Sensors

All of these sensors use a magnet but in different ways. The two-wire sensor just has a coil of wire wrapped around a magnet.

First a little theory: To generate a voltage with a magnet, (or electromagnet), you need three things, the magnet, a wire, (a coil of wire is better), and most importantly, movement between the two of them.

In the two-wire sensor, some rotating wheel has notches cut into it that pass under the end of the magnet. As the notch moves, it disturbs the magnetic field in the coil of wire. That is the movement that generates a pulse of voltage. The more loops of wire in the coil, the more total voltage is developed. The exact voltage isn't usually critical. Just the fact that a pulse is generated is sufficient to trigger something.

In the three-wire sensor, a special Hall Effect transistor is used. Do you understand how a relay works? A tiny current flowing through the coil causes the contact to trip which turns on a large current. Regular transistors work the same way. A real tiny current flowing from terminal one to terminal two allows a real high current to flow from terminal one to terminal three. In a Hall Effect transistor, that tiny current is not used. Instead, the high-current path is turned on by the presence of a magnetic field. That magnetic field could be from a magnet that is embedded in the part that's rotating under the sensor or it could again be a notch that disturbs the magnetic field in the sensor already.

Typically the Hall Effect sensor is fed 5.0 volts to the transistor and the other circuitry in the sensor. 5.0 volts is also fed to the signal wire. When the magnetic field is disturbed and the transistor turns on, it shorts that 5.0 volts on the signal wire to ground. That means the signal is not some random pulse as in the magnetic sensor; it is a nice sharp square wave that is either 5.0 volts or 0.0 volts. Digital computer circuits require these voltages with nothing in between. If the sensor sends out a 2.0 or 4.0 volt signal, there is no way to tell what the computer will interpret that as so it can get confused and not respond properly.

The three-wire sensor is used when the computer requires a nice steady 5.0 or 0.0 volt signal and when the signal produced is so small it needs to be amplified to be usable. Also, the signals from two-wire sensors are much more prone to interference from radiated signals. Current flowing through spark plug wires radiates out to all the other wires nearby. If one of those is a sensor wire, it can introduce those interfering signals. That's why it's important to always put wiring harnesses back where they were.

Magnetic sensors will only have two wires. The way to remember is there are only two ends to the coil of wire. There is no shielding wire, but you might find some with the wires twisted together. They call that a "twisted pair". That is sometimes done so any magnetic interference that radiates in will radiate into both wires equally and the voltages will cancel each other out.

I think you're getting hung up on the "inductive" terminology. Disturbing the magnetic field in a two-wire sensor "induces" a voltage difference between the two wires. In the three-wire sensor that same induced voltage is used to turn on the transistor and circuitry. Another way to look at it is the two-wire sensor develops a voltage pulse that triggers a circuit inside the Engine Computer. In the three-wire sensor, that same pulse triggers the same circuitry but it lives inside the sensor, not in the computer.

Two-wire sensors typically only fail when they become an open circuit, often from the flexing of the wires that are soldered to the coil. That used to happen to pickup coils in distributors a lot because the plate they were mounted on would rotate back and forth in response to engine vacuum to adjust ignition timing. In very rare cases the magnet can crack into two pieces. That caused multiple signals and multiple sparks with some backfiring. These sensors can become heat-sensitive but that is not very common. The plastic core that the coil is wound around expands and tugs on the ends of the coil. Eventually the wire breaks off the terminal it's soldered to. It might make just enough connection when it cools down to work again but it will soon fail completely. These are real easy to test with an ohmmeter. Every anti-lock brake wheel speed sensor I've ever seen is of this two-wire design.

Heat is always the deadly enemy of transistors so it's understandable that three-wire sensors with their circuitry will often fail when they get hot. It is real common for them to work again after they cool down for an hour or longer. These require a voltage test. There is no way to get inside to measure anything else.

I should mention or clarify if I haven't already that magnetic coil sensors induce a voltage in a coil of wire. Hall effect sensors ground out a voltage by switching a transistor on and off that turns on in the presence of a magnetic field. There is no coil of wire. Another type of three-wire sensor has the circuitry AND the coil of wire but it doesn't use the Hall effect transistor. I think that's what you mean by "inductive" sensor. Most three-wire sensors are of this design. The clip-on pickup for an inductive timing light uses that type of sensor with a coil of wire. In that case, the movement that is required to induce a voltage is the building and collapsing magnetic field around the spark plug wire.

As a point of interest, two-wire magnetic sensors and three-wire inductive sensors require the magnetic field to be moving to generate a signal. If you play with either one with a toy magnet, there will be no voltage pulse induced in the magnetic coil when the magnet is next to the coil or away from the coil. Voltage will only be induced when the magnet is moving closer to it or away from it. With the coil wrapped around the magnet, the movement occurs from the magnetic lines of force bending when the notch in the rotating member passes by that magnet. Different design, same result.

The Hall effect switch is the different one. The transistor is turned in by the presence of the magnetic field, not the movement of or in relation to the magnetic field. The most well-known Hall effect sensor was used in the Chrysler four cylinder distributors through the mid '90s. Unlike the other sensors that required movement to generate a signal, this one will turn on the transistor to ground out the 5.0 volts that is applied on the signal wire to the sensor, or it will turn off the transistor to create an open circuit, based solely on position, not movement. That means if you turned the distributor very slowly, you would see the signal voltage switch between 0.0 and 5.0 volts and stay in either state depending on here you stopped.

Another way to think of the Hall effect sensor is to imagine it uses a light bulb, optical sensor, and a shutter blade on the distributor shaft. That system actually IS used on a lot of Japanese imports. Depending on where the shutter blades are, light can shine onto the solar cell or it is blocked. The on / off signal is dependent on position, not movement.

See if that helps. These are a lot easier to explain when you have visual aids and hand gestures. Fortunately we don't have to know what kind of sensor is being used when we want to replace it because all we have to do is order the replacement part. It IS important to know how they act when you get into testing them. Many years ago it was easy to just throw a new part in based on those symptoms that we saw over and over. Today mechanics are a lot better trained so they can test and identify defective parts before ordering things to randomly try. Holler back with more questions.