Mechanical transmission and hydrostatic four-wheel drive control interlock

Claim 1

In a tractor having a variable ratio transmission means for driving the rear wheels, a transmission gear shift means shiftable into a plurality of positions for selectively establishing either a neutral condition or any one of a plurality of different drive ratios in the transmission, a transmission range shift means shiftable into a plurality of positions for selectively establishing a high gear ratio, a lower gear ratio, a reverse or a neutral condition in the transmission, a hydraulic fluid pump, hydraulic fluid motor means for driving the front wheels, conduit means including a direction control valve connecting said pump to said motor means, said control valve bEing normally in the first position-blocking fluid flow from the pump to the motor means and being selectively shiftable to second and third positions wherein fluid flow is established between the pump and motor means to drive the front wheels forwardly and rearwardly, respectively, electrical control means for shifting said control valve to its second and third positions, and electric circuit means including a current source connectable to said electric control means, the improvement comprising: first, second, and third series-connected, normally closed switches connected in said electric circuit means between said current source and said electric control means, said switches being operatively associated with said gear and range shift means for selectively connecting or disconnecting the current source to said electrical control means in response to shifting the gear and range shift means, said first and second switches being in said normally closed position when said gear and range shift means are positioned for establishing any of the transmission conditions except neutral, each of said first and second switches being movable between the normally closed position, a second closed position, and an open position, said first, second, and third switches, when in their normally closed position, connecting said current source to said electrical control means whereby said control valve is shifted to said second position, said third switch being movable to an open position in response to shifting the gear and range shift means to positions for establishing selected high gear ratios in the transmission, said second switch being movable to said second closed position disconnecting current flow to said third switch in response to shifting the range shift means for establishing said reverse condition in the transmission, thereby establishing current flow through said first and switched second switches to said electrical control means for shifting said control valve to said third position.