Up to Standards
- Subject: Electric shift motors in transfer cases
- Unit: NV136, NV246
- Essential Reading: Shop Owner, Center Manager, Diagnostician
- Author: Mike Weinberg, Rockland Standard Gear, Contributing Editor
The vehicles we are working on today contain a huge number of motors. Although there is one central power plant that provides power to propel the vehicle, there are increasing numbers of electric motors that provide power sources for other options. We have windshield-wiper motors, fan-blower motors, power-seat motors, convertible-top motors etc.
One of the most-complex and therefore troublesome motors is the electric shift motor for the transfer case. There are basically four types of transfer cases: mechanical-shift, all-wheel-drive (which has no driver input in most cases), electric-shift and electronic active. The major difference between electric-shift and electronic active transfer cases is simple.
With an electric-shift transfer case, the driver shifts to the desired mode and range with a cockpit-mounted switch or knob. This type of transfer case is really a mechanical-shift unit that instead of using a manual lever to engage uses an electric motor.
The electronic active transfer case, sometimes called automatic or intelligent, also uses a motor to shift, but that motor also applies an internal clutch in the transfer case to modulate torque split between the two axles in response to a computer-driven set of controls without further driver input. Once the driver puts the control knob or switch in “auto 4WD,” the computer will activate the internal clutch to adjust the torque split between the axles depending on driving conditions. These units are part of a series of vehicle controls that read road speed, wheel slip, braking level, yaw rate, steering angle, throttle opening and transmission gear range to provide the driver with the safest path.
If you understand and follow some basic rules regarding any type of transfer case, a lot of your diagnostic issues will go away.
Rule No. 1: “There is no transfer case of any type that can be driven on dry pavement locked in 4WD High or Low range.” If the driver attempts this, the vehicle will crow-hop and go into a bind in the transfer case that will release with a loud bang every so many revolutions of the wheels. This happens because there is a difference in wheel speed in turns, even slight ones, and the gear train in the transfer case binds.
Vehicles with electronic active transfer cases can be operated in the automatic mode of 4WD because in normal driving on dry pavement, the vehicle is in 2WD. When the computer senses wheel slippage, it activates the transfer case to send power to the axle that needs it, and when the wheel speeds equalize it returns to 2WD. 4WD high and low ranges are to be used only on dirt, sand, grass, loose gravel, mud etc., because this type of surface permits some wheel slippage and does cause the transfer case to bind.
Rule No. 2: “Thou shalt not have tires of unequal sizes or pressures.” You must measure tire sizes with a tape or stagger gauge, and all four tires’ circumferences must be within 1/4 inch of each other. The sidewall label means NOTHING; only an actual measurement will prevent you from wasting time and money on diagnostics. Mismatched tires create an appearance of wheel slip to the computers involved, causing false codes and damage to transfer-case components such as clutch packs and viscous couplings and to side gears in the differentials.
With active or automatic transfer cases, the driver electronically shifts through the modes and ranges by means of an encoder/motor that shifts the transfer case and applies the internal clutch pack to split power between the axles. These units have an automatic mode for 4WD in which the transfer-case control module (TCCM) automatically sends power to the axle that needs it with no driver input. The computer inputs come from prop-shaft speed, road speed, ABS, throttle position, steering angle, traction control, stability control and transmission range selection.
Generally, in the automatic mode 95% of torque is delivered to the rear axle. When the TCCM detects wheel slip it will send a pulse-width-modulated (PWM) signal to the encoder/motor to apply as much torque as needed to the front axle. When the shaft speeds equalize, the TTCM returns the transfer case to 95% torque bias to the rear. The encoder/motor is capable of applying several hundred pounds of torque to the clutch pack to achieve this, and it takes place in milliseconds (thousands of a second). If the driver wishes to go off road and selects 4WD High or Low range, the TCCM will signal the encoder/motor to lock the clutch pack full on, which creates a 50/50 torque split until the driver shifts back to automatic or 2WD.
