Wear at the 3-4 accumulator piston pin or bore.
The Sonnax piston is made from high grade billet aluminum to prevent wear and restores clearances at the pin.
It is common to have poor 3-4 shifts, no 4th gear or a burned 4th clutch in 4T65-E units. This can be caused by excessive wear at the 3-4 accumulator piston pin bore, which allows 4th clutch oil to exhaust and can result in harsh 4th, no 4th gear or a burned 4th clutch. Pin bore wear also reduces 3rd clutch pressure during 3-4 shift. The 4th shift quality depends on the condition of the piston and accumulator valves. If the 4th accumulator piston seizes, the 4th shift will be very hard. The Sonnax accumulator piston 84880M-01 restores accumulator pressure and eliminates cross leaks.