4-Wheel drive transmission control system

Claim 1

A control system for a 4-wheel drive transmission including a transfer unit capable of establishing 2-wheel drive mode and 4-wheel drive mode by operation of first hydraulic means and capable of establishing low-speed and high-speed gear positions in said 4-wheel drive mode by operation of second and third hydraulic means, said control system comprising: a source of oil at line pressure; a manual valve having: an inlet port connected to said line pressure source, a first outlet port, a second outlet port for supplying said line pressure directly to said first hydraulic means, a first drain port, a second drain port, and a spool mechanically connected to a shift lever and shiftable between first, second and third positions; said inlet port being in fluid communication with said first outlet port and said second outlet port being in fluid communication with said second drain port when said spool is in said first position, said inlet port being in fluid communication with said first and second outlet ports when said spool is in said second position, said inlet port being in fluid communication with said second outlet port and said first outlet port being in fluid communication with said first drain port when said spool is in said third position; an automatic transfer control mechanism for supplying oil at a control pressure which varies according to vehicle speed; an inhibitor valve having: a first chamber connected to said first outlet port, a second chamber connected to receive control pressure from said automatic transfer control mechanism, and a spool shiftable between a high-speed position and a low-speed position to selectively connect said oil pressure source and said second or third hydraulic means, said spool being held in said high-speed position when said line pressure is supplied to said first chamber through said manual valve or when said control pressure supplied to said second chamber is above a predetermined level, and said spool being held in the low-speed position when said line pressure is drained from said first chamber and the oil pressure supplied to said second chamber is below a predetermined level.