3-4 Shift Valve

The 3-4 shift valve either allows, or inhibits, transmission shift into overdrive (O/D). In all gear positions except O/D, the 3-4 shift valve is seated by spring tension.

In P, R and N ranges, line pressure is not applied to either end of the 3-4 shift valve, and spring tension holds the valve in the seated position.

In D range (3rd gear) shift solenoid B is energized. This removes line pressure from one end of the 3-4 shift valve, and spring tension at the other end is sufficient to keep the valve seated. Line pressure from the 2-3 shift valve is applied to the spring end of the 3-4 shift valve to assist spring tension (even though shift solenoid B is energized) in ranges D (2) and 2 (2), when engine rpm (and, therefore, line pressure) is relatively high.

In first gear (ranges D, 2, or L), shift solenoid B is de-energized, applying line pressure to one end of the 3-4 shift valve. This would normally be enough to overcome spring tension at the other end and seat the valve. However, in these gear ranges, line pressure is also applied to the spring end of the 3-4 shift valve (from the 2-3 shift valve). This allows spring tension to seat the valve.

Line pressure (from the oil pump) is then routed through the 3-4 shift valve to the O/D clutch, engaging the clutch.





When vehicle speeds warrant a shift into overdrive, shift solenoid B is de-energized, and line pressure seats the 3-4 shift valve against spring tension. This closes the passage to the O/D clutch (the O/D clutch disengages), and opens a passage to the O/D brake. As a result, line pressure (from the oil pump) is applied directly to the O/D brake, engaging it.