I'm trying to measure/test the square wave output of an active wheel speed sensor from a jeep wrangler for a project I'm working on. I do not have access to the actual vehicle just the sensor itself and so I was using a test rig with the tone ring attached to a lathe and bringing the sensor as close as possible to the teeth of the tone ring. At the moment I am having no luck in getting any type of signal from the sensor itself. I wasn't sure initially whether the sensor was active or passive so I tried first without applying power to the sensor and had the oscilloscope hooked up over the two pins of the sensor. No signal appeared so I assumed it must be an active sensor. I then did the same setup but this time connecting the positive pin of the sensor (I measured this on the vehicle before hand and saw it was at ~2V) to my power supply set at 2V and the other pin of the sensor to ground and then measuring over that. Again, no signal. I then read that sometimes these sensors modulate current instead of voltage so I attempted to add a shunt resistor and measure over that to no avail. I assume I am setting up my oscilloscope or power supply incorrectly since the secondary pin on the sensor is usually refereed to as the "signal pin" and not ground pin. I am almost certain the sensor I have isn't damaged as I have two of them and can't get a signal off of either.

My next train of thought was to incorporate the sensor into a wheatstone bridge and try to measure the signal that way. I haven't yet tried this but I figured I would post here first to see if anyone has a better idea of how to use these things or provide insight on how a vehicles ABS ECU does the measurement. I.e. Has anyone measured wheel speed with these outside of a vehicle? If so how did you do it?

  • \$\begingroup\$ do you have a question? \$\endgroup\$
    – jsotola
    May 7, 2020 at 23:50
  • \$\begingroup\$ Most likely an active Hall sensor \$\endgroup\$ May 8, 2020 at 0:34
  • \$\begingroup\$ Sorry from what I wrote the question may have been unclear. I’m wondering if anyone has used a wheel speed sensor in a lab setup and how they have gone about measuring the expected square wave. \$\endgroup\$
    – Tanner O
    May 8, 2020 at 0:55
  • \$\begingroup\$ If it is an older vehicle (>15 years) it is probably a variable reluctance sensor, or a 2-wire hall sensor (though less likely). If it is newer then it may be a PSI5 sensor, in which case you have to interrogate it using a digital protocol. \$\endgroup\$
    – Jon
    May 8, 2020 at 8:51
  • \$\begingroup\$ Also see this question electronics.stackexchange.com/questions/160113/… \$\endgroup\$
    – Jon
    May 8, 2020 at 8:56

2 Answers 2


A 2 wire automotive speed sender can only really be 2 types

Type 1 is a inductive sensor. It has a magnetic feild and metal moving through it generates a tiny voltage dependant on the speed. Usually signal levels are not higher than 1V AC when measuted with a multimeter. And should be buffered by an op amp if you wish to use it in your circuit.

Type 2 is a hall effect sensor. They do not commonly use this type in 2 wire. But they exist. They have a hall effect sensor powered by an external pullup on the signal pin. When metal is detected it pulls the signal pin low. All the way to 0V for a short period with some internal capacitance. Or more commonly to about 2V

How to tell the 2 apart? The inductive type will have a fixed resistance if there is no power to it. The hall effect type normally would use a 10K pullup to 12V and should not be damaged by a reversed voltage provided that pullup resistor is used to limit current.


I figured out why I wasn't reading a symbol. For anyone who might be wondering in the future: I was under the impression the sensors were meant to be oriented with the flat tip facing towards the tone ring. Instead I had to flip the sensor horizontally such that the long side was facing the ring in order for the ring to pass through the field lines properly. Also it's important to note that a series resistor needs to be used between the signal pin and ground in order to read the square wave since the sensor modulates current. When trying to measure this waveform you must then measure over this resistor.


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