R107 SL ABS Fault

[T-Bishop]

Can any body help me with knowing why the ABS warning light stays on on my 1984 280 SL I have checked the fuse on the ovp relay and that is fine.

 

[television]

Even though the fuse looks OK these relays do get badly dry jointed on the PCB somehow you need to be sure that you have 12v on pin 87.

They can be dismantled and re soldered

 

[T-Bishop]

Hi Malcolm,Thank you for your answer but how would I check if the pin 87 has 12v. Would it be wise to buy a new one?Is there any thing else I should look for ?

 

[wireman]

I have not dealt with a 107 but am led to believe that its Bosch ABS2 3 channel as fitted to 123/124/201 saloons.

Most common fault is a sensor or its wiring, then the OVP relay which has two functions (OVP and switching power to the ABS) is known to fail in a somewhat erratic/intermittent fashion due to poor soldering or fatigue of the internal circuit board where the relay coil itself is mounted.

To diagnose it
Examine the wire from the sensor for damage especialy as it joins the sensor body.
Take the front sensors off the hubs and clear any build up of rusty flakes off the end pole piece, with luck thats the fault. Test the resistance of the sensors ( at the coax plug under the bonnet just follow the wire up thru the wheel arch it will be some where atop the inner wings), expect around 1000 ohms which should be the same both (front) sides wiggle the wire as you check the resistance it should not change. Examine the teeth on the front hubs and ensure they have not rusted away or got clogged with rusty flakes.
Ensure the front wheel bearings are adjusted correctly and do not have any slop or play.

Check the rear sensors wire, the sensor is inside the diff and won't normaly get full of muck so removal may be pointless and can be somewhat oily, it should have a resistance around 800 ohms. There is a plug break or terminal block somewhere in its wire, on saloons its under the rear seat I do not know where its hidden on the 107 perhaps in the boot or under the carpets behind the seats.

Remove the cover from the ABS pump and examine the wire connector and relays (they unplug) for water damage or looseness.

Ensure the cable connetor on the ABS control unit is seated firmly. Trace the wiring looms path and examine it for physical damage.

If the lamp only illuminates after the car moves off(>3-5 Mph) its definately a sensor, if its at random and goes out on its own suspect the OVP relay.

With an AC millivoltmeter (DVM 1v ac range) connected at the plug break each sensor should produce at least 0.1v when the respective wheel is spun at around 1 rev/second.

The OVP relay may also power up some of the electronics involved in engine management tickover stability can be affected.

If you search the forum there is some more info on these systems, some folks have been kind enough to tell us what fault was eventualy found.

 

[television]

This is always a hard question,, that is to get someone to replace something that they may not need.

Lets put it this way, the OVP relay is the reason for 85% of these faults, but one must bare in mind that it could be a wheel sensor etc.
If you are keeping the car then it is handy having a spare OVP relay

I know its hard to measure the 12v from pin 87 as one needs a break out box. The car ios also before my system with the pin outs on the ABS pump so that you could connect there instead, the wiring diagram could be in the Haines 107 book

 

[mercman123]

Hi there.. Im by no means an expert, and ive just spent a couple of weeks trying to cure the same problem on my 97 C250 TD.. After numerous theories being put forward by different garages, it turned out to be a front speed sensor. I dont know if your model has these fitted but i just thought i would share my findings.

 

[woolf]

Hi Malcolm,Thank you for your answer but how would I check if the pin 87 has 12v. Would it be wise to buy a new one?Is there any thing else I should look for ?

Have you managed to resolve your problem yet? I had the same symptoms on my '85 280SL and spent weeks checking everything before I discovered it was simply a disconnected wire at the battery terminal - the ABS has it's own supply but whoever last changed the battery had failed to reconnect the wire to the post.

 

[coleyjf]

Hi all.

I've also got a problem with my ABS system (1988 300SL) which is causing a fail on my MOT and grief with my tool kit!!!!!.

Background:

the brakes work fine but not sure if the ABS does or not, tick-over(800rpm /starting/running are fine. This fault occurred a couple of months ago out of the blue.

Fault:

The ABS light comes on with the ignition as it should BUT stays on permanently with the engine running. No amount of re-starting will make it go out and it seems to be slightly brighter with the engine running than just ignition on ( I assume due to alternator running/higher voltage etc).

The ABS light stays on when driving and braking - no change whatever.

The brakes are working correctly according to the MOT test rollers -no pulling etc.


So Far: - Having read all the threads I can find on this and other forums I've:

Checked the OVP relay - tests OK and I've even wedged it operated and checked again after reading one thread but no change.

Tested the Battery/alternator output ( 12.4 static - 13.8 engine running)

Metered the sensors ( 1.03k ohms front two & 800 ohms rear drive)

Cleaned the connector(s) on top of the ABS pump

Checked the 2 relays on top of the ABS pump, cleaned the contacts and re-seated them.



I'm running out of ideas

Any new ones?

cheers

Jim

 

[television]

I feel that you must check to see if there is the 12 volt out on pin 87 on the relay that feeds the 12v to the ABS pump

 

[coleyjf]

I feel that you must check to see if there is the 12 volt out on pin 87 on the relay that feeds the 12v to the ABS pump

That sounds like a good idea - Thanks.

I've been reading various forum postings for ideas rather than using my normal electrical fault finding experience. Should have looked there 2nd after testing the OVP relay Dohhhhh!:Oops:

cheers

Jim

 

[Beegees]

ABS Sensor locations

I have an intermitent flashy ABS light....

In order to look for dirt etc...can someone describe exactly where they are and what the sensors look like...or I will be cleaning anything and everything down there :O)

Lee

 

[wireman]

They are on each hub with a cable attached that runs from the suspension to the bodywork, dont confuse them with the brake wear indicator wires which go to the calipers.

The sensors detect the motion of teeth cut into the hub by using a magnetic field and this can cause them to get covered in bits that will help bring on an intermittent warning, they have a bolt securing them to the hub, if you remove them be sure to loctite the bolts back when you replace them.

Examine the teeth on the hub for being rusted away and clean any accumalated debris off the sensors front face.

Check the front hub bearings for slop and adjust them to the correct settings if any is found.

If your warning occurs before you move the car suspect the OVP or wiring.

There are lots of other posts on here about this subject, have a good search around.

 

[thechasmanblue]

late to the party but just recently had and cured this with my 300SL. It was the ABS ECU which failed on mine. It's tucked up behind the (RHD) passenger footwell. An easy clip out and in fix once diagnosed. I got a £100 used replacement but they are freely available from Merc for megabucks.