In the early days, the driver controlled transfer case mode and range positions with a shift lever. Newer designs use a transfer case shift motor to change mode and range positions. The shift motor is bidirectional which can rotate clockwise and counterclockwise. Typically, a transfer case control module (TCCM) controls the shift motor. The transfer case control module has two drivers that can switch polarity to force the motor forward or reverse. One piece of information required by the TCCM is the shift motor position. Without this critical input, the TCCM cannot operate the transfer case shift motor.
There are many types of position sensors used in the automotive industry, such as crank position sensors, TPS sensors, and EGR pintle position sensors. Shift motor position sensors are slightly different from the ones we just pointed out. Borg Warner transfer cases typically use a series of on/off switches within the shift motor to determine motor position. The transfer case control module would send a reference voltage to each switch separately, and the switch would simply pull the reference voltage to ground depending on the motor position. These switches had a specific state associated with a specific position of the shift motor. To ensure the position switches were reporting correctly scan tool data could be compared to an OE chart found in service information. Figure 1 is an example of a service information chart.
Magna transfer cases use two position sensors—one position for the motor and another position sensor for the actuator shaft. The motor position sensor is referred to as the incremental encoder sensor in GM service information. The incremental encoder sensor is located inside the motor and reports the motor position. The actuator shaft position sensor is what GM calls the other position sensor in GM service information. It is located near the motor on the case and reports the position of the actuator shaft. The TCCM must learn the two sensor positions. In addition, the two sensors must stay within a seven-degree window of each other. Both sensors can be monitored and compared using scan data. Scan data can be confusing compared to GM service information. The sensors have different names in scan data than in the service information.
The incremental encoder sensor in scan data is referred to as the range actuator position sensor. The actuator shaft position sensor in scan data is referred to as the range sensor. The incremental encoder sensor is located within the transfer case shift motor. The shift motor may be referred to as the actuator in the service information. The incremental sensor is a variable position sensor. This sensor is used to report which range the motor is moving toward. The incremental sensor is a four-wire sensor. The TCCM supplies the incremental sensor with eight volts for a reference on GM and five volts on Chrysler vehicles. The sensor determines the degrees of movement made by the shift motor and can measure movements as little as .15 degrees. 
The sensor output voltage in scan data is pulled low as the motor rotates or is allowed to go high during rotation. The sensor may pull down to .75 volts on the low end as it may go as high as 4.2 volts on the high end. The voltage readings can be confusing because the shift encoder motor position does not equal a specific voltage. Figure 2 is an example of a chart from GM service information.
Each selected position (2WD, Auto, 4 High, and 4 Low) requires the motor to be at a specific degree, as shown in the second column of the chart above. The incremental sensor voltage or reference voltage is 7.5 volts. The impulse voltage is the actual signal wire. The signal wire will report either .75 or 4.2 volts, regardless of the motor position in scan data. It could be confusing to look at impulse voltage for motor position. Instead, turn your focus to incremental sensor degrees. Figure 3 is an example of scan data. You can compare command position in the chart to the actual scan data.
Scan data in the 2WD position reports 36.4 degrees, and the chart specifies 37 degrees (which is within tolerance). If we look at the incremental encoder sensor (the range actuator position sensor), the voltage would be 3.87. As we stated earlier, the voltage does not give us an accurate measurement. The voltage in scan data at 3.87 volts does not match GM’s service information chart at 4.2 volts. This is normal and nothing to be concerned with. If you were to measure the voltage with a meter, it would actually be 4.2 volts.
We also mentioned the two position sensors must stay within a seven-degree window. Scan data shows the two sensors are within .8 degrees, which is within the tolerance.
The incremental encoder sensor also includes a directional signal to indicate if the motor is turning clockwise or counterclockwise. The directional signal in scan data can also be confusing because it either reads .7 volts or 3.84 volts when the motor is stationary. The directional signal fluctuates between .7 and 3.84 volts when the motor moves, as seen in the scan data. The directional signal is in the green trace (Figure 4).
Another way to view the sensor is to connect a four-channel lab scope to the four wires at the incremental encoder sensor on the motor (Figure 5). The lab scope waveform represents a shift from 2WD to 4WDA position.
The scope has been connected to all four wires of the incremental sensor. Channel A, the blue trace, is the directional signal; the voltage is approximately .7 when the motor is stationary. The directional signal rises to a steady four volts with a couple of quick drops while the motor is rotating toward the 4WDA position from the 2WD position. Channel B, the red trace, is the eight-volt reference signal and stays steady at eight volts. Channel C, the green trace, is the impulse signal. The impulse signal is .7 volts when the motor is stationary and creates a square wave that travels from .7 to 4.2 volts when the motor is rotating from 2WD to 4WDA. The square waves report the motor’s movement to the TCCM and are represented as degrees on the scan tool.
Channel D, the yellow trace, is the sensor ground. The ground signal stays at ground during the entire process.
Now that we have explained how the incremental encoder sensor works, let’s turn our attention to the actuator shaft position sensor, referred to as the range position sensor in scan data. This three-wire sensor operates totally differently than the incremental encoder position sensor. One wire provides the five-volt reference, one wire is ground, and the third wire is the rotary position signal. This sensor can also be confusing in scan data. It shows the position of the actuator shaft as a percentage (Figure 6).
You might think that percentage starts at 0 percent in 2WD and ends at 100 percent in 4WDL, but that is not how it works. The percentage is an actual duty cycle of the signal. We connected our lab scope to all three wires to better understand the signal.
The signal wire always creates a five-volt square wave, regardless of whether a shift occurs or not (Figure 7). The frequency of the waveform is always 1000 hertz (1 kHz). How does the TCCM determine where the actuator shaft is using this signal? The signal frequency does not change, but the waveform’s positive duty cycle does. The Pico scope offers a unique feature, which is a math channel. A math channel can analyze the signal’s duty cycle and display a duty cycle graph (Figure 8).
The math channel is the purple trace and displays the positive duty cycle of the red trace. The duty cycle on the math channel can be compared to the scan tool’s duty cycle to ensure the two agree. Service information indicates that the duty cycle in 2WD is 52%, the duty cycle in 4WDA is 76-80%, and the duty cycle in the low range is 18%. The lab scope duty cycle graph shows the duty cycle changed from 52 % to 76% when the shift was made from 2WD to 4WDA. The lab scope graph, scan data, and service information agree. Hopefully, this sheds some light on some of the newer design position sensors.
About the Author: Jerry Stewart is the curriculum administrator at AVI and an instructor at Highlands College in Butte, Montana.
