ABS coming on - over sensitive!
 

Hi All

My 'other car' has a Bosch ABS system which is giving me a headache in resolving and so I would like to ask the experts here for some friendly advice. Hope you guys don't mind.

Here's the background:

Symptoms
During relatively light braking (say approaching a T junction) the ABS engages as if one were on ice. The hydraulic 'pumping' and ABS sensation can be felt at the wheels. The road surface gives no reason for the ABS to engage.


No ABS fault light

No fault code stored

Two of the toothed ABS sensors had become decoupled from the front drive shaft and was originally thought to be the cause, these were glued back in place but fault remains.

Changed all pads to ensure brakes wern't 'sticking', wheels tested on a test station and brake pressures measure equal forces.

Next thought I had was dropped counts from a sensor at slow speed (sensor's output voltage is proportional to speed), but by using diagnostic equipment which can measure each ABS sensor's count at the I/P to the ABS unit and compare with the other sensors, no count anomalies are shown! ie all counts are the same.

I'm now at a loss as to what else could 'trick' the ABS unit into thinking the car's in a skid??


Any thoughts would be most welcomed.
Lea

No thoughts?


Some additional info:

The brake pedal also sinks after around 30 seconds of pressure (engine running, rock solid when engine off), very much like a leaky master cylinder so I replaced the MC and it still sinks. It also occasionally sinks then takes around 10 seconds to come back up.

Strange. This may or may not be related.

Question. When the ABS system is operating and releasing hydraulic pressure from a brake, where does the fluid go? Someone said to me it's dumped into the modulator which I'm guessing is some type of reserve space in the ABS unit itself!?

Any thoughts most welcomed.
Lea

SOLVED

Even though the diagnostic equipment reported no sensor count errors new toothed rings on the front drive shafts (had to buy 2 new complete drive shafts as the rings are integral) solved the problem - weird!

Lea

hello LeaUK
I am happy you solved your problem so quickly.

but I have one question. Weren't the rings just so dirty that sensors did not feel anymore each single tooth?

thanks for reply.

All sensors and toothed rings were clean.

The fault didn't show on diagnostic equipment designed to count the pulses and compare wheels so I can only conclude that pulses were dropped (gained) intermittently due to the crack in two front rings.

Myself, my repair garage the car Manufacturer's dealer and multiple brake and my car's manufacturer's specialists cannot understand the symptoms but the new drive shafts incorporating the new rings have certainly cured the issue!

Thanks
Lea

<<car's manufacturer's specialists cannot understand the symptoms but the new drive shafts incorporating the new rings have certainly cured the issue!
>>

..and where are these 'Driveshafts" on that car ?????????????

For info the car is a Peugeot 406.

The sensor rings are mounted on the front drive shafts adjacent the the CV joints.

They are heat shrunk at manufacture and so complete new shafts were required (read expensive).

Lea

You may want to mention the make next time you ask for assistance.
You have two Benz models listed alomng with a Benz picture, so that surely does not indicate you were looking for info on Peugot.
Thanks

Arthur

I take your point but it was intentional. There are three/four well known ABS system manufacturer's shared amonst vehicles and so the manufacturer of car is rather incidental. Also, had I mentioned the manufacturer I probably would have got 'what are you posting here for' comments so I wanted to avoid.

I hope this thread will be useful for others whom experience the same phenomenon with their vehicles in the future.


Cheers
Lea

<<Also, had I mentioned the manufacturer I probably would have got 'what are you posting here for' comments so I wanted to avoid.
>>

Well, that sure does explain it .....................

Sorry for my confusion on the matter. I am just a Tech.
Good Day.