An Inflated Story about Erratic Shifting - Transmission Digest

An Inflated Story about Erratic Shifting

After a wide-open throttle forced downshift and then coming to a stop, the vehicle would take off in third or fourth gear. A scan tool confirmed that the wrong gear starts were commanded by the TCM. The transmission would upshift from third or fourth gear to fifth and sixth gears.

An Inflated Story about Erratic Shifting

Shift Pointers

Author: Wayne Colonna, Technical Editor
Subject Matter: 2006 Range Rover S, ZF6HP26
Issue: Tire size, pressure

Shift Pointers

  • Author: Wayne Colonna, Technical Editor
  • Subject Matter: 2006 Range Rover S, ZF6HP26
  • Issue: Tire size, pressure

Notes on strategies associated with active adaptive shift operations

A 2006 Range Rover S equipped with the ZF6HP26 transmission came into the shop with complaints of wrong gear starts and erratic shifting.

After a wide-open throttle forced downshift and then coming to a stop, the vehicle would take off in third or fourth gear. A scan tool confirmed that the wrong gear starts were commanded by the TCM. The transmission would upshift from third or fourth gear to fifth and sixth gears.

When the vehicle came to a stop and the Tiptronic feature was used, it would take off in first gear and shift up until a forced downshift after which it would return to wrong gear starts. No fault codes were present.

This is yet another vehicle that is highly sensitive to tire size. One of the tires on the vehicle had 20 pounds of air in it while the rest of the tires had 50 pounds of air. The Tire Monitor Lamp NEVER CAME ON.

Inflating the soft tire to match the rest of the tires on the vehicle cured all the wrong gear start and erratic shift complaints.

In retrospect, had a stagger gauge been used to check the circumference of each tire, a difference would have been seen when the measurement of the soft tire would have been compared with the other tires (Figure 1).

A difference in wheel speed RPM data could have also been observed through a scan tool.

Note: As an example, vehicles equipped with the NAG 1 or 722.6 transmissions that use rear wheel speed sensor input to calculate vehicle speed, can have complaints of not upshifting into higher gears due to different tire circumferences.

The use of a stagger gauge quickly checks the circumference of each tire in inches as indicated by the sliding ruler on the stagger gauge.

In odd complaints such as these it is always a good practice to check tire circumference. If a stagger gauge is not available a soft tape measure can be used to measure the diameter at the center of each tire. There should not be more than a quarter of an inch difference between all four tires.

This tire issue is one of the top causes of all wheel drive mechanical failures especially with “active” transfer case systems.

With this in mind, it is good to be aware that Mazda has a highly sensitive active adaptive shift (AAS) program that began with some Mazda 6 models from 2005 to present equipped with the AW6A-EL (TF81SC). They may have a complaint of no upshift to fourth, fifth or sixth gears under heavy throttle conditions. Under normal throttle conditions the transmission shifts through all gears.

The way the car may be driven is the cause of this complaint. If the driver accelerates rapidly, or accelerates and decelerates rapidly by operating the throttle and brake pedal for a certain time period with the shift lever in the “D” position, active adaptive shift control will be initiated by the TCM. When this occurs, AAS mode will maintain the transmission in the optimum gear, which creates the no-shift condition. This means that the TCM will keep the transmission from upshifting to prevent shift busyness and to ensure that upon re-acceleration there will not be engine lugging and that a smooth transition into the next gear will take place.

This information can be read in the owner’s manual of the vehicle.

There are a number of strategies associated with AAS operations such as:

Sport mode

When sport mode is selected, the AAS selects a lower gear compared to when normal mode is selected, this maintains that gear for a longer time period.

Throttle is closed suddenly and returned to heavy acceleration.

When the throttle pedal is fully closed suddenly and returned to a certain speed, upshifts are inhibited for a specified time by the TCM to improve speed control and re-acceleration performance.

Brake pedal is suddenly strongly depressed:

When decelerating at a certain speed, a lower gear is selected so that re-acceleration is performed smoothly.

Cornering:

While cornering in a turn with a radius less than a specified value, upshifts are suppressed to improve vehicle speed performance (Figure 2).

During high degree of vehicle ascent/descent:

During a certain level of downhill or uphill conditions, gears are selected according to driving conditions.

While ascending a slope of a certain grade or more, slope mode control prevents unnecessary upshift by maintaining the appropriate gear.

While descending a slope of a certain grade or more and depressing the brake pedal, the gears are appropriately shifted down according to the degree of slope for effective engine braking.

Higher speed merging:

Under a condition of temporarily decelerating while merging onto a high-speed expressway, upshift is suppressed directly afterward to improve re-acceleration performance (Figure 3).

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