Odd brake vibrations at low speed (E30 325)

[spiny]

Hi Guys,
got an odd problem with the brakes on my 325 tourer. At low speeds, less then 5mph probably, i get a weird pedal feel like the abs is cutting in, it feels like rubbing your thumb along a comb, and at the same time there is a 'sproingy' noise from the ABS pump area, a bit like when you spring a ruler off a table ...

in normal driving the brakes seem ok, but pulling up to a stop just before the car actually stops, is when the springyness happens. the car does stop, just with the weird feel, like if you try and brake on wet grass / snow etc.

sooooo, wheres the best place to start ?

presumably cleaning the toothed rings first would help, could the sensors themselves need cleaning or repositioning ?

none of the dash lights are on, and they all light up with the key.

cheers, Phil

[Brianmoooore]

Place to start is to unplug the ABS ECU and confirm that the problem goes away.

[spiny]

cool, i'll do that when its light tomorrow and report back

[fixedwheelnut]

If it does go with the ABS disconnected try and get access to a portable oscilloscope and scope the wheel speed sensors and see if one is irregular at slow speeds or has interference on it.

Irregular signal can be to *corroded* or dirty pulse wheels or faulty sensor.
*most common*

Interference can be down to poor screening on the sensor wiring, I've had one in the past Drivers side front nearest the ignition that needed re- shielding.

Temporarily cured it with the tin foil from my sarnies wrapped around the loom and earthed out

[spiny]

ok, pulled both relays from the ABS unit (couldn't get the plug off and didn't want to force it and break it ... ) and the problem has gone away. Obviously, the ABS light is now lit on the dash too.

are the speed sensors the same sensors that point at the toothed ring on each wheel ?

[Brianmoooore]

Yes, the ABS ECU determines road speed from the same sensors that it uses to determine wheel lock up.
The ABS should be non operational at very low speeds, and the chances are that 'noise' (false signals) from one sensor is fooling the ECU into thinking the car is moving faster than it is.

[spiny]

righto

so best plan is to wire brush the toothed rings and wipe off any crap / swarf etc thats on the ends of the sensors and see if that improves matters ?

also, is there a set position for the sensors ? I read in Practical classics mag a while back that someone had a similar issue to me which was caused by a sensor moving away from the toothed ring slightly (not on an e30 though)

the reason I ask is thta one of the sensors was replaced last June for the MOT, so could plausibly have worked itself loose perhaps ?

thanks for all the help so far