Cherokee 2WD L4-150 2.5L VIN H TBI (1988)
Electronic Brake Control Module: Description and Operation
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.