ABS problem driving me mad

[blockostar]

I have the following problem on a 1994 - E320 2 door coupe

After about 15-20 minutes driving from cold start my ABS light comes on. Now when it does it is then restricting the top end of the 5 speed autobox. So for instance: if I am close to a motorway and can get on it and up to speed before the light comes on, it is fine and at around 80 miles per hour you are doing about 3000 revs. However, if I have a drive to get on the motorway and on the way there the light comes on, when I get on the motorway the autobox will not kick up to the top gear. Therefore at 80 miles ph the engine over revs and you are at around 4-4500 revs. It is basically like doing 80 miles ph in a manual whilst leaving it in third or fourth gear.

If you get up to speed before the lights comes on, whilst you are fine, as soon as you lower you speed, in heavy traffic for instance, or stop at services, as soon as you switch back on the light will come straight on (as the car is now hot) and the problem kicks in.

I have been to a local merc specialist and whilst the diagnostics tell them there is a fault, it will not pinpoint exactly where. This week we tried the option of replacing the two front wheel sensors-but this has not solved the problem. The only solution they can suggest is waiting until it gets worse to the point where the light is on all the time – but this could weeks/months.

As anyone ever had this problem? I always thougth the light coming on whilst driving suggested a drop in voltage. Is it as simple as the battery no holding enough voltage? But why is the ABS knocking on to affect the gearbox - is the ABS, in turning itself off, trying to restrict the speed you can go at?

Any suggestions on how this can be resolved would be greatly welcome as it is restricting where I can travel at the moment.

Many thanks

 

[Bolide]

You need to find which sensor feeds the ECU for the gearbox. As you've changed the front two logically it's the rear one...

Does your car have ASR? Does it work?

Maybe the ECU for 5th gear reads road speed via the ABS sensor and uses this to determine when to shift to 5th. Other people have said there's a sensor in the speedo

Whatever, when you solve the ABS problem I'm sure 5th gear will come back. All you have left is the rear sensor, the ECU and the valve block

Nick Froome
www.w124.co.uk

 

[blockostar]

Thanks Nick

Not sure about the ASR-will find out.

What myself and a mechanic have just done is disconnect all the sensors and see what happened. As I pulled off and got it up to high speeds, instantly the gear box was rained in which never normally happens until the light comes on. Therefore it appears that when the ABS is off it is causing the fault. We did deduce that the ECU may be the issue now. I have asked around and people have siad if any sensor or sensors are going to go it is likely to be the front. A merc specialist said he had never known the rear one to cause a problem.

Any other options from people very welcome.

Many thanks

Stuart

 

[Bolide]

The 5-speed box is a 4-speed with an additional clutch & geartrain on the back of it. It's actuated electrically. The input signals, I'd assume, would be engine speed, road speed, kickdown cable position and gear lever position. I don't know if there's a separate ECU or whether is uses the engine ECU to activate it

If disabling ABS disables 5th gear then logically it takes a speed signal from an ABS sensor or a signal from the ABS ECU. I'd assume the former

The ECU rack is behind the battery. There's an engine management ECU and a Bosch ABS ECU and anything else is determined by the options fitted to the car

Nick Froome
www.w124.co.uk

 

[luders79]

What about the OVP relay? Could it just be dropping abit of voltage? I'm sure thay still had them on the later 124's still powering the abs ecu??

Sorry if i'm wrong someone please tell me otherwise?

 

[kth286]

I was thinking battery not in good condition and not holding charge, OR the OVR module.

It is one of those next to battery.

I would want to know the diagnostic fault code and then perhaps we can home in on the actual problem.

Let us know the following:-

1. full car VIN so we can se what options such as ASR you have

2. the fault code

3. whether you are using a merc specialist or which garage you are using.

4. what location are you - perhaps we can recommend someone close by.

 

[blockostar]

Thanks David

I will find out what the fault code was. I took this into MBS in Exeter: www.mbsmercedes.co.uk.

They took it for a drive and the light came on after 20 mins or so as per usual-but then when they hooked it up to diagnostics they could not trace the fault. Checked all the wiring etc and not obvious problem. They thought it may be a sensor but said they would not know until the fault developed (i.e. ABS light on all the time).

