ABS Sensors
 

I am working on 1991 300E 4matic ABS . I disconnected the round abs front sensors connector and hooked my DVOM up to the sensor end . Set the meter to 20 volt AC and tie strapped the meter to the wiper. I drove on the road at 70 km per hr . right sensor read .91 ac volt, left sensor .94 ac volt . unhooked rear flat connector under seat read 1.84 ac volt . What I need is the correct ac voltage from a working system from same 4matic . I can change my voltage by decreasing or increasing the air gap. Could any one please help? Dale

Pulses, not voltage
 

The sensor head provides frequency or number of pulses, and sends it to the ABS unit to determine how fast the wheel is rotating.
The voltage matters little, as long as the sensor can deliver an output over a certain threshold (satisfactory signal level) which the ABS unit can read.

abs sensors
 

Gerard yes you are correct . The speed is read by the module the faster or slower each wheel turns will change voltage. when the computer see`s one wheel turning slower then the others it will turn on the abs light or while braking it will hold some brake pressure from the slower wheel trying to match the slower wheel to the other wheels. You can read this speed with a DVOM set to the AC scale. My problem is rust the air gap has changed from sensor to reluctor [notched ring on axle] The computer see`s this as different speeds from each wheel .I would like some one to give me the AC voltage at 70km per hr from each off the three sensors from the same car on a working system. I will then adjust mind to match. Thanks Dale

Hi Dale,
I think I know what you are trying to do, to get equal voltages out of each sensor? but I dont think it matters. The ABS computer compares wheel rpm based on frequency or pulses from the sensor, not voltage. The voltage produced is not important.
While the voltage may change with speed, or by adjusting the air gap, this is not measured by the ABS computer. It's counting peaks and troughs, like on and offs.

hope this link works for you
 

go to page 40

http://books.google.ie/books?id=JVY8vvKRUxgC&pg=PA40&lpg=PA40&dq=abs+sensors&source=web&ots=QLjMKrYY1W&sig=GJPuRJluRFZcEXC1 CxcZOgHj8K4&hl=en&sa=X&oi=book_result&resnum=7&ct=result#PPA41,M1

Gerard

Are you saying the ridges on the hub have rusted a lot? I think that might be unrepairable if they are too far gone. The sensor relies on sharp ridges AFAIK. As said previously, the computer doesn't care about the RMS voltage. It just looks at each pulse and decides if it is "high enough". It is possible that by moving the sensor closer you could "fix" a missing pulse that was barely high enough. But I would guess that if the ridges/grooves on the hub are badly rusted you may need a new hub.

Look at the last few posts of this thread (search comes to the rescue again...)

http://www.peachparts.com/shopforum/tech-help/233453-95-e320-asr-problem-please-help.html?highlight=rust+hub+abs

This person had a rusted hub.

I searched on "rust hub abs"

Good luck

mpolli wrote;
Look at the last few posts of this thread (search comes to the rescue again...)

http://www.peachparts.com/shopforu...t=rust+hub+abs



A very informative post!! that and its related links. Thanks

what symptoms are you experiencing that have led to your investigation?:cool:

300 mv ac with the car moving very slowly just below the point the speedo registers.this i determined from testing and adjusting airgap with shims to the point where there was no false activation of the abs at slow speed.and yes you should clean out the teeth on the hub.a dental pick and wd-40 works well working thru the hole where the sensor mounts:)

Thanks for the links and help. Problem ABS light on speed sensors . I first ohm sensors they were fine. Next I checked teeth on axles badly rusted ,so I removed them both and sand blasted them ,squared them up and lightly painted. After installed could go 2 days before abs light on again . This time I removed sensors again and removed rust from hubs now as soon as I get on the road ABS light on. I do not care about matching AC voltage from side to side or front to back. I would like to get the AC voltage from the same car on a working system at 40 or 50 0r 60 or 70 km per hr . This will save me a lot of time .If I match these # from a good system and my DVOM increases smoothly I know this will solve my problem,It has in the past. Thanks Dale

A/C amplitude is simply a signal product of a faster wheel speed ..it is not the condition that the ABS monitors.. ABS looks at the a/c signals frequency of EVERY reluctor section as it passes the sensor.

http://www.picoauto.com/tutorials/diagnosing-abs.html

I had ABS trouble a few years back. I tried cleaning the sensors and and it worked temporarily, but always came back. I narrowed it down to 1 side at the front, (cant remember now) and I bought a 2nd hand sensor from ebay germany. It cost less than 20 euros (25 USD) and the problem went, now over 2 years.
G

Great tutorial. Complex set of issues, systematically and logically traced to a solution. Not every enthusiast is also a master mechanic, so it's nice to get a well-constructed, coherent description that makes sense even to the reader who doesn't have a mechanical pedigree. Thanks.

You are Welcome.
I find those once in a while and keep them for the proper occassion.
He does a great job and has that knack.........

Here he mentions a general sequential approach that I can appreciate ,having been to the same school.

http://www.hibtech.com/hibtech_tips.htm