Up to Standards
- Subject: Electronic operation & diagnostic procedures
- Unit: MP 3023/3024 transfer case
- Vehicle Application: Late-model GM trucks
- Essential Reading: Rebuilder, Diagnostician
- Author: Mike Weinberg, Rockland Standard Gear, Contributing Editor
Last month we discussed the design features and mechanical operation of the Magna Powertrain MP3023/3024 (RPO Code NQH), and this article reviews the electronic operation of these transfer cases and the diagnostic routines to troubleshoot problems. These are “active,” computer-controlled transfer cases, which are capable of biasing torque to the front wheels without driver input.
These transfer cases are equipped with five modes: Auto 4WD, 4HI, 4LOW, 2HI and Neutral, which the driver engages through a switch on the dash. Auto 4WD is an “active mode” that may be used on dry pavement; in that mode, the transfer-case control module reads the speeds of the rear wheel-speed sensors and the prop-shaft speed sensor. If the TCM sees a difference in rear-wheel speeds, it interprets it as a slip and will apply an internal wet-clutch pack to send torque to the front wheels until the wheel speeds are matched. The clutch is applied by a permanent-magnet motor that also shifts the unit through the mode selected by the driver.
4WD HI and 4LOW lock these units into a 50/50 torque split between the front and rear axles and should not be used on hard pavement. 2WD HI may be used on hard pavement at any time. There is an electronic front-axle disconnect that is activated either by the driver’s selection of any of the 4WD modes or by the TCM in the Auto 4WD mode when the computer sees a slip and needs to send power to the front axle. This system is calibrated to send 0-5 lb.-ft. of torque to the front axle in the auto mode until the vehicle’s road speed reaches 25 mph and then produce higher torque levels as road speed increases. This is done to prevent the crow hop and driveline windup produced at low speeds because of the difference in wheel rotation in turns.
There are many active-transfer-case designs in the market, some using viscous couplings, hydraulic-pump couplers or computer-activated electronics as we see with these units. The technology is not new, but the MP3023 and 3024 use an advanced design with adaptive learning capability and broader software in the TCM, which requires the VIN to be used in setting up the TCM when certain electronic components are replaced.
As with all electronic active transfer cases, any diagnostic routine should begin with the basics. You need to check battery voltage, all power and ground terminals, and fuses before looking for codes. Another must-do procedure before going further is to measure the tire sizes and pressures at all four wheels. Failure to do so causes more wasted time than any other function. MEASURE the tires and pressures using a stagger gauge or a tape measure around the center of the tire tread of each wheel. They must all be within 1/4 inch of each other. A worn or low-pressure tire will cause the computer to engage the internal clutch, as the difference in the rolling radius appears to be a slip. Disregard the sidewall labels, as they are not accurate to within 1/4 inch. Do this routine every time before you go any further and save your bottom line.
These units are equipped with a permanent-magnet electric motor that shifts the transfer case between the five operational modes and applies the internal clutch pack in the Auto 4WD mode. There is no detent to hold the actuator shaft in position when a mode is selected, but an internal motor brake on the actuator motor holds the selected mode. If this brake fails the motor will be unable to hold the selected gear and needs replacement. There is also a transfer-case position sensor that informs the TCM which position the actuator shaft is in at all times.
These units are capable of setting 19 diagnostic transfer-case faults, as you can see in Figure 1. Reading the codes should make it clear that there are P codes, C codes and B codes that need to be diagnosed because of the numerous systems involved, such as ABS, stability control, transmission controls and the front-axle connect. You will need a service manual to evaluate the circuits and codes involved.
- B0770 01 AWD indicator circuit short to battery
- B0770 06 AWD indicator circuit short to ground or open
- B0775 01 4 High indicator circuit short to battery
- B0775 06 4 High indicator circuit short to ground or open
- B0780 01 4 Low indicator circuit short to battery
- B0780 06 4 Low indicator circuit short to ground or open
- B0785 01 2 High indicator circuit short to battery
- B0785 06 2 High indicator circuit short to ground or open
- B0790 01 Neutral indicator circuit short to battery
- B0790 06 Neutral indicator circuit short to ground or open
- B2725 08 Active transfer case mode switch circuit
- C0306 02 Motor A or B circuit short to ground
- C0306 05 Motor A or B circuit short to battery or open
- C0321 01 Transfer case lock circuit short to battery
- C0321 02 Transfer case lock circuit short to ground
- C0321 04 Transfer case lock circuit open
- C0321 08 Transfer case lock circuit signal invalid
- C0374 54 General system performance temperature high
- C0379 01 Front axle system short to battery
- C0379 02 Front axle system short to ground
- C0379 04 Front axle system open circuit
- C0379 08 Front axle system signal invalid
- C0387 59 Unable to complete shift circuit/component protection timeout
- C0392 01 Rotational position sensor circuit short to battery
- C0392 02 Rotational position sensor circuit short to ground
- C0392 08 Rotational position sensor circuit signal invalid
Figures 2-4 provide some basic schematics, but you will need the diagnostic trees found in the service manual. Mitchell OnDemand has excellent manuals and schematics available online for a reasonable price.
The electronic controls on the motor are bidirectional, and most of the components can be operated through a correct scan tool. Figures 5-10 should help you get a feel for the control system, but anything this complex will require you to invest in obtaining the proper service manuals, as there are literally hundreds of pages and space is limited in the magazine.
The real need we face today to properly assess and repair these complex units is a thorough understanding of the theory of operations. They cannot be fixed with the old shotgun approach of just replacing parts. You need to make a fair profit from your work, and you can do this only by following a precise diagnostic routine that does not allow wasted time to eat into your profits.
Mike Weinberg, Rockland Standard Gear, Contributing Editor.
