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  #1  
Old 05-02-2002, 12:48 AM
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A Curly One for Benzmac, or whoever.

I hope that your as good as i belive you are, this is weird, 87 300e.

Abs dash light comes on aprox 5-10 seconds after starting if i let it stand idling, even if i bring the revs up.

But if i get under way even if it`s at idle crawling speed it never comes on until i stop for 5-10 seconds again.

So what i did was jack up the front start the car and slowly spin the wheel (R/H side) light stays off, stop spinning that wheel and the light comes on.

So i figured which sensor is playing up.

Cleaned it and the hub, but did`nt fix the problem.

Measured the resitance of the sensor to the other side and their exactly the same.

Please BenzMac have the answer!
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  #2  
Old 05-02-2002, 09:03 AM
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The ABS ECU & relays aren't getting enough power. Possibly the charging system isn't keeping up with the demand!
TRY switching all power consumers off & then see what happens.
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  #3  
Old 05-02-2002, 10:44 AM
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Same problem on our 1990 190E 2.6 a while back. Was advanced warning of alternator failure. New alternator solved the ABS light problem.

Bad sensors usually have the ABS engage at very low speeds for no reason. 190E had this happen too, later.
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  #4  
Old 05-02-2002, 11:40 AM
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You may want to...

check the brushes of the voltage regulator behind your alternator. Cheap part and easy replacement. How old is your battery?
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  #5  
Old 05-02-2002, 06:37 PM
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All you guys are thinking the same way i did.

Checked all the charge rates. 13v@idle 13.8@revs

Cleaned every contact pug and socket even the ovp

In the centre of the ABS wheel sensor there`s a magnetic insert that i take is the actual pick up, it has i knife edge on the end (looks like the end of a screwdriver). I`m wondering if maybe a rock can get thown up and upset it`s orientation, it seems that if you grab it that you may be able to turn it to change the angle of the knife edge to the wheel hub.
Can anyone comment on this before i break it by trying to turn it.
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  #6  
Old 05-02-2002, 06:44 PM
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A sensors ability to sense will not turn on the light until 10kph. The only way a sensor can turn the light on without movement is if one is totally open circuit (or at least open enough that the computer discriminator can tell).

Like the other replies I agree the problem is with the voltage supply either through the OVP or from the alternator.
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  #7  
Old 05-02-2002, 07:40 PM
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One benz mac here in australia said there was one instance were he had to put an extra sealing ring under the sensor to space it away further from the hub, because he believed that with age the sensors can become more sensitive.
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  #8  
Old 05-03-2002, 09:18 AM
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I'll say it again. The sensitivity of the sensors does not turn a light on untill the car is moving. Look only at the resistance of the sensors if your light comes on while sitting. Either that or look to some form of induced electromotive force (I've heard a CB radio will set the speed counter). With a scope you should have no activity on the sensor leads at rest. You get a AC sine wave style pattern with variable amplitude and frequency as the the speed ring teeth pass the blade of the sensor. Distance matters to amplitude but not frequency. The controller is more interested in frequency than amplitude but amplitude variations will set the light.

There is no amplitude or frequency to be judged at rest, unless its a stray signal.
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  #9  
Old 05-03-2002, 09:46 AM
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WOW! Steve you just blew me away with that last post, i did`nt know you spoke Hebrew. All jokes aside what i did today was to to take the car up to 60kph have the abs light off then slamed on the brakes so the abs functioned, i did this about 4 times and it helped and the light stayed off, but next time i got into the car and started it the dam light came back on again. i know your thinking electrics, but my digital meter is telling me all is correct. by the way i measured the sensors resistance again and they`re 1.2k left and 1.3k right the secs say 1to2k is the min and max.
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  #10  
Old 05-03-2002, 10:07 AM
moedip
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13V at idle seems a bit low - should be 13.4 minimum from the bosch alternators I have worked on. The higher rev voltage is acceptable but a little low. Some of my alternators went that low but most at higher rev produce 14.0 - 14.7 volts, at idle they never went below 13.4v. If you haven't done it yet - spring for a new regulator/brush pak for your alternator - simple to change and not all that expensive - with 13v idle charge - it is getting ready to let go. Like MB Doc says - not enough power getting through. While you are at it - let the car sit overnight and without starting it - check the static voltage on the battery - it should be 12.4 volts MINIMUM - anything lower (because of your 13V idle charging should bring it up to that) - indicates a bad battery that should be changed. If the battery has an internal short it will suck your charging voltage down and could damage your alternator.
1) check battery first to make sure it is good
2) then check alternator reg/brushes.
I am confidant - that the above should cure your problem
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  #11  
Old 05-03-2002, 10:39 AM
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Does the light come on when moving? The pump relay is monitored by the control unit. It is a simple 5 pin relay and might be worth shot gun replacement (it sure would be worth switching with other relays for test purposes). It is under the black plastic cover of the hydraulic actuator.
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  #12  
Old 05-03-2002, 06:07 PM
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Moedip, sory your right my idle voltage is 13.4v and at revs it gets to 13.8v.
Steve i`ll try your suggestion of the rely under the black cover of the abs unit for memory i cleaned up two under there i think ome was black the other was silver which one do you suggest.

One other thing guys, when i gave the abs some curry by locking up the wheels in the wet yesterday 4 or 5 times after doing that i did notice like a sewing machine sound comming from somewhere, it almost sounds the same as the vaccum pump under the back seat. is this normal after the abs has been thrashed a little, i could also feel it through the foot brake peddle as some very slight up and down movement, is this just the abs sorting itself back out after being hammered.
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  #13  
Old 05-04-2002, 12:09 AM
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No, this voltage is incorrect. The proper voltage is 13.8 to 14.2 volts. I believe this is a voltage problem. Pull out the voltage regulator as has been suggested and look for a bad brush.
Gilly
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  #14  
Old 05-04-2002, 03:35 AM
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Ok electrical experts, here are some more altenator measurements, idle 13.4 at revs steady 2mins @3000rpm will get up to 14v

At idle blower on full, high beam on, panic lights on, windscreen wiper on, 12v.
At 3000rpm 2mins just gets to 13v.
took out the brush reg assembly bushes look ok, long enough slip rings look ok.

how do i test the reg itself is it possible with a dig multimeter, probally have to remove the 68ohm across (if it were a trasistor) base and emiter. Is it possible to check, as the assembly is hard to get and expensive here in Australia.
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  #15  
Old 05-06-2002, 11:01 AM
moedip
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What is the static voltage on your battery? Let me repeat from previous (with more details) When a battery is just charged it has a "surface charge" which can give a false reading for the actual condition of the battery. When a battery is allowed to sit (like overnight) the surface charge disappates and the actual charge condition of the battery is then the voltage you read. So in reality - if you run your car for a couple of hours at revs and the battery is being charged by the alternator (all accessories off) and you turn the car off and read the voltage at the battery - it can be as high as 13.14 volts - which is the surface charge reading. Let the battery sit over night and without trying to start the car - read the voltage on the battery - it will be the true state of charge. This voltage should NOT be UNDER 12.4 volts (12.65 - 12.90 indicates a real healthy battery - at 12.56 battery is still quite usable but getting weak)- if it is you have battery broblems which could be sucking current from your alternator and not building up a reserve for when you need your accesories. CHECK YOUR BATTERY BEFORE DOING ANYTHING ELSE - It could be time well spent.
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