testing ABS sensors
 

I've learned from the archives that I needed to disconnect my battery first.
I've done that.

I also see mentioned in one place that I should do something to the ABS controller?
I assume this is it?

http://www.surfpick.com/benz/abscontrol.jpg

I've fiddled with the lid every which way... but can't figure out how to remove it.
I fear breaking it, if I yank on it with any more force.

What do I do to it, once I remove it?

I bought a multimeter.
I understand that I am checking the resistance, using the ohms scale?
I think I have the leads plugged in correctly, according to the instructions?
Is 200 the correct setting?

http://www.surfpick.com/benz/multimeter.jpg

Finally, exactly where do I place the tips of the tester?

http://www.surfpick.com/benz/absconnector.jpg

1 and 2 ? 2 and 3 ?

I've never used one of these before.


Thanks.

Looks like that is the ABS unit, but I may be mistaken, are there brake lines coming off of it that you can see? 200 is probably the right ohms scale, but it wont hurt anything to try other scales depending on what the resistance value. To figure out where you will be testing, you need to trace both of the wires pictured to find where they go. Test the side of the actual sensor, not the ABS unit itself.

You will be testing between either 1&2 or 3&4

It looks to me that the 3&4 side would lead to the sensor itself, but I may not be right because of where the picture is taken. Just follow the sensor wire and test the sensor side, not the ABS unit side.

After taking a resistance check, you will probably want to do a continuity check also. This can be accomplished by setting your multimeter 1 more counterclockwise than it is in the picture. If the sensor is good, you should here a beep when the probes are touched to the ends of the sensor lead.

Sometimes the wheel sensor wires fail because of repeated flexing, particularly those on the front wheels. With your multimeter set to ohms and with the contacts connected to the terminal on the wheel sensor unit, flex and twist up and down the length of the sensor wire. If you lose continuity while flexing the wire, you have a part failure.

Alrighty

The side with #3 and #4 leads to the controller,
so I tested #1 and #2.

Both sides gave a reading between 166 and 193...
but the number only flashed for a second.
Is that how it's supposed to appear?

I then tried the continuity setting
and got a reading of about 1300 on both sides.
I was again touching points #1 and #2, if that is correct.

What am I doing wrong?


Thanks for the help.

I guess I'll have to close her up and admit defeat.

At least I didn't make anything worse. ( I hope? )
I've got a history of snapping bolts, etc.

NO, the number's should stay there, could be that the resistance is higher than what you set the ohm meter scale to, so if you up the scale to 2K or 20K and the same thing happens (number appears then the display goes blank or shows inifinty) you have a broken line. And with your next quote below, my guess is that you have 1300 ohms, which is kinda on the high side, could be corrosion in the connector.

Good luck


www.benzbonz.biz

Marty

Thanks Marty,

I moved up to 2000 ohms and got a steady reading this time.
It was about 1300 on the driver's side.
The passenger side fluctuated between 1300 and 1800
and I noticed that the sheathing was pulled back a bit, next to the connnecter.

I tried Maki's trick of wiggling it while holding the multimeter on it
and was able to get it to hold at 1300.
I guess that's my bad sensor?
Unless someone suggests otherwise, I'll order a passenger side sensor wire.


Marty, I checked out your website.
Does my SDL share many of the same parts with your 420 SEL?
If so, what would the little flap door for the front towing position cost?
I recently noticed that I don't have the underbody panels that
shield the engine and transmission either.
I don't know if I really need them... but what would you charge for those?

Thanks again.

Actually, the goal of the wire flexing is to find a break in the wire while monitoring the resistance with the multimeter. If the ohm reading spikes while bending the wire at a specific point, it's likely to have a break there. Just wanted to clarify that...

The trick isn't really mine -- I learned it at a StarTech forum from the late Randy Durrance, a renowned Mercedes tech up in these parts. I recall Randy saying broken sensor wires was the most common ABS problem he came across, particularly on the front wheels where they have to flex to accommodate steering angles.

Good luck, HTH

I am not sure of your symptoms but I used the above method to troubleshoot. In my case the ABS light came on while turning left and the right front cable was the culprit.

To test resistance, you touch the probes to #1 and #2. You are sending a small current through the sensor, and the multimeter is telling you how much resistance the sensor (and the wires) are causing. You appear to have done this successfully, and got a reading of 1300 ohms on each side.

To test continuity (of the wires, not the sensor), you must be able to disconnect the wire AT THE SENSOR as well as at the ABS controller. You then place one probe at #1 (in your photo) and the other probe at the same point on the OTHER END of that wire. The reading should be zero ohms, no matter which ohm setting you have the multimeter on, or very nearly zero (certainly less than 1.0 ohm). If it's infinity, you have a break in the wire. (In your picture, the reading of "1" with space before the decimal point means infinity.) Then, with the probes in place, flex the wire as much as possible to be sure the reading doesn't change. Repeat with the probes on #2 and the opposite end.

You can also test whether the sensor is generating a signal. Place the probes at #1 and #2. Set your meter to read AC volts (on yours, it looks like the one o'clock position, with the lowest scale being 200V). Jack up the car so that wheel is free, and spin it. You should get a reading--probably very low, like around 1 volt--but something.

Bump
 

for customer

http://www.youtube.com/watch?v=m_YB0RKpQ1k

http://www.youtube.com/watch?v=8H-i_Bknn-c

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