Understanding the encoder/motor begins with its function. On electronically shifted transfer cases, the encoder/motor will inform the TCCM of the transfer case’s present gear position. It will respond to driver commands to shift the unit within design parameters, and it will hold the transfer case in a selected mode or range through an internal electric brake until commanded to do otherwise. On active or automatic transfer cases it will provide the same functions plus actuation of the internal clutch pack as commanded by the TCCM.
These are complex multi-functional parts and are expensive. The motor itself is connected to multiple sets of internal planetary gears that can provide sufficient torque to make the shifts and apply the clutch pack. The encoder part of the motor is an internal position sensor that varies voltage to the computer to inform it of the transfer-case gear position. A word to the wise: If you wish to test one of these be aware that the motor can handle 12 volts up to 20 amps, and the encoder never sees more than 5 volts at very low amperage. If you don’t know your circuits, you will buy a new one. To test the encoder motor safely, make sure it is on the transfer case, or it can over-travel and you buy a new one.
The encoder section of the motor is a four-channel switch (Figure 1). It consists of an inner ground ring that contacts a three-legged wiper arm. The legs are spaced 120° apart and make contact with the four conductive areas of the channels: A, B, C and P. Any contact by the wiper arm with voltage at one of the channels allows the circuit to complete to the inner ground ring and informs the TCCM of the position of the motor.
Figure 2 shows the positions and the voltage readings you should see as the motor sweeps. Always start by back-probing the TCCM connector pins for voltage, and then back-probe the motor pins. This will identify an open circuit or a problem with the encoder/motor.
The encoder section of the motor on the active transfer cases is more complex, because it has to apply the clutch pack at the correct duty cycle and shift and lock the motor to the correct gear. Figure 3 shows the encoder-motor operation for an NV136 transfer case and also applies to the NV246, except that the 246 has two internal relays because it is a two-speed unit; the 136 is a one-speed unit. The motor side of the encoder consists of the same four-channel, 5-volt system, but we will now review the clutch-apply function.
To apply the clutch pack at the desired duty cycle, the TCCM energizes a relay on its circuit board, sending 12 volts to the motor. To regulate clutch apply the TCCM pulse-width-modulates the ground side of the motor, controlling the amount of current the motor will see and controlling the clamp load the clutch fork applies to the clutch pack. This in turn regulates the amount of power and torque split to the prop shafts. The duty-cycle amperage will vary from zero amps = zero clutch apply to 20 amps = 100% clutch apply.
The TCCM determines slip by reading prop-shaft speeds through a set of vehicle-speed sensors attached to the transfer case. It continuously compares the prop-shaft speeds and the vehicle road speed and uses a programmed set of ratio comparisons to decide how much duty cycle to apply to the clutch pack. At 0-10 mph a slip must exceed 25 rpm before the TCCM takes action. The greater the road speed, the more slip there must be for the clutch pack to be applied. These units are capable of applying 100% duty cycle within 72 milliseconds and can revert to zero duty cycle within 48 milliseconds.
Now to the controversial part of the story: Numerous aftermarket encoder/motors are available through industry suppliers. A huge number of tech calls that we handle for our customers are generated by failures of the aftermarket encoder motors. We have seen the failures to be in two areas.
The driveshaft of the motor that actually turns the sector shaft on the transfer case comes from the OE manufacturer with a bearing. We have seen many aftermarket motors that use a bushing instead. The bushing is not up to the torque load placed on it and distorts because of torque deflection. This produces a flat spot in the bushing and binds the motor.
The second issue of repeat failures is in the encoder circuitry. We have seen multiple failures and a lot of wasted time because of shorted or open internal encoder circuits in the aftermarket motors. The aftermarket, of which we are all part, is the home of innovation and the oldest form of recycling. I don’t believe for a minute that anyone is trying to produce an inferior product, but whether by cost cutting or improper design, defects occur. The dollars you save are not worth the downside risk in going from OEM to aftermarket at present. I am sure that this will change, but you will rarely get a bad product from the OEM and we see a much greater failure rate on the aftermarket side.