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300SE 1990
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70 Posts
Discussion Starter #1
For about a month now I have been having intermittent problems with the ABS brakes on my 90 300SE.
When I was just about coming to a stop during normal city driving the brake pedal would sometimes become softer than normal and then start to pulsate just like they do when the ABS works on slick roads.  Then after that would happen the ABS light would come on and stay on until I shut the car down and restart it.   Also when the light was on I would still have the same pedal feel and pulsating until it was turned off and restarted.   Most of the time the light would go out after being restarted but sometimes it will not.   Also about the same time this started to happen, the ABS light would come on midway during a highway drive.  It does not come on when on the highway anymore but  it is starting to act up more frequently now.  When the car is first started the light it out but the first time I go to stop at 5mph and above the light will go on and the pedal acts funky again.
Any ideas of the cause of this?   If I can not fix it is there a way to disable the antilock system?
Thanks in advance
Dan
 

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1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
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5,334 Posts
Hi cam69,

Here are some images from the MB CDs regarding the ABS on your machine. I hope they are helpful.

There are some CDs on ebay the last time I looked. Might be something you can use in the future (2 cents).

Cheers, MBL
 

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89' 560SEL, 92' 300CE
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RE: to remove...???

Can you just pull them out or is it a trick to it? I need to replace mine on the front. I hope someone can give some details about taking them off and putting them on. Just don't wanna screw nothing up.
 

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88 560SEL
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114 Posts
MBL87560SEC has given you a nice print of how the system works. Just to shorten it a bit, the main culprits to your kind of ABS-problem are usually:
Clogged-up ABS-sensors,
Broken/faulty sensor leads/wires,
Faulty OVP-relay,
Charging disturbances from worn generator bushings
ABS-control unit going haywire.
Check them in that order. You certainly do not want it to be the control unit (or the ABS-pump). Those two components are among the five most expensive parts in these cars.
You have two sensors on the front axle and one in the differential. Unless your car has "anti spin control" (ASR) - Then you'll find two sensors to the rear as well. The front ones are fairly easy to take out. You see them next to the brake caliper, with a wire running up through the inner fender and into the engine room. Unscrew the wire guide, and then there is just one umbraco screw to hold the sensor. Pull it gently and evenly out AND DON'T PULL IT BY THE WIRE. If it jams it is easy to ruin it. Brush of the dirt, clean it with carburettor cleaner and electronic cleaner and put it back in. If the problem is still there you have to check the leads, and then the .... Just follow the wire and you'll see the two plugs in the engine room.
 

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300SE 1990
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Discussion Starter #5
MBL87560SEC
thanks for the diagrams that will help a lot.
Thanks for all the replys I will check the sensors this weekend and report my findings.
Thanks
Dan
 

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1990 300SE
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Yeah, if it turns out to be a dying control unit, it would be cheaper to buy another 126 then the ABS control unit itself. They run around $2,500 just for the unit. Gasp.
 

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'08 E63 AMG Wagon. C280 (320) track/drift slag. Wife has CLK500 Convertible
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Thanks for the insight in to the world of ABS folks. I too have a problem with my ABS, and your diagrams/comments will hopefully allow me to diagnose it.

Has anyone seen the following fault:

ABS works fine all the time, until I hit about 90-100mph or so, then the ABS light comes on the dashboard. I can cruise all day at 80 and it works fine - get above 90 (although not usually until I take my foot off the gas), and it fails.

I suppose the obvious answer is not to drive so bloody fast!

Is this likely to just be dirty sensors?
(Please say yes, please say yes!...)
 

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88 560SEL
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114 Posts
ABS works fine all the time, until I hit about 90-100mph or so, then the ABS light comes on the dashboard. I can cruise all day at 80 and it works fine - get above 90 (although not usually until I take my foot off the gas), and it fails.
According to the WIS there are three causes for ABS-light to be lit when you go beyond 160 km/h: Different tires front to rear, wrong rotors (for the ABS sensors in the hub) or dirty sensors. So you might be lucky?
 

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'08 E63 AMG Wagon. C280 (320) track/drift slag. Wife has CLK500 Convertible
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Autootter - 2/12/2005 7:07 PM
According to the WIS there are three causes for ABS-light to be lit when you go beyond 160 km/h: Different tires front to rear, wrong rotors (for the ABS sensors in the hub) or dirty sensors. So you might be lucky?
Oh thank you for that otter, it would make sense actualy - I am running 235 40 17's at the front and 235 45 17's at the rear - never had that problem with the last 2 SEC running those rims tho...

The rotors should be correct as it has always been fully dealer serviced.

Will put the original wheels back on and test your theory.

Still hoping on the dirty sensors though! [:)]
 

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88 560SEL
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I put your tire dimensions through a calculator just for the fun of it. Difference in rotation speed at 160 km/h is aprox 4%. If a speedo reading of 160 km/h is the same as your actual speed - measured with the 40 tires - the same speedo reading with the 45 tires will mean an increased actual speed of aprox 166 km/h. Probably enough to give the system a fright?
 

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300SE 1990
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Discussion Starter #11
This weekend I tried to get one of the front ABS sensor out to clean but I was not able to. Is there a trick to get them out? I twisted it L and R and used WD-40 then tried pulling the sensor straight out without any luck. It seem to turn freely but I just could not pop it loose? Is there something I am missing? I didn't want to but I even tried pliers and a flat blade screwdriver to pry it out with no luck.
Thanks for the info
Dan
 

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88 560SEL
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cam69ss - 2/14/2005 4:24 AM This weekend I tried to get one of the front ABS sensor out to clean but I was not able to. Is there a trick to get them out? I twisted it L and R and used WD-40 then tried pulling the sensor straight out without any luck. It seem to turn freely but I just could not pop it loose? Is there something I am missing? I didn't want to but I even tried pliers and a flat blade screwdriver to pry it out with no luck. Thanks for the info Dan
No, you went about it the same way I did - except for the pliers. They fit in the openings with almost no play, and as they are powerful magnets they stick like nobodys buisiness. The accumulated layers of brake pad dust doesn't help either. Just pour WD40/5-56 all around repeatedly. Perhaps they will already have come a bit looser by now, as the WD40 has worked its wonders for a while. Apply gentle force using two screwdrivers at the same time all the way around, until they are about 2 - 3 centimeters out. Then you should be able to gently twist and pull the rest out with your fingers as the magnetic counter pull drops. Just keep any sharp tools clear of the sealing and flange between the base and the cylinder part. The rear ones are much harder to take out without ruining them.[:(] New ones cost about 140€ a piece I believe. (In Europe that is)
 

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89' 560SEL, 92' 300CE
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RE: sensors out yet?

Just hoping to hear if ya finally got em' out. I'm suppose to try cleaning mine this weekend.
 

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300SE 1990
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Discussion Starter #14
No I did not try again. I hope to try this weekend. The ABS light is driving me crazy.
Anyway I hope to pry them out this weekend and report my findings
DAn
 

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Depends on the day!
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Be careful, you can break the sensors if you pull/pry/yank on them too hard.. I learned the hard way a long time ago on my SEL:-/ If possible try to get a bit of heat into the metal around the sensor and use a Real penitrating oil, ie screwloose, PB Blaster etc. WD40 sucks...
When you do have the sensor out, spray the hell out of the teeth as well.. Spraying in the hole where the sensor came out of while rotating the wheel. Do yourself a favour and buy a couple of cans of the Good brake clean (CRC etc..).

Jonathan
 
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