Recently I was working on an 04 Mazda 6 with an ABS code C1156, no signal from the right rear wheel. I checked the resistance in the right rear speed sensor, found it to be an open circuit so I replaced it and solved the issue. My question is to the way the system functions. In my investigation I found the sensor recieves a 5 volt supply from the ABS module on one of the wires, when you plug the sensor in this drops to 1 volt, grounding through the ABS module I assume, but the strange thing is when you spin the wheel you see what looks like an ac wave form on the scope, I was expecting to see a square wave. On other ABS systems I've recently checked that have an input voltage (06 F250) A square wave was produced. You would think the reluctor ring would be interupting the supplied voltage dropping it to zero and not converting it to an ac voltage. If any one has any insights to how this works I'd like to here them . THANKS.
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The ABS wheel sensor is an AC pulse generator, the PCM only supplies a 5 volt reference for circuit integrity check, the AC signal rides on top of the DC voltage signal the PCM sends out, the 5volts gets pulled down when the circuit is completed. if you have a open pcm will read 5 volts if its shorted to ground then read 0 volts. every manufacture is different so its very important to understand the system description and operation of each. The S/O Vantage, VERUS or MODIS component testing gives good information. Hope this clears things up for you..
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I would like to express an opinion please, but I don't wish to contradict any other views or opinions on this thread, but if anyone feels I get this wrong, please do advise me, thanks
ABS sensors are of two different types, one type is inductive, and the other is Hall Effect.
Inductive.
This is a two wire connection to the ABS sensor which is of the permanent magnet type, to test it, you can use an Ohmeter to check its continuity against recommended data, but a much better check is the scope test. These sensors produce an AC signal voltage which increases in proportion to the rotational speed of the wheel. To my mind there is no set AC voltage to look for, but a consistent smooth increase in voltage, which is comparable to other sensors signals from the other wheels is what is required.
Hall Effect.
These ABS sensors are a Three wire system, they have a reference voltage provided by the PCM. These sensors should not be tested for resistance as they can be damaged, again a voltmeter or scope should be used.
Hall effect sensors are digital by design and so one would expect a digital signal to be produced, i.e. square wave, but, depending upon the switching circuit in the PCM, the signal produced may appear on the scope differently. I think the main key to understanding here is in the comparison of each wheel sensor signal.
UK David
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Here is a screen shot of an ABS signal from an 07 F350. This is a two wire sensor that recieves a 12 volt supply from the EBCM and then returns a square wave signal. This is why I asked the first question about the sine wave. It seems that you don't need a three wire sensor to get a square wave. On this vehicle when the wheel is not moving a 1 volt flat signal is on the return wire.Comment
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Hall Effect Sensors
Actually, "Hall effect sensors" are not digital by them selves, but often are coupled with "switching" electronics to make them so. This is the case with many Crank and Cam Position Sensors which include a magnet, a "Hall effect" sensor and some transistors. This combination can create a very reliable "square wave" pulse. But to be clear, the "Hall effect" is an "Analog function" where a DC current thru a type of crystal is "bent" to one node or another because of a magnetic field. We see this in the Sun (Snap-On) VAT-XX series testers and others where DC current in a battery wire or starter wire produces a stable magnetic field which is in-turn measured by Hall effect analog sensor in a "Green" Amp Clamp.
Your point is accurate from a higher level, that most "Hall effect sensors" out put a square wave. I just thought clarification might be important for those who also know of the battery tester functions.Comment
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"Here is a screen shot of an ABS signal from an 07 F350. This is a two wire sensor that recieves a 12 volt supply from the EBCM and then returns a square wave signal. This is why I asked the first question about the sine wave. It seems that you don't need a three wire sensor to get a square wave. On this vehicle when the wheel is not moving a 1 volt flat signal is on the return wire."
bob, any chance the 07 f350 is a magneto resistive type abs wheel speed sensor? i couldn't find any clarification with mitchel on demand.Comment
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