W124 300TE no ABS function, no warning light

PiltdownMan

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Hello! First problem I've had with my 300TE: the ABS does not function, and the ABS light does not come on at any point (not when you turn on the ignition, or when driving).

Otherwise the car drives with no problems.

I have read some threads about the OVP relay failing, or the fuse failing, causing the ABS to go off, but they all mention the ABS light as being illuminated on the dash. Mine isn't, so could it be something else? The car doesn't get much use and has had a new battery in the past few weeks.

Any ideas greatly appreciated!

Ted.
 

wireman

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There is probably no power at all getting to your ABS.
The OVP relay has one of the fuses that power your ABS in its top face, it is located (on my 124) behind the battery under a black plastic cover which so to speak simply clips off (simple once you've done it) to reveal the OVP relay, there are relibility issues with some OVP relays but mostly these can be fixed. The OVP relay provides 12v to the more delicate electronics in the ABS controller and on some models engine management is also powered by it.

There are other supplies to the ABS, one goes directly to the pump and these are fused seperately.

What year is it?

Before you take it all to bits has the lamp ever illuminated whilst you have owned the car? because it is not unknown for some rogues to remove the bulb (or its wire) as a means of passing the car off as OK.
 
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PiltdownMan

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Hello, thanks for the reply. The car is a 1992 model, K registered. The light did used to come on and go out with all the other dash lights when the ignition was turned on, like it was supposed to.

Do you think it's likely to be a fuse, or perhaps that relay?

Thanks.
 

teddycatkin

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Did it used to come on when ign was turned on ?
How are you determining the ABS is not working ?
Easiest way is diagnostic check 1st to find the fault.
 
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PiltdownMan

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I know the ABS isn't working because I braked hard and the offside front wheel locked up. There was no ABS activity at all.

Ted.
 

teddycatkin

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OK --the wheels should not lock (on a good dry surface)--but you should have the light on permenantly if its not working and if any bulb blows then the warning light would show on all time too?
 

Number_Cruncher

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First thing I would do is to put a new ABS warning bulb in the dashboard, then check the supply voltage to the ABS.

If there's any problem with the supply voltage, look very closely at the OVP. If the OVP fails, the ABS lamp should come on.
 

meanie

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As above really,but change the bulb first.
Also,remove the connector off of the top of the ABS control module & check for corrosion on the contacts.The trims under the w/screen area are not always fitted/refitted correctly allowing water todrip onto these sensitive components.
 
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PiltdownMan

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Hello all and thanks for the help. The bulb in the dash had gone and the light was supposed to be on (it is now), so I'll have a look at the OVP relay and the fuse there first.

Ted.
 

wireman

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The OVP protects against battery voltage surges by blowing the fuse, if the fuse is blown replacing it will probably be the end of your problem, Do check the fuse contacts on the relay since they can get dirty or corroded if the covers are not properly installed or the thin seal on the OVP's clear plastic cover is missing/broken.

The OVP also cuts off if the battery voltage falls below 10.5v (ie a lead acid at 0% charge state), The circuitry that detects this is inside the ABS controller and signals to the OVP to switch on its relay thereby powering up all the sensitive electronics.

The controlled 12v output is on the red with white stripe wires that come from terminal 86(I think) on the relays base. I have seen one car with this wire trapped beneath the battery untill the insulation was compressed flat and the then exposed wire strands shorted to the battery tray blowing the OVP fuse when it did.

I have seen (and sorted out) several OVP units with poor solder joints on their internal circuit board, you need to open up the OVP to do this, cut off the edges of the lower corners of the can from just above the crimp line to the free/bottom end with a stanley knife so that the cans sides can be spread out alowing it to be removed whilst retaining some metal to hold it together once its been reclosed, reseal it with a smear of silicone sealant in the joint.
 
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PiltdownMan

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Thanks for the advice. I'll be having a go at it tomorrow. Wish me luck!

Ted
 
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PiltdownMan

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Hello all,

I tried replacing the fuse in the top of the OVP relay but that blew again immediately, so I went to the Mercedes dealer, got a new relay and fitted it, which was remarkably straightforward (although it'd probably be easier if you've got hands smaller than mine or if you took the battery out right out of the car instead of just disconnecting it).

I've just come back from a longish test drive and everything seems to be working as it should. The light comes on with the ignition and goes out when the engine starts, and (most importantly) the ABS works. I'm sure a relay from someone other than the dealer would've been cheaper than the £69 including VAT that I paid, but they had it on the shelf and I could at least be certain it'd be the right one.

There were a few things that weren't exactly clear in some of the other threads on this subject:

1. The OVP relay is under a plastic cover which tucks underneath the weatherstrip at the back of the bonnet. Under that cover you can see the ECU and next to that, towards the side of the car not towards the engine, is another oblong black plastic cover which goes over the top of the OVP relay itself. Take that off and then you see the red-topped OVP relay with the clear fuse cover.

2. If you get a new OVP relay from a dealer it will have two fuses in the top, not one as the originals generally seem to have.

Hopefully that should be the end of that! Thanks to everybody who wrote with help.

Ted.
 


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