Vibrating brake pedal at slow speed; machine or replace?

[thrillhouse]

Hi all,
Have recently noticed our paj’s break pedal shudders/vibrates as she’s coming to a stop, and even when idling too.

Doesn’t do it at any other time brakes are being applied.

Question is,should I just machine rotors and new pads? Or replace them all? Guess you eiildnt know until you looked. NT 134kms on the clock. Unsure if they’ve been replaced.

Cheers!


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[Keithyv]

Are you saying the pedal vibrates even after coming to a stop as well?

[thrillhouse]

Are you saying the pedal vibrates even after coming to a stop as well?

Hey Keithy, yeah mate that’s is right - it has done it a few times and a few pumps of the pedal seems to get it gone. Kind of follows the rhythm of idle.

I hadn’t found anything Pajero-specific, but on a BMW forum it was something obscure like a heater valve next to the brake booster that caused it.


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[old Jack]

Sounds to me as if there is a problem with the Hydraulic Brake Booster, possibly low accumulator pressure. Pedal pulsing whilst the vehicle is stopped can only be fluctuating hydraulic pressure.

When was the last time the brake fluid was completely drained, flushed and refilled?

Have you connected a Scan-tool to see if there are any faults?

Are there any warning lights coming on on the dash?

Here is a description on how the system works from the Technical Manual.

An electric pump motor (piston type) has been adopted. The accumulator is pressurised with nitrogen gas and collects the hydraulic pressure which is generated by the pump motor.
When the accumulator hydraulic pressure drops to approximately 16 MPa or below, the pressure switch (for pump control) turns off, and a pump motor drive signal is output from the ASTC-ECU to make the pump motor operate. When the pump motor operates, it pressurises the brake fluid, which is discharged through the check valve to be stored in the accumulator. The operation described above generates the hydraulic pressure (high pressure) which is required for normal brake operation and also HBB, EBD, ABS and ASC/ATC operation. When the accumulator hydraulic pressure drops to approximately 11 MPa or below, the pressure switch (for low pressure warning) turns off, and this causes the brake warning lamp to illuminate and the HBB buzzer to sound. Then, when the accumulator hydraulic pressure rises to approximately 17 MPa, the pressure switch (for pump control) turns on, and after approximately 1 second, the pump motor stops opening. If there is a malfunction of the pressure switch at this time, the release valve opens when the pressure reaches approximately 24 MPa to prevent excessive pressure from building up.


OJ.

[thrillhouse]

Hey OJ, thanks a lot for that information!

No warning lights or buzzing sounds yet. I’ve only noticed it this week, and unfortunately I don’t drive it too often now I have my own car.

We’ve owned it for close to a year and the brake system hasn’t been done since we’ve owned it. It’s due for a service soon so maybe I’ll get them to scan the ECU and check a few things out!


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[old Jack]

Hey OJ, thanks a lot for that information!

No warning lights or buzzing sounds yet. I’ve only noticed it this week, and unfortunately I don’t drive it too often now I have my own car.

We’ve owned it for close to a year and the brake system hasn’t been done since we’ve owned it. It’s due for a service soon so maybe I’ll get them to scan the ECU and check a few things out!


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Is the level of the brake fluid correct?

Are you losing brake fluid?

If it is the HBB then these are very expensive to replace, the Gen 3 Pajero's had HBB issues.

https://www2.pajeroclub.com.au/forum/showthread.php?t=46516

It could just be a dodgy electrical plug or dirty brake fluid.

Sounds like the HBB motor is fine but I am thinking it could be either the pressure sensor or its wiring, or the accumulator has lost its nitrogen pressure, if the fluid drain, flush and bleed does not fix the problem.

OJ.

[thrillhouse]

I will check the fluid level and how it looks tonight! If I get the chance I’ll also drain and bleed.

I do not like the $$ figures with replacing that unit hah! But I hope it is something simple as you say. Thanks again for your help it is much appreciated!


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[Kingbrown]

An element that's 'quite' closely connected to the brake pedal is the vacuum booster.

A fault in the vacuum booster, it's associated check valve or vacuum piping could lead to pulsation of the brake pedal whilst at a stop.

[old Jack]

An element that's 'quite' closely connected to the brake pedal is the vacuum booster.

A fault in the vacuum booster, it's associated check valve or vacuum piping could lead to pulsation of the brake pedal whilst at a stop.

Hi KB,

The Gen 3/4 do not have a vacuum brake booster it is replaced with the electro/hydraulic HBB.

http://faq.out-club.ru/download/pajero_iv/maintenance/Service_Manual_2008_2013/img/35/AC608397AC00ENG.pdf

OJ.

[Kingbrown]

Roger OJ, thanks for setting me straight.

[erad]

The only way I can see a brake pedal moving when the vehicle is stationary is for there to be some air in the system and the HBB is alternately increasing and decreasing the pressure. I suggest that you bleed the brakes until you have a solid pedal.

[thrillhouse]

Hi everyone,
So, got it serviced (at a new place) yesterday.

They did say the brake fault was an ‘ABS Module’, not specifically the HBB pump. But having a look online... seems the whole ABS unit/pump can be called a module.

I’ve sent the mechanic that link regarding rebuilding the motor so I’ll see what they say. Anyone used a second hand one and it’s been okay? I know prob not recommended... but I can’t really afford 2.5k+, or for it to be offf the road for a few weeks!


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[Kingmarz]

Rebuild would be the best financial option from what I’ve read as a second hand one is probably on the way out and $500nz from Aus.

[thrillhouse]

Rebuild would be the best financial option from what I’ve read as a second hand one is probably on the way out and $500nz from Aus.

Yeah that link where a fella got it rebuilt definitely seems the way to go. I hope maybe I can do that and the car not be off the road long - got three kids under four [emoji51]


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[BruceandBobbi]

I usually don't recommend companies or products here but this mob have rewound a few brake motors for people here and have been happy with the work and service.

I've not used them.

http://www.sydneymotorrewinds.com.au/