Help Please R129 ABS fault at very slow speed

[KPBangor]

Hi, this has been an on-going problem over the past three years and I cannot get to the bottom of it. Over the past couple of months it has taken yet another twist.

The brakes on the car work fine at normal speeds and it has passed the MOT each year. However when comming to a road junction or traffic lights when I slow down I brake in a two stage fashion one to slow down and one to bring the car to a halt from 5-10 mph. The first application of the brakes is fine, but when it comes to the final- slow speed - application the anti lock brake system is triggered and the brake pedal kicks back, causing the car to take longer to brake along with a graunching noise.

This origionally started when I was parking the car with a lefthand lock on the wheels and then progressed to almost every time I applied the brakes at low speed.

Over the past few months when I start the car first thing in the morning the ABS light goes on and I DO NOT have the problem. When I next stop and restart the car the ABS light Remains out and I DO have the problem!

I have replaced front left abs sensor, both front wheel bearings complete with abs senders, front discs and pads, rear calipers and had discs cleaned. I have also experimented with s/h abs ecu and compressor. Non of which has made any difference. An auto spark has checked out the resistance in the sensors and tells me he cannot find a problem.

Any thoughts please

 

[brianbrian]

Try not cleaning, but changing over load fuse.

 

[LostKiwi]

My '93 500SL does exactly the same. I've put it down to bad reluctors on the rear on mine (they're quite rusty...)

 

[KPBangor]

Try not cleaning, but changing over load fuse.

Hi Brianbrian, can you please tell me where I would find the overload fuse.

 

[KPBangor]

My '93 500SL does exactly the same. I've put it down to bad reluctors on the rear on mine (they're quite rusty...)

Mine is the early modle with only the one reluctor on the drive shaft, but I've been told that it is only used for the speedo and therefore should not interfere with the abs. Whether that is correct or not I don't know.

 

[mercedes13156]

I've just had a look at the EPC website and it looks like you've got front hubs like a lot of other Mercs of that year. That is, the reluctor is a series of ribs cast into the back of the hub. I've put a photo on for clarity. I had the same hubs on my old C Class and it had problems similar to what you're describing. The answer for me was to take the hubs off and stuff the races with rags then take a drill and wire wheel to the ribs to fully clean them down to bare metal. I also had to replace the backplates as they had rusted away and a lot of road rubbish had settled on the reluctors.

 

[LostKiwi]

Mine is the early modle with only the one reluctor on the drive shaft, but I've been told that it is only used for the speedo and therefore should not interfere with the abs. Whether that is correct or not I don't know.

It will almost certainly be used for ABS as thats the only way the car knows the rear wheels are locked. The point of ABS is to maintain steering ability when the brakes are operating at their maximum. If the rear wheels lock the rear will very likely overtake the front.

 

[mercedes13156]

Hi, this has been an on-going problem over the past three years and I cannot get to the bottom of it. Over the past couple of months it has taken yet another twist.

The brakes on the car work fine at normal speeds and it has passed the MOT each year. However when comming to a road junction or traffic lights when I slow down I brake in a two stage fashion one to slow down and one to bring the car to a halt from 5-10 mph. The first application of the brakes is fine, but when it comes to the final- slow speed - application the anti lock brake system is triggered and the brake pedal kicks back, causing the car to take longer to brake along with a graunching noise.

This origionally started when I was parking the car with a lefthand lock on the wheels and then progressed to almost every time I applied the brakes at low speed.

Over the past few months when I start the car first thing in the morning the ABS light goes on and I DO NOT have the problem. When I next stop and restart the car the ABS light Remains out and I DO have the problem!

The ABS coming in at slow speeds is because the system isn't getting the slow speed signal and thinks one of the wheels is locked. It then cycles the pressure to that piston hence the vibrating pedal and the slower stopping. If the ABS system is getting a good signal at higher speeds, there's no problem. The reluctors provide the alternating on/off signal to the magnets in the sensors and that creates an electrical voltage across the sensor wiring. The voltage is read by the ABS system and if there is a voltage, consistent with the voltage from the other three wheels, then the ABS assumes that wheel is rotating normally. When it stops reading a voltage, the ABS assumes that that wheel has stopped rotating. If the other three wheels are rotating at a consistent speed, the ABS then cycles the pump to that piston and "releases" the wheel which is felt as the brake pedal vibrating. In reality, that wheel is rotating at the same speed as the others, it's just not getting the signal voltage If you've changed, checked and cleaned everything on the sensors and ABS system and they're all good, then the only thing left is the reluctors.

When the ABS light comes on the brakes will apparently work as normal except that the ABS won't work if you ever need it. A bit like driving a Triumph Herald I suppose.

 

[KPBangor]

Hi, thanks for your answers, and my appologies for the delay in getting back but following a spell in hospital and subsequent recouperation have only just felt like getting back to dealing with this challange.

