It is very difficult to get a concise diagnosis from wheel speed sensor/reluctors output without a scope to examine the ac sine wave pattern. You can ohm them and you can take DMM output voltages , but you really want to see what the ABS control module sees ..and that is the sinewave.
...which is why I posted the article......
It take one little glitch to activate ABS , specially at low rpm [ output]

you can see the rms a/c voltage output of the sensors this way.crude but effective.my right sensor had 1/10 the a/c output of the left despite a new sensor on that wheel.thats how i tracked it down.your dvm probes wi:)ll fit into the female side of the coaxial connector.drive at sloww speed below where the speedometer will register

Have you ruled out 4matic?
 

My 91 4matic sedan has a 4matic test lever under the hood used to disable the system. On your car, I believe that it will also disable the leveling system, but that shouldn't be a problem for a short period to rule out a 4matic issue.

I would disable the 4matic and see if the symptoms are the same.

Grrrrr!
 

Getting really frustrated now. Pulled the right front this morning, cleaned up the teeth that the WSS reads, no change.

To recap:

1. 1991 300TE 4matic, 147k miles
2. Getting ABS engagement on every stop when speed falls below ~1-2 mph
3. Both front WSS resistance readings are within spec - checked again today: 1.598kΩ (left) and 1.020kΩ (right)
4. Both left and right front teeth were crapped up with rust. Both cleaned. No change.
5. ASR light (mistakenly called 4matic light by me yesterday) comes on sometimes when pulling away after ABS has engaged on previous slow roll stop (definitely NOT due to wheel slip)
6. If I do a panic stop from 20/30/40/50/60 mph, ABS functions normally
7. If I hang one side off in the gravel and stomp the go peddle from a dead stop, ASR functions normally
8. Flipping the 4matic into test mode (lever on right front fender wall) makes no difference to any of the above

Questions:

1. Where can I find the connectors for the rear WSS's (would like to confirm 'under the back seat' is correct for my wagon before I start tearing up carpet)?
2. Lacking a scope to get more meaningful readings off the front WSS's, what are my options for further troubleshooting?
3. Does it make sense to pull the rear wheels and do the same 'clean out the rust' routine on the teeth there?

W/O a scope , you can try a DMM as Cole suggest..
I would jack each front wheel and take a V reading at 60 rpm. [ 1 rpm/sec]
This can be done with a little practice. You get the tire going smooth by putting a finger on the side wall and spin the tire with your whole arm and count in seconds...You can actually get it pretty close if you do 5-10 sec rotations.
Then compare the V of each side. [ @ approx same rpm]
The problem with hall effect sensors is the amplitude [ voltage] increases with rpm , so slow rpms show the first drop to the ECU [ which is why you have activation coming to a slow stop.]
So, that indicates either a glitch [ only scope will show that]..or a V drop from a sensor [ caused by sensor /reluctor/spacing/etc ..you know the drill.].....
My concern would be the % of ohm differential reading you have between the two fronts..they are both spec, but I would suspect a difference in ohm values that large to possibly show up as a lower V on the lower R side at low RPM..[ in other words, that one would likely drop V to zero sooner than the other ]...just a possible
Scope is the tool, no matter what other test are done..but when you don't have one , you punt.

Ramonjim.your front sensor ohm readings are good.nothing to worry about there.i suspect the teeth on one or both of the front axle tone rings are worn down a little bit from corrosion over time.the air gap between the teeth and the sensor is the critical dimension here.some ac voltages would be handy now to further troubleshoot.dont worry about the rear sensor too much.you can get the ohm reading at the plug which is under the right rear seat under the carpet.carefully lift the carpet the edge nearest the door.you will find it right under there.the rear sensor mounts into the rear differential and as such is protected from rust.the sensor is easy to remove for inspection and cleaning.ive tried the scope method on the front wheels but found the ac sine wave difficult to interpret so im recommending you try hooking the dvm to the connectors in the engine compartment and driving the vehicle with the dvm taped to your windscreen.you will see the ac voltage fluctuate fairly rapidly.have an assistant watch the meter and keep your speed below where the speedometer begins to move.:)

<ive tried the scope method on the front wheels but found the ac sine wave difficult to interpret so im recommending you try hooking the dvm>

There is NO comparison to a sine wave and a DMM reading when diagnosing hall sensors.
I am also recommending the DMM , but only b/c he has no scope..certainly not b/c a DMM is the better diagnostics tool for hall sensors...for sure.

First off, thanks again to all who are helping out.

Second - Michael, your note about the mounting of the rear WSS opens another set of questions in my mind (and your thinking that I could be facing a front axle replacement ($1156 each!) frightens me - two of those would be damn close to what I paid for the car!).

Gretel has a noisy rear end (noise when loaded either from accel or decel, quiet when unloaded). My guess is that the rear WSS is NOT looking inside the diff - and therefore the noisy rear end shouldn't be a factor here. Can anybody confirm or correct that guess please?

