ABS Coming on at slow speeds W126 500SE

[jontyash]

Hi All,

I've got a problem on my W126. I've just had the bearing changed on the front left wheel and since I got it back from the garage, the ABS keeps activating at slow speeds.

Just as I come to a complete stop, it turns on and makes the (drrrrrrr) noise and shakes the pedal.

It's a pain cos I need to sell my car this month and need to get it fixed.

The brakes and discs are fairly worn but will this have anything to do with it? It had a new ABS pump only a year ago so I'm hoping it's not that.

Any ideas guys?

Thanks

 

[kwakdonut]

Speed sensor on the LH hub been disturbed/disconnected?

If this fault has shown itself since the bearing was changed, chances are it was caused by something the mechanic did. I imagine the hub would have been removed to press out the old bearing.

May be worth having a friendly word with them and taking the car back to be checked over first.

 

[jontyash]

yeah, I already had a friendly chat with him. He was totally non responsive. I'm quite annoyed about it as I'm sure it's something he did.

Do you know how much a new speed sensor is? or even where it's located? Is it on the hub or attached to the brakes?

 

[kwakdonut]

Might be worth looking round the hub, with the wheel off, to see if any wiring is detached/chafed/damaged.

If you didn't know, the ABS works by receiving signals from a sensor from each wheel. There is a 'phonic wheel' attached to each hub - looks a bit like a toothed gear. This and/or the sensor may have got dirty too so maybe worth cleaning it up.

I take it the garage that carried this work out is a standard one not a Merc indy? Don't think I'd be too impressed with the mechanics response either!!

Don't think you need to go down the route of sensor replacement quite yet!

Anyone else on here have any ideas?

 

[kwakdonut]

The sensor may be attached to the shock absorber or possibly the brake calliper btw. It will be a fairly small item with a couple of wires coming out the back.

 

[PhilJ]

I had that on my cosworth, but only every now and then. It didn't end well!

 

[jontyash]

Thanks kwakdonut, I'll have a look and see what I can see. Nice one.

 

[jontyash]

what was the outcome?

 

[PhilJ]

Unfortunately, it fixed itself for a good few months, was fine to the point I forgot about it, then with a bit of rallysport down a country lane it reared its ugly head and unfortunately I parked the car at high speed into a Toyota hiace. I 'd get it looked at! Tempted to take the ABS off my new one.

 

[jaymanek]

Its pretty safe to say they disturbed something... got to either have stretched the wires or got muck on the sensor.

 

[jontyash]

Thanks guys. I'll have a look and see what I can see....I hope a new sensor isn't too expensive... eek.

 

[IanW]

My guess is that the sensor is just a little too far from the phonic wheel. A gentle push home might just cure it.
The output from the magnetic sensor is proportional to frequency/wheel speed. As you come to a halt, the signal level falls until it is too low for the ABS ECU to register it, at which point the system decides that the wheel has locked & releases the brakes.
If it were more serious (sensor open circuit etc), I think the ABS light would be lit on the dashboard.
Ian.

 

[kwakdonut]

Unfortunately, it fixed itself for a good few months, was fine to the point I forgot about it, then with a bit of rallysport down a country lane it reared its ugly head and unfortunately I parked the car at high speed into a Toyota hiace. I 'd get it looked at! Tempted to take the ABS off my new one.

I wouldn't, ABS is there for a good reason!! That's why it has been standard equipment on all cars for some years now....

 

[jontyash]

OK guys, in the end I bought a new ABS sensor and had it fitted £109 - eek. The problem is still there though. Only now, the ABS comes on sometimes. When the ABS light comes on, there is no problem with the brakes at all (except the ABS doesn't work of course).

I don't know what to try next guys? I've already put the car on ebay and I need to sell it like yesterday.

What would you try changing next to fix it? The ABS ECU?

What do you reckon guys? Any help would again be appreciated! Thanks

 

[grober]

1989 300E 4Matic abs problem - Mercedes-Benz Owners' Forums

 

[johnsco]

I've been in just the same position with a W126.
I had a worn bottom ball joint.
I took the front suspension leg off and took it to my mate's to get the new joint pressed in.
The ABS sensor was fastened to the suspension leg.
I didn't want to disturb it, so I disconnected the cable connector under the bonnet and carefully unthreaded the cable as I removed the leg.
I got the new joint pressed in and reassembled the front suspension leg and reconnected the cable.
Did it work ?
Did it hell.
The new ball joint was fine, but the ABS was then cutting in whenever I touched the brakes.
Disconnecting the sensor didn't help.
Disconnecting both front sensors didn't help.
I changed the sensor.

No thanks to MB main dealer who checked my VIN number and declared that the car had not been supplied with ABS and it must have been retro-fitted.
RHUBARB !!!

Got one from Euro Car Parts.
Fitted it and the problem was cured.
If that hadn't worked, I don't know what I would have done.