I got some 2nd hand front sensors from www.mercedesparts.co.uk in Leeds and had a local garage put them on and they looked rusted in.

Tried these and again, regular as ever, the light came on. It is now 15 mins rather than 20. It just seems like a programmed fault because it comes on at the same length of time each time. Not after 5 mins one day, and then 10 the next - after a specific length of time. You know when it has come on if you are not looking at the dash board as you will feel the car 'tug' slightly, as if you have touched the brake, and you will notice it is on.

In terms of the battery: there are no obvious signs that there is an issue with the battery. Starts first time. Could it still be a problem therein? Also people have mentioned the OVR - what exactly is this?

I am near Exeter and so if you know of anyone this way let me know.

Thanks

 

[bigasotonuk]

I think before looking at the ABS ECU, I would be looking at replacing the rear sensor as I believe you should only have 1 which is in the Differential, and this is where I believe the speedo signal is derived from, besides 1 new sensor is a bit cheaper than an ECU.
Some aftermarket diagnostic equipment (in my experience) have problems reading the 1 rear sensor as it expects to see 2, so "may" not show up.

 

[kth286]

Stuart

Just a quick thought - does the car have non standard wheels tyres on it ??

OR perhaps are any of the tyres worn more than others.

Is the car new to you ?

In my previous post I should have said OVP relay and not OVR.

The OVP relay is "Over voltage protection" relay and is indeed next to battery.

It actually has a fuse on top.

They do go faulty with dry solder joints on certain generation of earlier cars, and a modified part number was introduced.

As far as the consistent timing for the fault to occur it may be that as the OVP relay heats up, then the soldered joint opens up and goes open cicuit, hence your light coming on.

 

[blockostar]

David

Yes it does have non-standard wheels and tyres on it. 18 inch alloys and low profiles. The car has also been lowered at some point. This was lowered and the wheels on it added before I took on the car. I have had it for about 12months.

Funnily enough the tyres were worn on the front about 3 months ago when the problem started. I had these replaced for the MOT in May. However these did not have a 'full' tread on but were good part worns. They are however worn more than the rear ones.

I had seen somewhere else on the web someone mention in passing that occasionaly incorrect tyre pressures and tread etc can cause an issue but thought this to simple a solution. Also I would not have thought it would cause the regular timing of the problem. Could it be when the tyres get hot?

The OVP suggestion sounds interesting.

Stuart

 

[Bolide]

The ABS system compares the rotational speed of the front & rear sensors to see if there's wheel slippage. If the relative wear front-back is enough to reduce rolling radius to the point the ABS notices it then this will cause problems

I'd try it on a set of standard wheels & tyres for a few days to see if the problem goes away

BTW this is how some tyre pressure monitoring systems work - by comparing rotational speeds

Nick Froome
www.w124.co.uk

 

[Iain the gadget]

I had same thing with a '94 C180
Changed the OVP relay and this cured the problem.

 

[anas2912]

I had the same prblem with ABS always on

this is my car VIN WDB1240321C161281

does it had ASR

HGS control unit changed -then no fault codes
selnoid valve checked- OK
5th switch checked - OK
OVR replaced by a used one and not cure the problem

Iam not sure about ASR sensors at rear

 

[television]

Cars fitted with ASR do need the 9 pin OVP relay as the ASR uses the extra outputs

 

[anas2912]

I had the same prblem with ABS always on

this is my car VIN WDB1240321C161281

does it had ASR

HGS control unit changed -then no fault codes
selnoid valve checked- OK
5th switch checked - OK
OVR replaced by a used one and not cure the problem

Iam not sure about ASR sensors at rear

i found that ASR or ABS sensor at rear unfortunately broken and I search at many shops new and used but i did not find
i will search at ebay thanks for guidance and grateful help

 

[bigasotonuk]

Try Euro car parts approx £40.

 

[anas2912]

i did not find unfortunately

 

[television]

How many pins on your OVP relay

 

[Quick Silver]

Had the same problem on my 1990 300SL last year and with the information and help from Television narrowed it down to the OVP Relay . Replaced OVP Relay cost £45 from euro car parts and problem solved a 5 minute job to replace.

 

[anas2912]

How many pins on your OVP relay

9 Pins
the ninth is small like this

and sorry for late reply