DEPRESSURIZING THE HYDRAULIC UNIT
The ABS pump motor assembly will keep the accumulator charged to a pressure between approximately 2000 psi (13,800 kPa) and 2600 psi (18,000 kPa) any time the ignition is in the ON or RUN position. The pump/motor cannot operate if the ignition is OFF or if a battery cable is disconnected.
- With the ignition OFF and the negative battery cable disconnected, pump the brake pedal a minimum of 20 times using at least 50 ft. lbs. (68 Nm) of pedal force each time.
- A definite increase in pedal effort will be felt as the accumulator becomes discharged.
- After the increased pedal effort occurs, continue with 5-10 additional brake applications to release any remaining pressure.
Before any system diagnosis is begun, the brake system should be inspected visually for common faults which could disable the ABS or cause a code to set. Check the vehicle carefully for any sign of:
Carefully inspect the multi-pin connectors at the EBCM for pushouts or poor connections.
Once the visual check has been performed, perform the functional check (found in the diagnostic charts) to determine if the problem is truly ABS related or arising from common faults.
DISPLAYING ABS TROUBLE CODES
See Figure 1
The 1st generation system found on the 6000 AWD does not display trouble codes; diagnosis is performed through symptom analysis and circuit testing.
Only certain ABS malfunctions will cause the EBCM to store diagnostic trouble codes. Failures causing a code will generally involve wheel speed sensors, main valve or the inlet and outlet valves. Conditions affecting the pump/motor assembly, the accumulator, pressure switch or fluid level sensor usually do not cause a code to set.
The EBCM will store trouble codes in a non-volatile memory. These codes remain in memory until erased through use of the correct procedure. The codes are NOT erased by disconnecting the EBCM, disconnecting the battery cable or turning off the ignition. Always be sure to clear the codes from the memory after repairs are made. To read stored ABS trouble codes:
- Turn ignition switch to ON . Allow the pump to charge the accumulator. If fully discharged, the dash warning lights may stay on up to 30 seconds. If the ANTI-LOCK warning light does not go off within 30 seconds, make a note of it.
- Turn ignition switch to OFF .
- Remove the cover from the Assembly Line Diagnostic Link (ALDL) connector. Enter the diagnostic mode by using a jumper wire to connect pins H and A or to connect pin H to body ground.
On 1993 and later models the ALDL is also known as the Diagnostic Link Connector (DLC).
- Turn the ignition switch to ON and count the light flashes for the first digit of the first code. The ANTI-LOCK light should illuminate for 4 seconds before beginning to flash. If, after 4 seconds, the light turns off and stays off, no codes are stored.
- The light will pause for 3 seconds between the first and second digits of the first code and then continue flashing. When counting flashes, count only the ON pulses.
- When the EBCM is finished transmitting the second digit of the first code, the ANTI-LOCK light will remain on. This last, constant ON should not be counted as a flash. Record the 2-digit code.
- Without turning the ignition switch OFF , disconnect the jumper from pin H and reconnect it. If an additional code is present, it will be displayed in similar fashion to the first. Record the second code.
- Repeat the disconnection and reconnection of pin H without changing the ignition switch position until no more codes are displayed. The system is capable of storing and displaying 7 codes, the ANTI-LOCK warning light will stay on continuously when all codes have been displayed.
- After recording each code, remove the jumper from the ALDL and replace the cover.
The ABS trouble codes are not specifically designated current or history codes. If the ANTI-LOCK light is on before entering the ABS diagnostic mode, at least 1 of the stored codes is current. It is impossible to tell which code is current. If the ANTI-LOCK light is off before entering the diagnostic mode, none of the codes are current.
Although the ABS trouble codes stored by the EBCM are not identified as current or history codes, these codes may still be useful in diagnosing intermittent conditions.
If an intermittent condition is being diagnosed:
- Obtain an accurate description of the circumstances in which the failure occurs.
- Display and clear any ABS trouble codes which may be present in the EBCM.
- Test drive the vehicle, attempting to duplicate the failure condition exactly.
- After duplicating the condition(s), stop the vehicle and display any ABS codes which have set.
- If no codes have been stored, refer to the Symptom Diagnosis Charts. A good description of vehicle behavior can be helpful in determining a most likely circuit.
Most intermittent problems are caused by faulty electrical connections or wiring. Always check for poor mating of connector halves or terminals not fully seated in connector bodies, deformed or damaged terminals and poor terminal to wire connections.
Most failures within the ABS will disable the anti-lock function for the entire ignition cycle, even if the fault clears before the next key-off occurrence. There are 3 situations which will allow the ABS to re-engage if the condition corrects during the ignition cycle. Each of these will illuminate 1 or both dash warning lights.
CLEARING TROUBLE CODES
Stored ABS trouble codes should not be cleared until all repairs are completed. The control module will not allow any codes to be cleared until all have been read. After reading each stored code, drive the vehicle at a speed over 18 mph. (31 km/h).
Re-read the system, if codes are still present, then not all the codes were read previously or additional repair is needed.
In the following diagnostic charts, certain special tools may be required. Use of the J-35592 pinout box or equivalent, is required to avoid damage to the connectors at the EBCM. Use of J-35604 pressure gauge and J-35604-88 adapter (or equivalents) will be required to measure accumulator pressure. The use of a high-impedance Digital Volt/Ohmmeter (DVM or DVOM) is required at all times.