Isuzu Cars and Trucks 1981-1991

System Self-Diagnosis

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The ABS is equipped with a self diagnostic capability which is used to isolate ABS failures.

Electrical failures in the ABS are detected by the ECBM (located under the passenger seat) and result in the "Anti-Lock' warning lamp illuminating. The "Anti-Lock' warning lamp is intended to inform the driver that a condition exists which results in the ABS being disabled. The "Anti-lock' warning lamp is connected to a Light Emitting Diode (LED) on the EBCM. The LED assists the individual servicing the system by flashing trouble codes which pinpoint the defective component.


NOTE
If more than one failure in the system is detected, the first failure to occur will be flashed in code on the LED. Once this failure has been corrected, the next failure code will be will be flashed.

The EBCM enters diagnostic mode any time a trouble code is set (the "Anti-Lock light illuminates). To read the ABS trouble code, count the number of times the LED on the EBCM turns ON and OFF. Trouble codes are erased from the EBCM memory when the ignition key is turned OFF.

The ABS requires a large quantity of electricity to function. The engine may stall if the battery has been discharged and the vehicle is jump started. In this case, wait until the battery is recharged or disable the ABS by removing the ABS fuse from the fuse box.

After the battery has been recharged, reinstall the fuse, start the engine and check for the presence of ABS trouble codes.

DIAGNOSIS

Diagnosis of the Anti-Lock Brake System consists of three basic steps:



ABS Visual Inspection
 
ABS Functional Check
 
Additional tests as specified by the Functional Check
 

All diagnosis should start with the ABS Visual Inspection and proceed in order through the Functional Check and additional tests until the failure is detected.


NOTE
Some diagnostic procedures require that the ignition switch remain in the ON position for extended periods of time. Low battery voltage may result, causing erroneous voltage readings. Connect a battery charger and apply a slow charge while testing to avoid this problem.



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Fig. Reading the ABS trouble codes Impulse



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Fig. ABS diagnostics Impulse

Pin Out Checks

Before performing the following tests, connect a pinout box (J35592) to the EBCM connector. Use a high impedance digital multimeter to probe all connections of the affected system.


NOTE
In the diagnosis charts, circuits (wires) are labeled with the designation "CKT' and the circuit number. In the wiring diagram, circuits are labeled with the circuit number and a color code. The circuit number should be used to identify the circuit. If an individual wiring diagram is not furnished with a diagnosis chart, refer to the ABS WIRING DIAGRAM.



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Fig. ABS testing Impulse



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Fig. ABS testing Impulse



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Fig. ABS wiring diagram Impulse

Trouble Code Diagnosis
SPEED SENSOR CIRCUIT CODE 11, 12, 13, 14

As each wheel turns, the wheel speed sensor for that wheel creates a small AC voltage as a toothed ring, mounted on the drive axle, passes by a stationary sensor. The frequency of the speed sensor is determined by vehicle speed.

The EBCM uses the wheel speed sensor signal to calculate vehicle speed, individual wheel speed, acceleration and slip values for each wheel. These values are used to determine when anti-lock control is needed.

The following conditions which result in a lack of continuity or low wheel sensor output produce trouble codes:



open or short to ground in sensor high/return wire
 
open or shorted sensor coil
 
short to voltage on wheel speed sensor line
 
improperly installed sensor
 

If a failure is detected which causes a wheel speed sensor code to set, the "Anti-Lock' lamp will remain on for the duration of the ignition cycle.



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Fig. Speed sensor circuit code 11, 12, 13, 14



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Fig. Speed sensor circuit code 11, 12, 13, 14



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Fig. Speed sensor circuit code 11, 12, 13, 14



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Fig. Speed sensor circuit code 11, 12, 13, 14

MISSING WHEEL SPEED SENSOR SIGNAL CODE 15

The EBCM compares the wheel speed sensor output signal to expected values and the output of other wheel speed sensors. When a missing signal is detected, the EBCM sets trouble code 15.

If a failure is detected which causes a wheel speed sensor code to set, the "Anti-Lock' lamp will remain on for the duration of the ignition cycle.



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Fig. Missing wheel speed sensor signal code 16

G-SENSOR CIRCUIT CODE 16

The G-sensor judges high or low road friction. Normally, the switch is in the ON position. With a "G' of more than 0.35G the switch turns OFF. Conditions causing G-sensor code 21 to set are:



wire opens in the G-sensor circuit
 
stop light circuit shorts to ground
 


NOTE
When G-sensor will not turn OFF due to a malfunction, braking distance may be increased on high friction roads. The G-sensor ON malfunction can only be diagnosed through physical inspection.



