ABS & Speedometer Sensor

[BigDusty]

Hi,

Hopefully this should be a simple one to answer.

On a 1996 (W202) C230K C-Class, is the rear ABS Sensor (located on the differential) also the Speedometer Sensor, if yes then that may solve a problem I'm having, if no, where is the location of the Speedometer sensor?

Cheers in advance,

BigD

 

[andy27168]

Early 202s ABS system only had three sensors, and yes the rear sensor is in the diff.
The later cars had a sensor at each wheel.
The speedo from what I understand is fed from the front drivers side sensor (Right front wheel).

 

[BigDusty]

Thanks for that info andy,

The problem I have is the ABS light was always on and the ABS not working at all, then all of a sudden the light went off but when braking the ABS would kick in with the usual vibrating brake pedal etc and always happens just as I was coming to a stop.

The other problem I have is the speedometer, it only seems to work when driving at about 30mph plus, any slower than that and the dial drops to zero.

So, me thinks faulty ABS and speedometer = faulty sensor and if what you say is correct the front O/S ABS sensor should be at fault.

Obviously the best way to find out is to have it tested with a fault code reader, but when taking my car to any garage I like to have the heads up first so I don't get duped.

Any other thoughts regarding this problem would very much be appreciated.

Cheers,

BigD

 

[andy27168]

Hi,
You could try cleaning the sensor and checking and cleaning the reluctor ring on the drivers side wheel, as this might save you a trip and the expense of a garage visit, and won,t cost anything but an hour of your time.
I presently have a similar issue with my AMG though not lighting the ABS light I lose my cruise control and speed limit function, and speedo needle wavers and I get a similar occasional pulsing of the brake pedal when coming to a stop.
I had the codes read by PCS and it pointed to a faulty speed pulse from the drivers wheel, I have replaced the wheel sensor and its still the same and as the reluctor ring appears intact though rusty (after inspecting it through the sensor hole) so my next course of action will be to give the ring a good clean with a wire brush or the like.

 

[BigDusty]

It sounds like you and I may have a similar problem, my speedometer, after 30seems fine but below that and it jumps about a bit before going to zero.

Just took the kids to school this morning and now the ABS light is permanently on and the ABS doesn't kick in at all and this ABS problem is failing my MOT

I have already removed both front sensors and the hubs and gave them a good clean. The reluctor ring, I used a small screwdriver and cleaned each groove out of any dirt and rust.

Unfortunately this has not cured the problem and is why I thought about the rear sensor but I can't remove it from the diff as it's rusted in and I know it will mean breaking it to do this and at £50 for a new one unless it is faulty there's no need to waste money on replacing it if it's OK.

So it looks like I'll have to book it in to pin point the problem but saying that, you were pointed to the drivers front replaced the sensor and still having problems.

One other thing, I read elsewhere that on some Mercs (but I don't know which Mercs they're talking about) the front wheel sensors work in conjunction with the wheel bearing seals, this it what an expert quoted

"OK, the problem with your car is that you damaged the front wheel seals when you repacked the wheel bearings. The wheel seals contain the magnetic segments that the ABS speed sensor reads"

Does this apply to our cars or was this design used on later models (I think it's the later). I have just recently replaced bother front wheel bearings and of course their seals.

Because I'm under pressure with time because it's an MOT failure I may have to book it in for the repair.

Cheers,

BigD

 

[andy27168]

The 202 only uses the sensor and reluctor ring, the later 203s use the bearing seals, so have no worries there.
I think I agree your only route is to get the codes read, I will say that when I had my C230K codes read with a generic code reader it had problems identifing that it only had three sensors, and identified I had a rear driver side sensor gone, though we took this to mean that the rear sensor had gone in this case and it had, but it would make me think twice about the accuracy of the codes it would bring up. Might be worth seeking out an Indy with STAR so you can be sure of the diagnosis.
You could try disconnecting the front sensors and spinning the wheels with a multimeter connected to the ends of the sensors and check the outputs from the sensors and if they are different then this would seem to suggest that one of them was U/S and as your having speedo issues then I would replace the drivers side.

 

[Danny DeVito]

Hi,
, and won,t cost anything but an hour of your time.

Oxymoron spotted there

Does anyone also know the position of the speedo sensor on a vito 108 or similar or is it like some people think from the wheel sensors ?

 

[BigDusty]

I tested all 3 sensors with a multimeter and all are reading roughly the same, the front O/S and rear sensor read about 1.083 and the front N/S read 1.075. So they seem OK?

I have yet to test them with the wheel turning, what result will I get on the multimeter by doing this, obviously a change in the number on the multimeter but what am I looking for.

I also found this on YouTube, might be helpful so I'm trying it out

ABS Sensor Diagnostics and Replacement Honda - YouTube

I managed to get a wiring diagram (I think it's for my car) from here

http://www.autolib.diakom.ru/CAR/Mercedes-Benz/1996/C220/SYSTEM WIRING DIAGRAMS/

So I'm attempting to follow the YouTube guide but the problem is I'm not sure what the ABS Control Module is and where it's located, it says Right rear of engine compt, so first which side of the engine is the right (looking at the engine or from the drivers seat position) but even so, on the one side is the ECU and the other the fuse box and relays, I have looked at both but found nothing that corresponds with the numbers on the wiring diagram and if the ABS Control Module it is part of the ECU there are no numbers to be seen either on the ECU or either connectors.

Scratching my head on this one!

I appriciate your help and advice with this Andy.

