Grade Logic Control (GLC) in the PCM keeps the A/T from making frequent and unnecessary shifts when you’re going up or down a hill, or going around corners. This allows the vehicle to ride smoother, have more power available when needed, and makes the car operate more normally. The PCM adjusts the shift points by comparing actual driving conditions—based on input from the vehicle speed sensor, throttle position sensor, and engine coolant temperature sensor, along with the brake switch signal and the shift lever position signal—to the values stored in the PCM memory.

Ascending Control. When the PCM determines that you’re climbing a hill with the vehicle in one of the drive positions, for example D or D5 (shift position names vary by model and model year), GLC may delay up-shifting from 2nd to 3rd, 3rd to 4th, and from 4th to 5th (if you’ve got a 5-speed trans), to keep the trans from shifting too soon or too often.

Descending Control. When the PCM determines that you’re going down a hill with the the vehicle in one of the drive positions, for example D or D5, GLC may delay up-shifting from 2nd to 3rd gear, 3rd to 4th, and from 4th to 5th (if you’ve got a 5-speed trans), to keep the trans from shifting too soon or too often. This shift pattern and deceleration lock-up help control the speed so you don’t need to brake. If you do need to brake, GLC may down-shift the trans from 5th to 4th (if you’ve got a 5-speed trans), 4th to 3rd, or 3rd to 2nd, depending on how steep the hill is, how fast you’re going, and how fast you’re decelerating.

Deceleration Control. When going around corners, the vehicle needs to decelerate first and then accelerate. GLC anticipates corners when you’re braking from speeds above 27 mph and then accelerating. When this is detected, the GLC shifts into 2nd for better acceleration.