# ABS Module vs. ABS Sensor



## tincher_b (Jul 31, 2009)

Need some advice...My 2003 VW Jetta (155k miles) has the typical ABS/traction control light on. I had a bad wheel speed sensor a while back and replaced it. The light came back 10k miles later. It is an intermittent light but becoming more regular. I took the car to the dealership and they said the sensor was fine, but there appeared to be a short in the wiring going to the ABS from the right rear ABS sensor (I hate dealers). So with that load of crap I went on home. I checked the resistance thru the wiring on both the right rear and left rear wiring and came up with the same number (8.5Mohms) for each side. The voltage measured 2.4V on both sides. I think that rules out the wiring, but doesn't fix my light issue. I check voltage on the wheel speed sensor and it is producing plus/minus voltage (don't have a scope so can't tell if there are broken spikes in the wave). How can I tell if the ABS module is bad vs the speed sensor? Wouldn't the dealer have a code saying its one or the other (did I mention how much I hate dealers)?


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## GTijoejoe (Oct 6, 2001)

*Re: ABS Module vs. ABS Sensor (tincher_b)*

Usually the codes should tell you if you have a bad sensor (100% of the time), or if the module is faulty (I'm not actually 100% on this on vw's). 
From your explaination I can tell you have some experience/education, so I'll tell you there is no need for a O-scope, but good idea. Since you are dealing with an '03 (right?) the likely hood of you having a passive abs sensor is very very slim, meaning passive abs sensor's output is voltage in a large sine wave range (~50~150mV... not standard from ABS suppliers it varies) based on frequency of the wheel spinning from the pick-up of the sensor's encoder gear....meaning you need a moderate wheel speed before the sensor is going to work properly, this is the downside to passive sensors and why the industry does not use them anymore...low wheel speed = poor ABS/VSA/TCS capability
The other type (most likely what your have, MR or HALL) is an active sensor, where the output signal is actually current, square wave, high/low, 7mA/11mA... this is actually a standard value amoung ABS sensor suppliers. Meaning you can spin the wheel and measure the current output. Of course for a good output signal you need a proper voltage, and a proper airgap between the sensor's IC and the encoder gear/ring. 
Honestly 2.4V is very very low, leaves me sorta







, the best readings come from atleast 10-16v. Anything less than 10 will start to taper off the 'airgap' and get improper pick up signals...meaning your airgap will have to be decreased with such a low voltage to get the proper output signal.
Get all that? http://****************.com/smile/emthup.gif


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## tincher_b (Jul 31, 2009)

Let me clarify the 2.4 volts...that was the measurement of the voltage on the wire side of the system. I pulled the plug on the speed sensor and measured the voltage supply to the sensor. I tried to measure the voltage on the wheel speed sensor and it varied (wish I had that scope) but in the 5-8mV range. I just noticed you said the computer reads an amperage signal. Do i need to read the amperage, or can I just read voltage as both should change going back to the computer?
Both sides were typically the same. With that said, should there be 10-16V measured at the wheel speed sensor wire connection (where I measured 2.4V)?



_Modified by tincher_b at 6:25 PM 7-31-2009_


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## GTijoejoe (Oct 6, 2001)

*Re: (tincher_b)*


_Quote, originally posted by *tincher_b* »_Let me clarify the 2.4 volts...that was the measurement of the voltage on the wire side of the system. I pulled the plug on the speed sensor and measured the voltage supply to the sensor. I tried to measure the voltage on the wheel speed sensor and it varied (wish I had that scope) but in the 5-8mV range. I just noticed you said the computer reads an amperage signal. Do i need to read the amperage, or can I just read voltage as both should change going back to the computer?
Both sides were typically the same. With that said, should there be 10-16V measured at the wheel speed sensor wire connection (where I measured 2.4V)?
_Modified by tincher_b at 5:44 PM 7-31-2009_

I believe you correctly measured. 
If you pull the plug of the sensor, or tap into the back of the harness (this maybe the best solution to keep the circuit complete), and determine which wire is the supply voltage, you get a pretty consistant voltage (I mean your bat voltage does vary regularly by mV), but it shouldn't be as low in mV to supply the sensor








Basically, when we test ABS sensors, we supply them with a constant voltage, 6,12, and 16v. We start with an airgap of 0mm, spin the encoder on a lathe type machine, and pull the sensor head away from the encoder by 5-10micron increments and read the output current of the sensor until we get poor signal. This determines the maximum airgap at the various voltages. We use this information to guarantee the design of our system.
The modulator will be looking for the amperage signal to make a square wave function, and by time increments and a little programming mathametics it will determine what your wheel speed is... so in IMO you'll be looking to understand your output current to determine if the sensor and pickup are working properly.
Of course you will usually not get more than 14v on a car's electrical system, we test them above and below a normal voltage range to get an understanding of the sensor, of course we plot all this information on graphs.
Normal ABS sensor are a two wire system, one is a supply lead and the other is an output, three wire systems do exist, but usually are mission speed sensors, direct supply, direct ground, and direct output.
Try measuring another newer vehicle's system and see what you get. I honestly have never done the measurements on an actual vehicle, they are all on bench testing, but of course the system needs to work the same either way. Be sure you have a good chassis ground when you are measuring voltage. Subframe bolts are good grounds, don't use the other end of the sensor http://****************.com/smile/emthup.gif 
This type of measurement should basically go for EVERYTHING you try to measure, chassis ground is always the best.


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## tincher_b (Jul 31, 2009)

OK...I went down and gave it another go. I measured the supply voltage and it is 2.4 volts. I measured the resistance on both sides of the car for both the supply circuit and the return circuit to ground (ground at the same place). The wires are fine as the resistance is the same on both sides for the respective circuit. I measured the resistance in the ABS sensor and they are different from side to side. Actually the one I suspect is bad is reading all over the place. The good side is a solid 1.136Kohms. Now the reading that is all over the place...I don't know why it does that, but it is different. What is the cause of constant fluctuation in the that sensor?


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## GTijoejoe (Oct 6, 2001)

*Re: (tincher_b)*


_Quote, originally posted by *tincher_b* »_OK...I went down and gave it another go. I measured the supply voltage and it is 2.4 volts. I measured the resistance on both sides of the car for both the supply circuit and the return circuit to ground (ground at the same place). The wires are fine as the resistance is the same on both sides for the respective circuit. I measured the resistance in the ABS sensor and they are different from side to side. Actually the one I suspect is bad is reading all over the place. The good side is a solid 1.136Kohms. Now the reading that is all over the place...I don't know why it does that, but it is different. What is the cause of constant fluctuation in the that sensor?

I still think your voltage is strange, but perhaps just how your circuits are (car off right?)... but if they are the same I would expect them to be normal.
Now your resistances be different is usually an indication of a bad sensor.... why would it jump all over the place?
The only thing I could determine is if the actual internals of the circuit or IC is broken, causing bad connection and changing the resistance.
I think you found your sensor sir http://****************.com/smile/emthup.gif


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## tincher_b (Jul 31, 2009)

Thanks...and GO BUCS!


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