Mitsubishi Montero (2004+). Manual - part 1018

HYDRAULIC BRAKE BOOSTER (HBB) DIAGNOSIS

TSB Revision

BASIC BRAKE SYSTEM

35A-71

STEP 15. Recheck for diagnostic trouble code.
(1) Erase the DTC memory.
(2) Recheck for diagnostic trouble code.
Q: Is DTC 55 set?

YES : Start over at Step 1.
NO :  The procedure is complete.

DTC 56 : Pressure Switch (Open or Short Circuit at the Pressure Switch)

.

CIRCUIT OPERATION

• The pressure switch (for low-pressure warning) 

monitors brake fluid pressure inside of the 
hydraulic brake booster (HBB). When the brake 
fluid pressure decreases abnormally, the pres-
sure switch (for low-pressure warning) is turned 
off and the brake fluid low-pressure warning sig-
nal is sent to the M-ASTC-ECU. 

AC204497AB

Pressure Switch Circuit

AC204545 AC

CONNECTORS: B-17, B-18

B-17 (GR)

B-18 (B)

AC203901AD

CONNECTORS: E-119, E-120

E-119

E-120

HYDRAULIC BRAKE BOOSTER (HBB) DIAGNOSIS

TSB Revision

BASIC BRAKE SYSTEM

35A-72

• The pressure switch (for pump motor control) 

monitors brake fluid pressure inside of the 
hydraulic brake booster (HBB). When the brake 
fluid pressure decreases lower than the specified 
value, the pressure switch (for pump motor con-
trol) is turned on and outputs the pump motor 
drive signal to the M-ASTC-ECU. 

• The M-ASTC-ECU monitors an open circuit of the 

pressure switch (for pump motor control) (at 11 
terminal). 

• The M-ASTC-ECU monitors an open circuit or 

short circuit of the pressure switch (for low-pres-
sure warning) (at 57 terminal).

.

HBB DTC SET CONDITIONS
DTC 56 is set if the pressure switch (for low-pressure 
warning) circuit is open or short, or the pressure 
switch (for pump motor control) circuit is open.
NOTE: 

.

•

DTC 55 will be set if the pressure switch (for 
pump motor control) is stuck on. 

•

DTC 57 will be set if the pressure switch (for 
low-pressure warning) is stuck off. 

.

TROUBLESHOOTING HINTS

• Malfunction of the master cylinder and hydraulic 

unit assembly [Integrated with the hydraulic brake 
booster (HBB)]

• Damaged wiring harness or connector

• Malfunction of the M-ASTC-ECU

.

DIAGNOSIS

Required Special Tool:

•  MB991958: Scan Tool (MUT-III Sub Assembly)

• MB991824: Vehicle Communication Interface (V.C.I.)

• MB991827: MUT-III USB Cable

• MB991911: MUT-III Main Harness B

• MB991223: Harness Set

STEP 1. Check the following connectors.

• HBB connector B-17 and B-18

Check connectors for loose, corroded or damaged terminals, or 
terminals pushed back in the connector.
Q: Are the connectors and terminals in good condition?

YES : Go to Step 2.
NO :  Repair the faulty connector or terminal and then go to 

Step 8.

AC204629 AB

CONNECTORS: B-17, B-18

B-17 (GR)

B-18 (B)

B-17

B-18

HBB CONNECTORS

HYDRAULIC BRAKE BOOSTER (HBB) DIAGNOSIS

TSB Revision

BASIC BRAKE SYSTEM

35A-73

STEP 2. Check the pressure switch (for pump motor 
control) circuit inside the HBB.

(1) Depress the pedal repeatedly more than 40 times until the 

pedal depressing force feels heavy with the ignition switch 
at the "LOCK" (OFF) position to release the pressure of 
HBB power supply system.

(2) Disconnect HBB connector B-18.

(3) Measure the resistance between the terminals 33 and 35 at 

the HBB. The resistance should measure approximately 1 
kilo ohms. 

Q: Is the resistance approximately 1 kilo ohms?

YES : Go to Step 3.
NO :  Replace the master cylinder and hydraulic unit 

assembly (Refer to 

P.35A-140

). Then go to Step 8.

AC204629 AD

CONNECTOR: B-18

B-18 (B)

HBB CONNECTOR

B-18

AC204685

HBB 
CONNECTOR

B-18

AB

HYDRAULIC BRAKE BOOSTER (HBB) DIAGNOSIS

TSB Revision

BASIC BRAKE SYSTEM

35A-74

STEP 3. Check the pressure switch (for low-pressure 
warning) circuit inside the HBB.

(1) Depress the pedal repeatedly more than 40 times until the 

pedal depressing force feels heavy with the ignition switch 
at the "LOCK" (OFF) position to release the pressure of 
HBB power supply system.

(2) Disconnect HBB connector B-17.

(3) Measure the resistance between the terminals 1 and 5 at 

the HBB. The resistance should measure approximately 5.7 
kilo ohms. 

Q: Is the resistance approximately 5.7 kilo ohms?

YES : Go to Step 4.
NO :  Replace the master cylinder and hydraulic unit 

assembly (Refer to 

P.35A-140

). Then go to Step 8.

AC204629 AC

CONNECTOR: B-17

B-17 (GR)

HBB CONNECTOR

B-17

AC204686 AB

HBB CONNECTOR

B-17