Since my last post I have replaced the drivers side sensor with a good secondhand one but no change. I then had the electrical output checked with an osciliscope and it showed that the drivers side showed much more noise but also an output of 1.4volts compared with 1.9volts from the passanger side. As a result I have replaced the driver's front hub with a good quality used one and the output is now a lot cleaner than even the passanger hub, but there is still the same difference in voltage output. Surely this should not be the case? The output has been measured by turning the wheel by hand whilst on a ramp.

Could it have something to do with the wireing loom and if so is it a difficult job?

I would appreciate any thoughts on why there should be a difference, and if this is likely to be the culprit in my problem.

 

[mercedes13156]

What you didn't say was whether replacing the hub had cured the problem. Turning the wheels by hand to get a voltage isn't the ideal method of measuring the output. Try reading them altogether on live data and if you see the rotation of all four wheels is the same, then that's not your problem. Your ABS is not, as far as I know, reading the voltage but reading the signal (the frequency of the ON/OFF messages it gets as each ridge / dip on the reluctor passes the pickup. They should all be within a specific tolerance for the ABS to assume that all four wheels are rotating as expected.

 

[KPBangor]

What you didn't say was whether replacing the hub had cured the problem. Turning the wheels by hand to get a voltage isn't the ideal method of measuring the output. Try reading them altogether on live data and if you see the rotation of all four wheels is the same, then that's not your problem. Your ABS is not, as far as I know, reading the voltage but reading the signal (the frequency of the ON/OFF messages it gets as each ridge / dip on the reluctor passes the pickup. They should all be within a specific tolerance for the ABS to assume that all four wheels are rotating as expected.

Hi thanks for that, just to confirm changing the hub made no difference to the problem.

 

[LostKiwi]

Have you tried cleaning the rear reluctors and sensors yet?

 

[mercedes13156]

If it's not the reluctors, check the pickups for damage. if it's not that, try getting you ABS pump checked on STAR. That's the only thing that will read a Merc like yours. I think you'll have the old round 38 pin connector. It might be a sticky solenoid or a dodgy electrical connection. After that I'm out of ideas.

 

[XJ600]

Hi, Am new to the site.
Without a doubt, the symptoms described are a faulty reluctor ring, probably split, this makes the sensor think that wheel is rotating slower than the rest. Only happens at slow speed, especially at junctions.
Had this on 2 cars and several lorries. Worth taking the wheels off and looking at the reluctor rings. They get rust under them between the hub / driveshaft and split. Easy change, fit new one by heating in the oven (When the wife is out preferably) and tapping it on.

 

[KPBangor]

Have you tried cleaning the rear reluctors and sensors yet?

Have done better than that have replaced them!

 

[KPBangor]

Hi, Am new to the site.
Without a doubt, the symptoms described are a faulty reluctor ring, probably split, this makes the sensor think that wheel is rotating slower than the rest. Only happens at slow speed, especially at junctions.
Had this on 2 cars and several lorries. Worth taking the wheels off and looking at the reluctor rings. They get rust under them between the hub / driveshaft and split. Easy change, fit new one by heating in the oven (When the wife is out preferably) and tapping it on.

Thanks for that. That was my first thought, but on my car the reluctor ring comprises a set of 92 teeth which are part of the hub, (two years later this system was updated to seperate reluctor rings as is common in cars now). I have already replaced one hub because of poor read out on the osciliscope and whilst the readout has improved, it has not solved the problem, the other side was a much better reading so reluctant to replace it, have been told the problem is seldom with the hub???

 

[KPBangor]

If it's not the reluctors, check the pickups for damage. if it's not that, try getting you ABS pump checked on STAR. That's the only thing that will read a Merc like yours. I think you'll have the old round 38 pin connector. It might be a sticky solenoid or a dodgy electrical connection. After that I'm out of ideas.

Thanks,replaced one hub (reluctors being intragal on my model), have swaped abs pump - no change. Unlikely to be any fault reading on STAR as the abs light does not come on, (presumably because it is responding to what it sees as a legitimate situation) so won't register a fault. I have been told that it needs a seperate ABS reader because of the age of the system and I cannot trace anyone with one in Northern Ireland.

 

[mercedes13156]

If the reluctors are ok then the next cheapest thing to check are the pickups. Take them out of the hubs and check they're not damaged or decayed. Give them a good clean before you put them back in. I've seen a cracked pickup letting in water that was failing but still picked up a signal, albeit poorly. Check the cables and the connectors for damage and decay. Spray both ends of the connectors with electrical contact cleaner before reassembly. See if that helps.

 

[KPBangor]

Have you tried cleaning the rear reluctors and sensors yet?

Thanks but my car only has one rear built in reluctor in the drive shaft/ diff, it has been tested and is giving out a good signal.

 

[KPBangor]

If the reluctors are ok then the next cheapest thing to check are the pickups. Take them out of the hubs and check they're not damaged or decayed. Give them a good clean before you put them back in. I've seen a cracked pickup letting in water that was failing but still picked up a signal, albeit poorly. Check the cables and the connectors for damage and decay. Spray both ends of the connectors with electrical contact cleaner before reassembly. See if that helps.

Thanks, have replaced both sensors no change!