w124 abs

[ncollin]

Hi, the ABS warning light came on yesterday on my 1995 e220. The brakes still work fine (without ABS). The light is on all the time. The only other symptom is a slightly erratic idle, when you brake to a standstill in Drive the revs dip and it sometimes stalls. I replaced the OVP relay with a new unit about a month ago to cure a starting problem. the car now starts first time without any problems.
any thoughts? I see from previous posts that the wheel sensors often accumulate rust and other crud. would this cause the ABS to fail and if so how do you go about cleaning them.
thanks for your help
Neil

 

[C220GJS]

Check the fuse/fuses on top of your OVP relay, the symptoms you describe are consistent with a blown fuse.

 

[kth286]

You may well have a poorly battery or it needs a good charge.

Once voltage falls below a certain threshold the ABS is designed to disable itself.

May well be connected to bad starting you experienced.

 

[wireman]

The ABS is seperate from all the other electronics with the exception of the OVP relay, so if that is faulty there will be some effect on your injection (if its petrol).

The ABS unit connects via a big fat 36way plug to all the wheel sensors and the pump unit, in addition there is an input from batt+ to monitor the supply voltage and an input from the brake lights switch in some cars this switch has two contacts one for stop lamp one for ABS others (earlier) use just one contact. Check the switch is intact, fitted correctly into its bracket correctly and does work.
Check that any local body earth wires (brown) are in good order around the ABS computer unit, near the battery.
The alternator output should keep the battery voltage high enough regardless of its condition so check the voltage is 13.8 to 14.4 volts with the engine running and rear screen + head lamps on.

The front sensors are mounted onto the hubs with an M6 bolt (locktite it back in afterwards) remove it and pull the sensor out, its face has a magnetic pole piece that can attract some bits of material and reduce its sensitivity clean it off (sticky tape will stick to the muck better that the muck sticks to the magnet and is therefore quite good for cleaning magnets of swarf etc). Each front probe is wired up the front strut and thru the wheel arch to a coaxial (a bit like a radio aerial plug) connector in a clip atop the inner wing near the servo and battery, break the connection and check the resistance of the probe/sensor, compare the resistance side to side (I think it should be around 2K ohm but I'll have to check and get back on that) if one is different or open circuit thats your fault.

The rear sensor (only one) is on the nose of the diff again held in by an M6 bolt, it runs inside the clean diff and should not require any cleaning (if it has swarf on it your diff is about to go very wrong) check its wire for breakages and sharp bends making sure that the seal where the wires enter is intact (do that for the front as well). the connections to the wiring loom are under the rear seat this time terminal block is used, disconnect the probe and check its continuity/resistance (about 1K?). Wobble the wire whilst you check in case its tried to mend itself during the dismantling process.

Remove the cover from the pump unit (T15 torx screw) beneath it the are two relays pull them out and check them for corrosion and firmness insertion, similarly examine the cable and plug for damage or looseness.

Make sure that the teeth on the front hubs are deviod of rust and not rotted away, if they have rotted it may be a new hub thats required (unlikely but later cars with a four channel system have steel plate reluctor rings on the drive shafts and these can and do rust away).

Some of the electronics is in the pump unit and cannot be easily accessed for asssesment or repair, if the fault is in there it may mean a new pump.

The ABS computer can be opened (but must be resealed afterwards) It is known that a few very similar units on similarly aged BMW 3/5 series have poor soldering on the internal circuit boards (power supply section) which can be repaired by a competent electronics bloke, dont just go at it with an iron use the proper tequniques to remove and replace the solder joints if they are fountd to be deffective.
I have not found anybody reporting this on a Merc yet but time will tell, similar poor soldering is known in OVP relays though.

There is and offical Bosch test set which will help diagnose which unit is at fault, however substitution of parts may be an easier approach.

If the lamp comes on before you move off the fault is unlikly the be muck on the sensors since without motion the computer cannot detect the fault.

I have seen the wiring loom from the OVP relay crushed by a replacement battery on one 124 saloon, it had slipped beneath the battery and was not actualy at fault but almost certainly would have become broken or shorted if it had been left squashed between the tray and battery.

good luck and let us know what cures the fault please.

 

[ncollin]

Thanks for the suggestions.
George, the fuse is OK. took it out and cleaned the connections just to be on the safe side.
David, battery seems to be OK. 12.4v with the engine off.
Wireman, thanks for your very detailed reply. I haven't had time to check through it all and as its just started pouring with rain I probably won't get it done this evening.
The plot thickens however, on the way home tonight ABS light came on with ignition and stayed on as it has for the last few days. revs dipped and stalled at a junction. restarted fine then the ABS light went out and no more problems with erratic idle. miraculous cure? so I thought, but unfortunately the light came back on with associated idle problem after picking the kids up from nursery. On the way home from there the light stayed on for about a mile then went out again.
Clearly an intermittent fault now, so I'm thinking maybe loose connection or dodgy wire? I'll post back when i've had a chance to go through wireman's suggestions. any other tips welcome
Neil

 

[wireman]

If its affecting the engine management and ABS the fualt is not likely to be ABS related, the wiring is to me most suspect, the wiring per circuit is dead easy but since loads of stuff has wiring associated with it identifying which piece of the wiring is at fault can be long and painstaking, Pay close attention to the condition of terminals and the wires where they are crimped together, it is not going to be easy now that you have found it to be of an intermittent nature since you are never sure that any repair which left it working was not the fault clearing itself.
You need the wiring diagram specific to your car to be able to accurately identify where the circuits are common, it seems likely that it is either a power supply to the OVP or a ground/earth on any of the circuits it feeds. Bear in mind that a power feed may be taken to and from any of the circuits and not just from the fuse box or OVP.

