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Testing ABS sensors ?


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Long story short, I swapped final drives, installed a used one (different ratio) along with the ABS sensor that came with it.


Before the swap, the ABS & servo assist worked perfectly.


Now both the blinking warning light on the right side of the brake warning lights blinks (starts out fast, then slows after the servo quits running for the self test (?) and the left light stays lit.


There is no servo assist for the rear brake, and the pedal has to be applied very hard to get any braking effect at all. Same master cylinder, different rear brake caliper which appears very nearly new. I've bled it thoroughly, and do have the correct fluid level in the reservoir (I've got the little plastic funnel from BeemerBoneyard).


The servo runs when I bleed the back brake and pumps fluid thru the system and out the bleed screw on the rear caliper, and when I step on the rear brake pedal in normal operation, there's just apparently no servo assist.


I looked in the operator's manual and it gives a little troubleshooting table containing some "if this happens and this doesn't happen, then check this" kind of things. It says if the servo doesn't run when the rear brake lever is applied then there's some type of problem (sorry, don't have it in front of me right now) but the servo does run when I apply the rear brake.

And I've bled the control circuits also along with the rear brake before the swap on my annual brake fluid change, and things worked fine after that. And the rear control circuit reservoir is at the proper level.


Doing some research, I found an explanation of how to test ABS sensors on BMW's (cars, but shouldn't they be about the same?)



It said to make sure the clearance is correct (it is), and that the detecting surface is clean (have to check that). Then to unplug the sensor and hook up a DVM on ohms and if it shows infinity then the sensor is bad. If it doesn't show infinity ohms, then to hook it up on the 12vdc scale and it should show 5 volts or less.


My question is how can that be done with the sensor unhooked and no apparent voltage source? I don't imagine the sensor generates any voltage by itself? And there's no access to check voltage when the sensor is plugged in.


And if anyone has any suggestions on something I may have missed it would be humbly appreciated.


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Danny caddyshack Noonan

Almost sounds like a collapsed rear hose, the rubber part. The inner liner can fail and behave something like that but, it'll eventually give pressure feedback but no braking.

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That's a valid idea, but it's a brand new Spiegler braided SS hose, and I even unscrewed it from the caliper to make sure fluid was coming thru it when bleeding with the servo unit.


I hope to find the time to test the ABS sensor maybe Saturday.


If I do, and can determine anything, I'll post.


In the meantime, thanks!

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Danny caddyshack Noonan

Tried to do the sensor test with the two wire units. Pretty darn hard finding the miniature pins and make a circuit.

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Morning Macx


As a rule when it comes to diagnostics go back to the last thing that was done prior to the problem appearing.


So that is final drive install and/or brake bleeding.


I presume since you are talking about servo assist that you are working on something in 2002-2005 range?


In any case it’s possible you have a rear sensor failure problem or maybe air in the rear system.


I have the ABS sensor resistance and wheel spin data somewhere out in my shop but I am no where near there now. Problem is: that would be with my high end Fluke meter so might not be same as your meter.


A good rule when testing wheel speed sensors without a scope (scope is really needed for a proper test) is to compare the resistance and wheel spin output voltage to a known good sensor. You have that on your front wheel. So unplug the front sensor and use your ohm meter and measure the static resistance. Then set the meter to DC on a real low setting or even m/v then give the wheel a spin. Depending on your meter’s response time you might get a usable reading, if not switch to the AC scale to see if the meter will do a better job using that.


In any case you want to see about the same resistance on the front and rear sensor and close to the same voltage, and close to the same voltage toggles or cycles as the wheels spin about the same speed. Your sensors are not the newest hall type so do produce a small (pulsed) voltage without needing a base power supply. Problem is: different meters respond differently to the voltage pulses so about your only option is to compare front to rear under same conditions.


If the sensors test out close to each other then look for possible wire pins pushed back into the connectors or if you had the large connector off the top of the ABS controller to allow access to the bleed screws look for a terminal pin pushed out of place there.


Can you ALWAYS hear the servo run when pushing of rear brake pedal (obviously before ride off)?


Are you sure that the rear master cylinder is full (inside the ABS controller itself)?


Added: I see another post from you about swapping final drives and rear wheels as well as many other parts.


If you installed a new rear wheel is the sensor gap the same as it was on the old set up? Also does the tone ring have the same number of raised/depressed segments on it? (you need a proper tone ring to go with your system)

If not the tone ring needs to be the same (exactly) as came off the old set up. The ABS controller needs to see the same wheel sensor inputs from front and rear at the same road speed.


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One more thing----



Also look at the tone ring now installed for ANY signs of damage like wobble or bent segments. Just one bent segment can cause an error at ride-off.


If this is your new rear wheel it looks like the ABS tone ring is different than the old R tone ring. If the tone ring segment count is different I think you have your problem identified.




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