Vacuum Pump or Brake master cylinder problem - PeachParts Mercedes-Benz Forum

turbo97se · #1 10-16-2010, 10:12 PM

I hate to start so many new threads cos I know how hard it is to find info in multiple threads with the same title but anyway thought it would get better attention.

I have been checking boost from turbo and vacuum from vacuum pump with a gauge I acquired yesterday. Boost seems to be fine from the turbo

My issue is recently my Anti-lock light has been coming on. Thought it may be brake pads that are low at first but they aren't bad actually. So I checked vacuum by teeing into the big brake booster line from the vacuum pump to the brake master cylinder. When idling, the gauge reads 18-20 Inches Hg. When I press the brake pedal, the amount of vacuum drops 1-2 Inches Hg and takes a couple of seconds to recover. If I press the brake pedal like 10 times, I lose all vacuum maybe 2 inches Hg. It takes several seconds to get to 18 Inches Hg again. Any thoughts? Maybe I should put a mityvac on there to see if there's a leak and try to isolate?

Thanks! Sorry for all the threads! I tried to tag this on to another thread with what sounded like the same issue recently.

Thanks!

vstech · #2 10-16-2010, 11:57 PM

OK, first off, why are you pumping your brake pedal? it's a HUGE vac user, and it'll suck a perfectly functional pump out in no time.

sixto · #3 10-17-2010, 12:26 AM

ABS doesn't check booster vacuum. A frequent cause of the ABS warning light is low system voltage such as when the voltage regulator is on its way out. It might also be triggered by an overvoltage protection relay fault.

Sixto
87 300D

turbo97se · #4 10-17-2010, 03:08 AM

I started this whole thing because of the ABS light coming on (and I can feel it kicking in frequently when braking). I pumped it more than once because when I pressed the pedal down I saw the vacuum drop 2 Inches Hg and took a while to come back up. I was wondering if the vacuum pump was weak because it took a while to come back up. On a mechanical pump I would not have expected In-Hg to drop and stay down.

Sixto, I can actually feel the ABS kicking in (even under light braking occasionally) and not just the light coming on, is that still a voltage regulator issue?

fruitcakesa · #5 10-17-2010, 07:39 AM

While I am not familiar with MB abs, on my GMC p/u when the wheel sensor fails the abs would come on immediately on pushing the brake pedal

sixto · #6 10-17-2010, 01:36 PM

As fruitcakesa suggests, check the ABS pulse wheels. There's a cog at either front wheel and a sensor in the rear diff. The front wheel sensors can be damaged when working on the bearings or rotor. I don't know if caked on dirt throws the sensors. Make sure the sensor wires are properly retained and not binding or stretching when the wheels turn left and right. Not much to check visually in the rear other than the condition of the wires.

As for the vacuum, losing vacuum after 10 presses of the pedal seems weak to me but I have no technical information on this. I don't know of any volume tests to determine whether the pump and booster are tight. Thinking out loud, I'd set a vacuum gauge on the engine shutoff nipple on the brake booster line check valve and see that vacuum holds for days after shutting off the engine. I suppose you can pump the booster using a MityVac but I don't have a reference for how many strokes a tight booster needs.

Sixto
87 300D

turbo97se · #7 10-18-2010, 11:09 PM

Thanks for sharing your thoughts. I will check on these things. I am wondering if there are any particular pulses I can scope from the sensors? I will see if I can find them in the WIS I just got. I'll also see if it says anything specific to brake master cylinder/vacuum pump testing. Appreciate the help here. The ABS seems to only kick in under light braking .. if that is any clue too.

Quote:

Originally Posted by sixto

As fruitcakesa suggests, check the ABS pulse wheels. There's a cog at either front wheel and a sensor in the rear diff. The front wheel sensors can be damaged when working on the bearings or rotor. I don't know if caked on dirt throws the sensors. Make sure the sensor wires are properly retained and not binding or stretching when the wheels turn left and right. Not much to check visually in the rear other than the condition of the wires.

As for the vacuum, losing vacuum after 10 presses of the pedal seems weak to me but I have no technical information on this. I don't know of any volume tests to determine whether the pump and booster are tight. Thinking out loud, I'd set a vacuum gauge on the engine shutoff nipple on the brake booster line check valve and see that vacuum holds for days after shutting off the engine. I suppose you can pump the booster using a MityVac but I don't have a reference for how many strokes a tight booster needs.

Sixto
87 300D

turbo97se · #8 11-19-2010, 02:26 AM

Thought I would follow up with some interesting findings. My buddy recommended I spray the ABS wheel sensors with electrical cleaner to clean them up. It seems to have done the trick! My ABS light no longer comes on! Just thought I would share this with other people that encounter this issue!