Finding a good Auto electrics chap with Bosch ABS and Merc experience may be a worthwhile undertaking.

 

[ncollin]

thanks, Ive had a look over the wiring and it looks OK but I'm no electrician. The relays on the ABS pump were a bit coroded and I've given them a bit of a clean-up. On the drive to work today all was functioning normally so fingers crossed it was a short or some such that has sorted itself out. Theres an independant garage near me that advertises itself as a bosch service centre, so I'll drop it in there if the problem recurs.
Again many thanks to you all for your help
Neil

 

[Kaye]

I have got an 'N reg 'E' class mercedes and the ABS light came on a couple of weeks ago - previous before that I have my head gaskit replaced went on holiday so the car was idle for two weeks when I came back to move it - reversing - the ABS came on. I have since had a new battery put in and the light is still on and I am now getting worried. Can anyone help me please. Kaye

 

[television]

Check the fuse on the Over voltage relay, its on top of the relay just behind the battery.

Was the car junp started

 

[Kaye]

Yes it was car jump started but then I went a bought a new battery and the light was still on but I shall check the fuse on the Over voltage relay. thanks Kaye

 

[teddycatkin]

teddycatkin--ABS-OVP

Hi, the ABS warning light came on yesterday on my 1995 e220. The brakes still work fine (without ABS). The light is on all the time. The only other symptom is a slightly erratic idle, when you brake to a standstill in Drive the revs dip and it sometimes stalls. I replaced the OVP relay with a new unit about a month ago to cure a starting problem. the car now starts first time without any problems.
any thoughts? I see from previous posts that the wheel sensors often accumulate rust and other crud. would this cause the ABS to fail and if so how do you go about cleaning them.
thanks for your help
Neil

----------------------------------------------------I have exacttly the same poblem --How did you sort it ?

 

[television]

The give away tell tale on post 1990 cars is that when the OVP relay fails, not only does the ABS lamps come on, but the idle is erratic as well, and this applies to cars without an idle relay, and just fitted with One engine ECU

 

[akkizma]

W124 E220 ABS light is on.

Exactly the same problem.
ABS light is permanently on in my car, and does not go off after starting.
Once in a blue moon, it would go off for a few seconds, only to return.
I have replaced the OVP relay and cleaned the wheel and differential sensors.
Check and cleaned the fuses on top of ABS pump as well.
I read that scanning using a diagnostic tool, will not yield any results on a W124 E220. How do i go about fixing this?

 

[television]

cars from 92 to 95 can be checked on a blink code reader

 

[grober]

The OVP feeds the following -----the ABS control unit., the motorised idle control unit, and the engine ECU. A fault in one may drag down the voltage in others? I would give the motorised idle valve a careful clean up internally in case its bunged up. While the fuses on the OVP may be OK the relay itself may develop component dry joints in the little internal circuit board due to vibration over the years. Try taking off the relay top can and remelting/resoldering all the component connections on the PCB- or get a friend who is handy with a soldering bolt to do it for you.

 

[akkizma]

cars from 92 to 95 can be checked on a blink code reader

Will the blink code reader, help me reach the root cause of this issue?
How about using a diagnostic computer with OBD 1 connector for 124, will it work in sorting this issue?

 

[akkizma]

The OVP feeds the following -----the ABS control unit., the motorised idle control unit, and the engine ECU. A fault in one may drag down the voltage in others? I would give the motorised idle valve a careful clean up internally in case its bunged up. While the fuses on the OVP may be OK the relay itself may develop component dry joints in the little internal circuit board due to vibration over the years. Try taking off the relay top can and remelting/resoldering all the component connections on the PCB- or get a friend who is handy with a soldering bolt to do it for you.

I already have a new OVP in place.
The thing that i need to check is, if there is any wiring mess up in its slot.
Btw, my car is running a 65AH battery, is that OEM spec, or it needs a better/more powerful one?

 

[television]

Will the blink code reader, help me reach the root cause of this issue?
How about using a diagnostic computer with OBD 1 connector for 124, will it work in sorting this issue?

All I know is if the car falls between 1992 and 1995 it should, later cars communicate better.

See here, its about ½ way through.
http://www.mbslk.de/files/artikel/r170_fehlercodes.pdf

 

[grober]

http://www.diakom.ru/CARS2/pribor/cs1000/manual/cs1000_mb.pdf
page 58 confined to models with the 38pin plug.
Unfortunately earlier 8/16 pin diagnostic sockets mainly gave out ENGINE DATA so you may not be able to access ABS system data/fault codes. The early control modules were tested by special Mercedes test breakout boxes and without them its difficult to interrogate these early systems.