A/T - 440T4 Coast-Down Clunk
BULLETIN: # 256TRANSMISSION: 4T60/440-T4
SUBJECT: 3-2 Coast-Down Clunk
APPLICATION: GM
A 3-2 coast-down clunk on a GM vehicle equipped with a 4T60/440-T4 transaxle can occur for several reasons. This condition does not necessarily mean that there is a problem, it can occur simply due to mileage and time. Take the following measures to correct this condition.
1. Verify that line pressure and engine idle are normal. Most transaxles operate at 60-70 psi in drive at an idle. High line pressure can be caused by low vacuum, the wrong modulator or a stuck modulator valve just to name a few.
2. Check/modify the 1-2 servo. Servo travel should be 1/8"-1/4". Lean toward the looser specification for this condition. The servo can be modified by grinding the servo pin stop by approximately 1/8" (figure one). This will allow the cushion spring to be compressed further before the pin bottoms out on the piston. If the cushion spring is fatigued (figure two) replace it. GM has made several upgrades of this spring based on the application. Contact your local GM dealer for the correct spring.
3. Check the 3-2 coast valve. It may be stuck. If the valve is not stuck, install the end plug backward (figure 3). This will compress the spring more than normal. This will offer a quicker release of the third clutch during the 3-2 coastdown shift.