The Speed sensor for the TCU is important to determine the correct shift speed. This will keep the engine in its optimum gear at all times. It will also prevent erratic shifting and over-revving of the engine.

The Speed sensor is normally situated at the tail piece of the transmission. In 4×4 vehicles it is at the back of the transfer case. If there is no sensor available for the TCU, it may be tapped into an existing sensor and some cases a sensor can be installed to sense the prop-shaft bolts. The speed signal can be any pulse that is equivalent to prop-shaft rotation. The pulses can be adjusted up to 60 pulses per engine revolution. See below the different connection diagrams.

Speed Sensor Wiring

There are three types of sensors. The Inductive sensor, Optic sensor and Hall type sensor. The inductive sensor is a coil wound over a magnet. The teeth of a gear will disturb the magnetic field and that will generate a voltage spike over the 2 wires. Hall and optic sensors both give a square wave output and are treated exactly the same. They have electronic components in the sensor which convert the signals to square wave. The Hall sensor uses magnetic field where optic uses infrared light. In both cases a beam is broken and detected.

Operation of an inductive sensor:
The inductive sensor generates a voltage between the coil wires when the magnetic field strength is changed by a tooth passing the sensors. The sensor must be wired for a Falling waveform, this is best determined using an oscilloscope. The output voltage amplitude increases with increased RPM. The output voltage amplitude also depends on the gap between the sensor and the Tooth. The Minimum acceptable output voltage under cranking conditions is 3v.

Testing an Inductive Sensor:
Inductive sensors has a wired coil with a magnet. The sensor either has two or three wires from which the third wire is not connected to the coil. The sensor coil resistance may vary from 150ohm to 1200ohm. Multi meter polarity will not change the coil resistance reading. A multi meter switched to AC voltage can be used to test the sensor polarity without an oscilloscope. Connect the wires from the coil to the multi meter and move a metal object to the sensor. Look closely at the polarity indicator, if it indicates positive when moving closer and negative when moving away from the sensor then the red wire of your meter is on the positive of the sensor and the black wire of your meter is on the ground of the sensor.

Operation of a Hall Effect Sensor:
Hall sensors use a magnetic field effect to switch between a low voltage (usually 0 V) and a high voltage (5 V or 12 V) to form a ‘square wave’. Both the rising and falling edges are valid reference points for the ECU input.

Testing a Hall Effect Sensor:

Hall Effect sensors normally have three wires. A supply wire (5v + or 12v +), an earth wire and a signal out. This sensor is equipped with electronic components that require power to operate. Incorrect connections will cause damage to the sensor. First test for resistance on all the pins and swop the test leads around to make sure it is not an inductive sensor. Put the multi meter on diode test and measure voltage drop over the pins. You should get a voltage drop between 0.5v and 1.9v. This is an indication that it is a Hall Effect sensor.

Now take two 1k ohm resistors and tie one end of each to 12v +. This will ensure that if you connect the supply wrong the sensor will not be damaged. Connect ground to the one sensor pin and the two resistors onto the remaining two pins. Connect the multi meter black wire for the remainder of this test. Connect the multi meter red wire onto any of the other pins. Now move a metal object to the sensor or in the gap and away. If the signal varies between 0v and 12v then this pin might be the sensor signal output. Now put the red wire on the other pin and repeat the metal process. The voltage should not change, but might be less than 12v due to the drop over the resistor. If so then this is the supply pin.

Now change the earth pin to the next and repeat the process. Note that you may get a similar reaction if you have the earth and signal pins swopped, the pin that reacts most to the iron pulse is the signal and the other the earth.

Remember this is a guideline to black box testing and not a failsafe operation.