Mitsubishi Montero (2002-2004). Manual - part 997

HYDRAULIC BRAKE BOOSTER (HBB) DIAGNOSIS

TSB Revision

BASIC BRAKE SYSTEM

35A-13

HYDRAULIC BRAKE BOOSTER (HBB) DIAGNOSIS

INTRODUCTION

M1351012800059

The hydraulic brake booster (HBB) accumulates the 
high-pressure brake fluid in the accumulator by oper-
ating the pump motor. Even though the working 
sound of the pump motor can be heard during opera-
tion, this sound is usually generated at normal opera-
tion of the HBB and is not an indication of a 
malfunction.

HBB Diagnostic Trouble Code Detection 
Conditions

HBB diagnostic trouble codes (HBB DTCs) are set 
under different conditions, depending on the mal-
function detected. Most HBB DTCs will only be set 
during vehicle operation. Some HBB DTCs will also 
be set during the HBB self-check immediately after 
the engine is started.

After erasing an HBB DTC, duplicate the HBB DTC 
set conditions to see if it resets. Depending on the 
detection timing and set conditions for the specific 
HBB DTC, you must either drive the vehicle or turn 
the engine off and restart it. To set the proper condi-
tions for that DTC again, refer to "HBB DTC SET 
CONDITIONS" for each HBB DTC that you are trying 
to reset.

DIAGNOSTIC TROUBLESHOOTING STRATEGY

M1351012900056

Use these steps to plan your diagnostic strategy. If 
you follow them carefully, you will be sure that you 
have exhausted most of the possible ways to find an 
HBB fault.
1. Gather information about the problem from the 

customer.

2. Verify that the condition described by the 

customer exists.

3. Check the vehicle for any HBB DTC.
4. If you cannot verify the condition and no HBB 

DTC sets, it can be assumed that the malfunction 
is intermittent. Refer to GROUP 00, How to use 
Troubleshooting/Inspection Service Points 

− How 

to Cope with Intermittent Malfunctions 

P.00-6

.

5. If you can verify the condition but no HBB DTC 

sets, or the system cannot communicate with the 
scan tool, check that the basic brake system is 
operating properly.

• If the basic brake system is not operating prop-

erly, refer to "Basic Brake System Diagnosis" 
(

P.35A-6

).

• If the basic brake system is operating properly, 

refer to "Symptom Chart" (

P.35A-80

).

6. If an HBB DTC sets, record the number of the 

DTC, then erase the DTC from the memory using 
the scan tool.

7. Duplicate the HBB DTC set conditions to see if 

the same HBB DTC will set again.

• If the same HBB DTC sets again, perform the 

diagnostic procedures for the DTC. Refer to 

P.35A-15

.

• If you cannot make the same HBB DTC set 

again, it can be assumed that the malfunction is 
intermittent. Refer to GROUP 00, How to use 
Troubleshooting/Inspection Service Points 

− How 

to Cope with Intermittent Malfunctions 

P.00-6

.

HYDRAULIC BRAKE BOOSTER (HBB) DIAGNOSIS

TSB Revision

BASIC BRAKE SYSTEM

35A-14

BRAKE WARNING LIGHT CHECK

M1351013000056

Check that the brake warning light illuminates as follows:
1. Check that the brake fluid level in the reservoir is between 

the "MIN" and the "MAX" lines.

2. Choke the wheels and release the parking brake.
3. When the ignition switch is turned to the "ON" position, the 

brake warning light illuminates.

4. When the ignition switch is turned from the "START" position 

back to the "ON" position, the brake warning light illuminates 
for approximately 3 seconds and then switches off.

5. If the illumination is other than the above, check the 

diagnostic trouble codes.

TROUBLE CODE DIAGNOSIS

M1351011200054

Retrieving HBB Diagnostic Trouble Codes

Required Special Tool:

• MB991502: Scan Tool (MUT-II)

CAUTION

To prevent damage to scan tool MB991502, always turn the 
ignition switch to the "LOCK" (OFF) position before con-
necting or disconnecting scan tool MB991502.
1. Connect scan tool MB991502 to the data link connector.
2. Turn the ignition switch to the "ON" position.
3. Use scan tool MB991502 to check for HBB diagnostic 

trouble codes.

