ABS Warning Light ALWAYS On

[Alan]

Hi All.

I wonder if anybody can please advise on my best course of action to rectify an ABS issue I face on my M535i (M30B34).

I've read lots here about ABS troubleshooting, and I'd like to get to the stage where actual troubleshooting occurs, but my initial issue is unusual, that being, the ABS warning light stays on. As in, always on, when the ignition is switched on, and when the car is running. The warning lamp goes off when the ignition is switched off.

The ABS light/system, does not do a check where, like on other posts I've read, the lamp briefly turns off, and then back on, to highlight that there is a fault. Mine just stays on.

Any of you good folks able to point me in the right direction? I have the Bentley manual too, just to mention.

The actual brakes feel firm, but do stop the car when enough pressure is applied. All new discs and pads. I don't hear any prining of pumps either when the ignition is turned on.
Fuses are fine in the fuse box beside battery.

TIA

[vinceg101]

Off the top of my head:
1. One of the ABS wheel sensors has gone bad (wire nick, etc.)
2. The Electronic control unit has gone bad (bad solder joints, etc.)
3. The relay on the ABS unit has gone bad
4. One of the wires to either the ABS unit is damaged

The latter two are relatively easy to troubleshoot and fix.
The first two are a bit more involved since you need a known good sensor to test. Since these are NLA from BMW, you have to source a good used ones (borrowing one from someone is likely the best route). If the wheel sensor is bad, Ivo Christov sells new ones. Ebay for the control unit is best.

You should test the ABS to see if it indeed not working, it could be a bad cluster or light.
And yes Bentley has a whole section in the Brake chapter to testing the ABS system.

[Blue Shadow]

When the light comes on helps diagnose the problem. If a wheel sensor is bad the light will come on with engine start and go off until the car starts moving forward and the problem sensor does not provide a signal...prior to that the computer thinks things are ok.

I would expect that the light on all the time means the abs system knows there is a fault in the computer or relay at the pump and the light just does not go out.

If the light doesn't come on (after start up sequence) until you hit higher mph (kph) then there is a bad tone ring sending an intermittent signal to the computer that is more correct at lower speeds. This is usually a rusty tone ring on a rear wheel hub.

[Chimi-Changa]

Tone rings in the rear can easily be checked if you jack up the rear end there is a open space that when you spin the wheels you can see the entire ring from underneath. If there are any that are chipped or corroded that could be causing the fritz. I think Ivo sells the tone rings separately too.

[Alan]

No idea as of yet, but thank you for the helpful advice.

I'll start with relay swapping and connections to the ABS unit.
The car is in a Carcoon now, and I'm tight on time, but there's always the Christmas holidays for working on the car!

Logically, yes I agree, the system isn't even going into diagnostic mode, so the problem may very well be obvious.

Out of curiosity, is there an overhaul/repair kit I can buy for the ABS unit? I looked and didn't see anything too obvious for sale.

[stuartinmn]

Tone rings in the rear can easily be checked if you jack up the rear end there is a open space that when you spin the wheels you can see the entire ring from underneath. If there are any that are chipped or corroded that could be causing the fritz. I think Ivo sells the tone rings separately too.

If the tone rings are clogged with dirt or rust, cleaning them with a small wire brush may help.