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Fig. G-sensor circuit code 16

STOP LIGHT SWITCH CIRCUIT CODE 22

A brake light switch input is provided to EBCM pin 25 whenever the brake pedal is depressed. A 12 volt signal is routed through circuit 20 (grn/yel) from the normally open stop light switch.

When the stop light switch stays ON for more than 15 minutes, the EBCM judges the switch to be defective, the "Anti-Lock' light illuminates and the ABS is shut OFF.



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Fig. Stop light switch circuit code 22

WHEEL SOLENOID VALVE CIRCUIT CODE 41, 42, 43

The two valves in the hydraulic unit reduce, hold or increase the brake fluid pressure of the right and left front wheel circuits according to the signal received from the EBCM, to control braking power. When a wire opens or shorts occur in the valve circuit, the "Anti-Lock' light illuminates and the ABS is shut OFF.

Code 41 will set any time an electrical failure is detected in the solenoid valve circuit. Failures include:



open or shorted solenoid coil
 
open or short in front valve or valve relay monitor wire
 

If a failure is detected, the "Anti-Lock' lamp will remain on for the duration of the ignition cycle.



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Fig. Wheel solenoid valve circuit code 41, 42, 43



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Fig. Wheel solenoid valve circuit code 41, 42, 43

VALVE RELAY CIRCUIT CODE 51

The signal from the EBCM causes the valve relay to turn ON/OFF, and establishes the power circuit of the solenoid valve. Any opens or shorts in the circuit, or a defective relay, will illuminate the "Anti-Lock' light and shut the ABS OFF.



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Fig. Valve relay circuit code 51

PUMP MOTOR AND MOTOR RELAY CIRCUIT CODE 52

The signal from the EBCM causes the motor relay to turn ON/OFF, the motor to rotate and the pump to drive. When wire opens or shorts occur in this circuit, or when the motor or motor relay is defective, the "Anti-Lock' light illuminates and the ABS is shut OFF.



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Fig. Pump motor and motor relay circuit code 52

EBCM PROBLEM CODE 55

ABS code 55 indicates that a failure has occurred either in the EBCM or in a circuit which can affect the functioning of the logic circuits. In most cases, code 55 indicates the need for EBCM replacement. It is necessary, however, to check the EBCM ground circuit before replacement. It is possible for code 55 to set intermittently due to external electromagnetic disturbances.



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Fig. EBCM problem code 55

Symptom Diagnosis

Charts should be used as directed by the ABS functional check to assist in the diagnosis of a problem involving a trouble code.



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Fig. ABS wiring Impulse

IGNITION ENABLE CIRCUIT CHART A1

The ignition enable input initiates a signal for the EBCM to perform its self checks and initialization routine. Failure of the EBCM to receive this signal will prevent initialization. The "Anti-Lock' lamp will illuminate and remain ON continuously and no ABS operation will be available. This condition may be caused by:



a blown fuse C-15
 
a short to ground or open in the ignition enable wire
 
an open in the ignition switch wire
 



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Fig. Ignition enable circuit, chart A-1



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Fig. Ignition enable circuit, chart A-1

WHEEL SPEED SENSOR OUTPUT CHART B, B1, B2, B3, B4

This chart identifies wheel speed sensor circuits which may be demonstrating low wheel speed sensor output. Sensors which demonstrate output of less than 100mV in chart B should be investigated. Additionally, if there is a large difference between sensor outputs on the same axle, then the difference should be investigated.

The signal strength of the wheel speed sensor depends primarily on the sensor gap (assuming the internal coil windings and permanent magnet are functioning properly). Low output can be traced to the sensor (including excessive air gap) or the sensor wiring harness. The EBCM uses sensor frequency to determine wheel speed. If the sensor signal strength is large enough for the EBCM to read (min. 100mV AC at 60 RPM), the system will operate. Low wheel speed sensor output can be caused by:



a lack of continuity in the sensor circuits
 
excessive sensor air gap
 
a sensor with low output
 
a wheel speed sensor cable intermittently going open
 
toothed sensor rings which are damaged or have missing teeth
 


NOTE
Install pinout box prior to performing tests. Raise and support vehicle safely so that wheels clear the ground.



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Fig. Wheel speed sensor output chart B



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Fig. Wheel speed sensor output chart B1



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Fig. Wheel speed sensor output chart B2



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Fig. Wheel speed sensor output chart B3



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Fig. Wheel speed sensor output chart B4

ANTI-LOCK LIGHT INOPERATIVE WITH KEY ON CHART C

The "Anti-Lock' light will turn on during engine starting and will usually stay on for approximately three seconds after the ignition switch is returned to the ON position. Lack of power or ground could prevent the light from illuminating.



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Fig. Anti-lock light inoperative with key on chart C

 
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