W124 ABS light on, tachometer off

[wraltd]

Hi guys,

My 1991 W124 E class 300TD is driving me nuts - just changed both of the front springs and now the ABS light is permanently on and the tacho doesn't work. Local garage checked it out and couldn't find anything wrong, car returned to me with ABS light out and tacho working. At next start up ABS light was again on and the tacho didn't work. Back to local garage, again found nothing wrong but couldn't get ABS light to go out / tacho to work. Reassembled everything (?) and suggested I collect car and try a specialist. When I arrived to collect the car they reported ABS light out / tacho working but again, on start up, ABS light was on and tacho not working. What the dickens could be wrong !??

 

[Thegemmesabogie]

Quite likely the OVP relay situated to the left of the ECU and behind the black cover behind the battery.

 

[wraltd]

Thanks - I'll take a look. Presumably a replacement is the only way to test this ?

 

[Thegemmesabogie]

Yes, if it's faulty internally, unless you are up for resoldering the board. Try cleaning the contact pins first and if you have the version with the fuse in the top give that a clean up. They do go commonly faulty though and show-up as symptoms just like yours ie ABS light and random dash or instrument failures.

 

[kth286]

wraltd

I would suggest you do not go looking for another problem that is unlikely to exist at this tiome, BUT that you concentrate on the likely problem at hand, ie. your ABS light, after having work done in that area.

Think about it this way.

The garage suggested you now take car to a specialist.

It would seem therefore you had the work done by a general garage that did not often work on Mercedes.
As the suspension of a Mercedes requires a very specialised (and expensive)workshop tool, it is unlikely the garage had this tool for the very few Mercedes they worked on.

Therefore, to remove the front springs without this special tool,they would have had to "fudge" the job.

They would have had to undo the top shock absorber (strut) nut to allow the wishbone to be lowered sufficiently to allow the tension off the springs.
The wishbone drop required is substantial, and unfortunately the ABS wire is often not long enough to allow the full drop required. Therefore my calculated guess is that the ABS wire has been stretched to the point where it has actually snapped inside the outer covering, so you cannot see it.
Therefore your ABS circuit is open circuit at this point, probably on one side of the car only because they would have learnt their lesson by the time they got to the second side spring.

They can probably temporarily get the circuit to make contact again by pushing/taping the wire together.

Where do you go from there ?

I would personally get each wheel off in turn and feel the wire inside the outer covering to see if there was an obvious break. You need someone sitting in the car with ignition key off and on whilst you fiddle with each wire pushing them together to see if you can make the circuit.

The wires are avilable from Mercedes if required and you could charge the cost to the garage I suppose.

Next time always use a Merc specialist.

 

[television]

I am with Keith on this one, do not go diving in further. Stick with the sensors

 

[premmington]

keep me in touch with tacho repair, have 1990 300D with same fault, unable to trace fault.

p@remmington.plus.com

 

[wraltd]

Quick update - I've replaced the wheel sensors with known good ones but the problem persists.
Interestingly, the battery went flat during the week it took me to locate replacement sensors - after removing the battery, charging and replacing it, the car started OK and the ABS light was out / tachometer was working. For about three minutes. It was at this point I changed the sensors with no effect. I have not changed the OVP relay yet - should this be the next step or could the state of the battery / alternator be having some effect ?

 

[kth286]

wraltd

Are you saying you changed the actual sensors that bolt into the hub ?

If so. why did you do that ?????????????

Nobody on here mentioned to do that.

They have not been touched in the change of your springs, so were unlikely to be faulty.

Why dont you read what people have suggested in the above posts, and action those recommendations.

Otherwise, why waste your time asking for suggestions if you are going to totally ignore them ????????????

 

[wraltd]

Keith,

The sensors and cable up to the underbonnet connector were changed this being much easier than trying to locate and replace the cable alone. As I said, and whether this was the right or unnecessary thing to do, the sensors and cable have now been eliminated as a potential root cause and the problem persists. At your earlier suggestion I did not replace the OVP relay -should I now do so ?

 

[Stevie A]

If the OVP relay being faulty effects / shuts down the tachometer circuit then that could be the problem.

ABS light on, erratic idle and hard starts are symptoms of faulty OVP relay. ABS light comes on as the OVP relay shuts down the juice to the ABS system I think.

Malcolm or someone should be able to say why ABS light and tachometer faulty is showing in tandem. Could it be an electrical problem with weird voltage causing OVP to cut in so then ABS shuts down?
Are OVP relay and tachometer linked somehow?
Is ABS system and tachometer linked?

 

[wraltd]

Stevie,

The symptoms of ABS light and tachometer are most definitely linked - light is on when tacho is off and vice versa. Voltage to operate the tachometer seems to be linked to voltage required to to turn on the ABS system. With the battery fully charged the system operated for a few minutes hence my question could the root cause be in the battery / alternator circuit ? Perhaps the voltage in the fully charged battery was sufficient to operate the OVP relay ?

 

[BlackC55]

I would say the OVP is at fault in this case.

 

[television]

As a guide line on these things.

If the battery is low voltage this can put the ABS light on from the start, The ABS runs from the engine and not the battery. So if the battery has a short circuit cell and the voltage is down to 11 ish volts this will put the lights on as the battery can drag the running volts down, the running voltage should be 13.8 volt.

The OVP relay also plays a big part in this, there is a fuse on top that might have blown or the relay could be dry jointed. When the OVP relay is blown the voltage on pin will be down to 7.5 volt it should be 12.5 volt the correct battery voltage.

The ABS also needs, and is part of the speed signal, always hard to separate and find faults on, is the two faults or one ?? the Tach needs a speed signal as does the cruise and ABS.

Always start with basics, that is make sure that you have 12 volt where there should be

 

[wraltd]

Thanks everyone. I'll have the battery checked out for cell broken down before moving on to the OVP relay. Incidentally, how do I identify this relay -as I look at the ECU (on nearside bulkhead) looking from the front of the car there are two relays in front and to the right of the ECU (one is clearly marked 'kick down') with a third, taller, relay in front of and to the right of these. This last relay is devoid of any markings. I cannot see any sign of a fuse in this area.

 

[Thegemmesabogie]

The relay marked kickdown is just that: not sure on a diesel if there is an elec fuel pump which this relay also does. Try pulling the black cover vertically of the 'tall' relay. The ovp relay on my 320 124 had a tall black plastic cover. The fuse (if it's the correct relay) is underneath.

 

[wraltd]

Got it - I'll give this a try. Thanks.

 

[edba2000]

Hi,

The same problem with my W124. ABS light on and tachometer off. A new OVP relay and the problem is gone! (37.5 euros in Portugal) .

Best Regards

 

[television]

Hi,

The same problem with my W124. ABS light on and tachometer off. A new OVP relay and the problem is gone! (37.5 euros in Portugal) .

Best Regards

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