Also - I'll throw a meter on in either or both Michael's and Arthur's suggested methods. Assuming I find one corner different enough to be deemed the trouble spot - how do I positively determine if I need to replace the WSS vs. the axle?

If you find the problem w/DMM , then you are good...but before I spent any $$$ for parts, I would go to someone to get a scope pattern for a proper diagnosis.
A scope pattern with a flattened sine wave peak on the amplitude of the pattern would point to airgap excess of reluctor. Just one of the many possibles that a sine wave shows and a DMM won't. BC this is such a low rpm wheel speed complaint, I would want to look at EVERY single tooths sine
wave, b/c that wave will show what is the cause of ABS activation when the complaint is ABS when coming to a very slow stop.....even loose wheel bearings will cause that complaint.

i do concur with arthur on possible wheel bearing issue being a root cause and would recommend checking all the easy stuff before axle replacement.to be honest i havent resolved my own problem with this yet but i have nailed down to the right front wheel.the axles are mucho$$$.all teeth appear to be in good shape but they have been coated with rust before cleaning.im thinking now about possible measurement of the air gap with a feeler gauge at the known good wheel and comparison with the bad.i can see that a loose wheel bearing could give several thousands of an inch play and depending upon the air gap tolerance could be a factor.the airgap can be accessed at the rear of the hub so you dont need to remove the sensor.its also easier to blow compressed air in that way for periodic cleaning.:)

Well, crap.

Checked both front WSS AC voltage readings as described by Arthur (probe WSS connector, female side, spin wheel by hand at ~ 60 rpm).

Left: ~ 0.028 to 0.031 VAC

Right: ~ 0.250 to 0.290 VAC

Pulled the left front sensor, and checked the mating surfaces (sensor and hub). Sensor was clean, but the hub had a couple small mountains (eyeballing it, I'd guess maybe 0.050 - 0.060" high) of rust within the mounting area. Eureka, me thinks! Cleaned those up (small grinder) and replaced the WSS.

No change in the left front reading. Crap.

So:

Left WSS reads higher static resistance (1.598kΩ) and lower low speed voltage (0.028 to 0.031 VAC).

Right WSS reads lower static resistance (1.020kΩ) and higher low speed voltage (0.250 to 0.290 VAC).

Options for determining which component is the culprit?

1. Chuck the DIY mode and take it to a Benz dealer. What are my chances of getting a dealer who will actually hook up a scope and DETERMINE which part is the fault, rather than just have me pay to replace parts until it is resolved?
2. Replace the wheel bearings. I have no records indicating when they were last done, so I could maybe convince myself it 'needs' done now.
3. Actually CHECK the wheel bearings. How do I determine their health?
4. Temporarily replace the axle with a known good unit. Anybody got one I could borrow?
5. Other ideas?

My only other request (for right now :D): where do I fax a tall cold one in appreciation of your help?

Stick the depth gauge rule from a vernier in both sides w/sensor removed and see how the reluctors measure from the top of the sensor mount hole. Compare..side/side.


<<Left WSS reads higher static resistance (1.598kΩ) and lower low speed voltage (0.028 to 0.031 VAC).
>>

Flex that cable around where it enters the sensor while you have the ohm meter hooked up and look for that value to drop/change ..you could have a poor connection in the harness , causing higher R / lower V........
That would fit the slow stop complaint while no other ABS problems b/c as soon as WSS sees a higher rpm [ even 3-4 mph], the amplitude of the sensor [ V] is high enough and verified for the ABS module to be satisfied..

How much V can you get out of that side if you hand spin that wheel as fast as possible by hand?.... and does it drop V before stopping earlier than the other side ?

No dramatic difference in spacing side to side: a 20 teeth from around the full diameter on each side, all fell within +/- .004 of 1.025" from the mounting surface to the top of the tooth.

No obvious open/short in the cable - wiggling produced no more than a .005kΩ change.

Max voltage left: 0.051 VAC
Max voltage right: 0.345 VAC

Left dropped to meter reading of nominal zero (0.002 VAC) pretty much right as the as the wheel stopped spinning.

Right didn't drop to a meter reading of nominal zero until just after the wheel stopped spinning - didn't put a stop watch to it, maybe a full second, maybe two - but markedly different that the left.

Does this mean funky left WSS?

<Does this mean funky left WSS?>>



That would be my guess.
Reminding all that w/o a scope reading , that is a logical guess only...

Might want to try that measure with tire suspension loaded [ some blocks under tire so you can get to hole again.
How do the working height of the 2 sensors compare ??? [ does the end of the sensor look worn down from debries/reluctor rust]

Are those MAX readings when you increased the hand spin tire speed??

Yes - spinning the wheels as fast as I could by hand:

Max voltage left: 0.051 VAC
Max voltage right: 0.345 VAC