During a rebuild of a GM 6L80 transmission, while the valve body was being serviced, an incorrect spacer plate was accidentally used. This blocked supply pressure to the 3-5-R regulator valve which prevented the 3-5-R clutch from applying.
One of the more significant changes made to the valve body for 6L80 series transmission is related to the 3-5-R clutch feed. The early first design valve body had a direct feed from the #5 shuttle ball to the #7 metering check ball as seen in figures 1 (above), 2 and 3.
The #5 shuttle ball would receive pressure from one end for reverse and the other end for third and fifth. This pressure would then feed the #7 check ball where it was pushed onto its seat in the spacer plate forcing the oil pressure to go through the orifice along side the seat. This metered fluid pressure is then sent to the 3-5-R regulator valve to control the rate of 3-5-R clutch apply (figures 4, 5 and 6).
The later design blocked this passage at the #7 check ball location with a wall as seen in figure 7. The spacer plate received an additional slot prior to the wall forcing the circuit into a slot provided in the channel plate (figure 8).
This pressure is now routed to and through the orifice and ball seat for the #7 check and onto the 3-5-R regulator valve (figure 9). What this means is the pressure circuit between the 3-5-R regulator valve and the #7 ball will have a controlled exhaust rather than a controlled apply.
If a late plate is used on an early valve body, you will lose 3-5-R feed as seen in figure 10. It will also flood SS2 (figure 11). If you use an early plate on a late valve body, you will block 3-5-R feed as seen in figure 12.
One note concerning the 6L45 valve body is that it was first produced in 2010. To affect this valve body, a mismatch using 6L50 parts can cause the same complaint.