Cheers,

BigD

PS, Sorry, I know nothing about the Vito 108

 

[Danny DeVito]

ABS control module is attached to the abs unit maybe under black plastic covers. You will see all the brake pipes going into the abs unit. The wheel sensors feed into the abs so that it can control the abs and things like traction control. Right side is always sitting in the car facing forward.

 

[C30077]

++++1

 

[killick1]

Had the same problem on my '96 w202 eventually found it was the rear sensor, replaced by a local Indie for £185

 

[Danny DeVito]

Is it possible to switch the sensors at the front ? If the speed is sent from one wheel then it should correct ur speedo problem and also tell you that one sensor is faulty.

 

[BigDusty]

Hi all,

OK, I'm going in the right direction.

After much testing with a Multimeter and finding that the sensors seem to be working OK, but one thing I did notice, when doing the rotation test the front wheels gave a slightly different reading than the rear. The front sensors started at about 1.083 and when turning the wheel the numbers did change a bit, but the rear, again starting off at about 1.083, when spinning the wheel the numbers changed allot more.

Anyway, I decided to remove the front sensors to check that they were earthed correctly and in doing so realised a mistake I had made when I reassembled the front hubs after an overhaul of all the suspension, bearings etc, the sensors were bolted on the wrong side of a bracket (this bracket acts as a shield and holds cables and flexi brake pipe in place), when bolting on a sensor to the hub carrier it goes directly onto the hub carrier and the bracket on top, not the bracket first and then sensor

So, my problem was that the front sensors were to far from the reluctor ring hence why my speedometer was acting up and ABS light on.

But, now my speedometer is working fine, but my ABS is still not quite sorted yet. When I test drove my car the ABS light went out (I thought, YES!!) :bannana:, I drove down the road but unfortunately the ABS light came back on again. It seems to be doing this a few times, start the car ABS light goes out, drive forward a little light comes back on.

I feel I'm almost there, how sensitive are these sensors and their positioning?

 

[andy27168]

Glad you have sorted the speedo, but with the ABS light going out then coming back once moving seems to suggest you have still a sensor issue, which from your results of the multimeter brings us back to the rear one, and the fact you have tried to remove it and it is seized in the diff, the metal casing of the sensor has been known to swell through corrosion/water ingress causing it to fail.
I think you should get the codes read to confirm the rear sensor failure, though you could just replace the rear sensor and take a chance.

 

[BigDusty]

I was thinking of changing the rear sensor but it will be my last resort. I have until Wednesday to learn, understand and test the whole of the ABS, if I do take it to a garage at least I have done my homework and all I'll be asking is for them to read any fault codes and to look into one or two other electrical problems I have been having.

I have yet to see if the actual hydraulic ABS unit is working OK, the car has been idle for over three years (garaged) but it may have seized or is on it's way out. I knew the sensors were playing up because of the speedometer but have now put that bit right.

I still want to find the ABS control unit, as far as I can see it's not in the engine bay but I haven't looked under the bulkhead covers.

I found a picture of the ABS control unit on eBay as follows;

So, anyone know where this is located, think I'll start a new thread for this one.

Cheers,

BigD

 

[BigDusty]

Found it, located in the passenger foot-well behind a large flap of carpet and behind that you find a larger metal plate screwed to the foot-well with 3 plastic 10mm nuts. To release the ABS control unit undo the two screws that hold it onto the large plate, unplug all the wiring and you have one ABS control module in ones hand.

FYI, there are other things behind this plate including two fuses, don't know what they are but I will investigate them later.

 

[andy27168]

The alarm control unit is mounted on that plate, and the Transmission ECU if the car is auto 5 speed, the fuses you are talking about from memory are for the alarm, can be prone for causing alarm/immobiliser issues.

 

[BigDusty]

More FYI,

Now that the speedometer is working I decided to find out and confirm which ABS sensor is being used for the speedometer, to do this I simply disconnected in turn each sensor until I got a result.

I found that on my car the front N/S (front Left wheel if sitting in drivers seat) wheel or ABS sensor is what controls my Speedometer. And just in case someone asks, my car is a righthand drive.

BigD

 

[Danny DeVito]

More FYI,

Now that the speedometer is working I decided to find out and confirm which ABS sensor is being used for the speedometer, to do this I simply disconnected in turn each sensor until I got a result.

I found that on my car the front N/S (front Left wheel if sitting in drivers seat) wheel or ABS sensor is what controls my Speedometer. And just in case someone asks, my car is a righthand drive.

BigD

Useful info there. No if or maybe about which wheel it is

 

[BigDusty]

OK, it's all working again, good news is the auto electrics found the fault, fixed it and now hopefully I can get the car passed the MOT.

Bad news is it cost me £225. Could be allot worse some might say, my main irritation though is because I didn't have a fault code reader I couldn't pin point what was causing the problem. It turned out to be problems with the return pump and was caused by a dry solder or bad connection.

To fix the issue they had to dismantle the unit attached to the side of the ABS Hydraulics, see picture below

and mod it to get it to work. If I had known it was this I would have simply bought a replacement for about £20 and fitted it myself or purchase a complete ABS Hydraulic Unit, prices range from about £30 to £150.

I did ask them to simply find the fault and let me know so I could decide what to do but they went ahead and fixed it without saying.

So, on the one hand I have a working ABS but at a cost plus a modded ABS unit with bits of wires cut off and plastic glued together or on the other, if by some miracle I could have found the fault myself would have replaced the unit rather than hack up and repair and without the hefty bill.

It's a case of stuck between a rock and a hard place.

I will now get the car through the MOT but sometime in the near future I will eventually get a replacement ABS unit and keep to hand in case the repair job fails.

Thanks to all who helped.

Cheers,

BigD