4. Turn the ignition switch to the "LOCK" (OFF) position.
5. Disconnect scan tool MB991502.

Erasing HBB Diagnostic Trouble Codes

NOTE:  If the M-ASTC-ECU functions have stopped due to the 
fail-safe function, the HBB diagnostic trouble code cannot be 
erased.

Required Special Tool:

• MB991502: Scan Tool (MUT-II)

AC000682 AB

BRAKE
WARNING
LIGHT

IGNITION
SWITCH

ILLUMINATED

NOT
ILLUMINATED

APPROXIMATELY 3S

LOCK, ACC

ON

START

BRAKE WARNING LIGHT

ACX01539

16-PIN

MB991502

AC

HYDRAULIC BRAKE BOOSTER (HBB) DIAGNOSIS

TSB Revision

BASIC BRAKE SYSTEM

35A-15

CAUTION

To prevent damage to scan tool MB991502, always turn the 
ignition switch to the "LOCK" (OFF) position before con-
necting or disconnecting scan tool MB991502.
1. Connect scan tool MB991502 to the data link connector.
2. Turn the ignition switch to the "ON" position.
3. Use scan tool MB991502 to erase HBB diagnostic trouble 

codes.

4. Turn the ignition switch to the "LOCK" (OFF) position.
5. Disconnect scan tool MB991502.

DIAGNOSTIC TROUBLE CODE CHART

M1351013700066

Follow the inspection chart that is appropriate for the 
diagnostic trouble code.

NOTE: If the trouble marked by * has occurred, the M-ASTC-ECU sounds the buzzer continuously to inform 
the trouble of the driver and protects the pump motor by activating the pump motor intermittently.

ACX01539

16-PIN

MB991502

AC

DTC

INSPECTION ITEM

DIAGNOSTIC CONTENT

REFERENCE PAGE

16

Battery positive voltage

M-ASTC-ECU power supply voltage is 
extremely low or high

P.35A-16

31

Ignition switch circuit

Open circuit at the HBB power supply 
sub circuit

P.35A-18

53

Pump motor

• Seizure of the pump motor*

• Abnormality at the current detection 

circuit of the M-ASTC-ECU

P.35A-23

54

Motor relay

Open circuit, short circuit or failure of the 
motor relay coil

P.35A-31

55

Pump motor circuit

Pump motor energized abnormally for 
long period*

P.35A-56

56

Pressure switch

• Open circuit at the pressure switch (for 

pump motor control)

• Open circuit or short circuit at the 

pressure switch (for low-pressure 
warning)

P.35A-69

57

Accumulator

Accumulator low-pressure abnormality*

P.35A-77

58

M-ASTC-ECU (Power supply 
drive circuit)

Pump motor drive circuit in the M-ASTC-
ECU abnormality

Replace the M-ASTC-
ECU (Refer to 
GROUP 35C, M-
ASTC-ECU 

P.35C-

205

).

HYDRAULIC BRAKE BOOSTER (HBB) DIAGNOSIS

TSB Revision

BASIC BRAKE SYSTEM

35A-16

HBB DIAGNOSTIC TROUBLE CODE PROCEDURES

DTC 16: Battery Positive Voltage (M-ASTC-ECU Power Supply Voltage is Extremely Low or High)

CAUTION

If the battery voltage drops or rises while making this check, DTC 16 is set as an existing problem, 
making it impossible to perform correct trouble diagnosis. Before carrying out the following diagno-
sis, test the battery and charge it if necessary.

AC204492AC

ASC-ECU Power Supply Circuit

AC204688 AB

CONNECTOR: D-116

D-116

AC204547AB

CONNECTORS: D-208, D-212

D-208

D-212

JUNCTION BLOCK
(FRONT VIEW)