Suburban’s ‘Jerking’ Baffles TechnicianPosted 5/1/2008
By Ralph Dahlen
The technician was tired of seeing a particular Chevrolet Suburban that would show up at the write-up area every few months. This morning was not looking good as the white Suburban pulled into the shop with a less-than-happy face on the driver. “Back to jerking again” were the first words from the customer’s mouth. Without further explanation, the tech knew instantly what the customer was referring to.
The “jerking” occurred while turning at low speeds, such as parking lot maneuvers, and with the transfer case in AUTO mode. This vehicle was equipped with the popular New Venture Gear (NVG) 246 transfer case. This transfer case has been an optional or standard equipment transfer case on GM trucks since 1998, dependent upon model.
What makes the AUTO feature unique on the NVG 246 and other GM AUTO type transfer cases is the use of a clutch pack. The clutch pack is a stack of steel plates and fiber discs such as those found in an automatic transmission clutch pack, the difference being that the transfer case clutch pack is applied with a motor-driven lever rather than hydraulic apply. The clutch pack is modulated by the transfer case motor, based on commands from the transfer case control module (TCCM).
The TCCM monitors two speed sensors, one at each output shaft, front and rear. When in the AUTO mode, the TCCM can fully apply the clutch pack within 200 milliseconds to drive the front prop shaft in accordance to rear prop shaft slip. When the AUTO mode is selected, the TCCM commands the front axle to engage and the axle remains engaged while AUTO is selected. In the AUTO mode if no rear drive slip is detected, the TCCM does not apply the clutch pack and the transfer case operates as it would in 2WD, with only the rear output shaft receiving torque.
In the AUTO mode, if the rear output shaft speed increases at a higher rate than that of the front output shaft, the TCCM takes action by commanding the transfer case motor to apply the clutch pack. The TCCM modulates the clutch pack via the transfer case motor, until the rear shaft and front shaft speeds are equal, at which point the motor reverses and releases the clutch pack. The duration and force of application of the clutch pack varies with demand. The motor and lever have the capability to apply the clutch pack with a 2,600 psi force.
Problems with the system show up most often when the vehicle is driven in the AUTO mode, and all four wheels are not of equal rolling circumference. It is of utmost importance that all four tires remain equal with any 4WD vehicle, but is most critical with vehicles driven on dry pavement while in the AUTO mode. If one or more tires has unequal pressure or wear or has a defect in sidewall construction, then its rolling circumference will not match the others. This inconsistency will then cause the TCCM to continually cycle the clutch pack on/off in an attempt to compensate for the perceived slip. This repeated cycling overheats the clutch pack and transfer case fluid, resulting in clutch pack and/or transfer case failure.
To verify that the tires have equal rolling circumference: 1) Check and adjust air pressure of all four tires to specification. 2) Check and verify that all four tires are of equal wear. 3) Mark all four tires at 6 o’clock with a chalk mark. 4) Drive the vehicle straight ahead for 10 revolutions. 5) With one mark back at 6 o’clock, verify that the others are within one inch of 6 o’clock also.
After many trials and tribulations, the problem with the white Suburban was found using this test procedure ... three transfer cases later! The jerking that the customer complained of was the result of a failed clutch pack.
The new tires on the Suburban were the culprit although an assumption had been made that, since the tires were new, they couldn’t be the problem. One tire’s rolling circumference was 2.5 inches out and another tire was just over an inch out. A second set of tires from another manufacturer was the answer. A recheck showed that all four tires had equal rolling circumference, well within an inch. And that kept the white Suburban from haunting our technician.
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