faults include:

^ Modulator Circuit
^ Modulator Solenoid Under Voltage
^ Brake Switch Circuit
^ Low Fluid Level Circuit
^ Relay Circuit
^ Right Rear Wheel Speed Circuit
^ Left Rear Wheel Speed Circuit
^ Right Front Wheel Speed Circuit
^ Left Front Wheel Speed Circuit
^ Pressure Differential Circuit
^ Boost Pressure Switch Circuit
^ Low Accumulator Switch Circuit
^ Booster Pump and Motor Circuit
^ ECU Self Check

The ECU is also equipped with a mercury switch mounted at a specific angle which senses vehicle deceleration and provides input to the ECU for improved system operation on slippery surfaces.

NOTE: Proper mounting angle of the ECU is critical to correct and accurate operation of the mercury switch.

The stored faults can be read by the DRB II tester to assist in diagnosing system faults.