The scope of this paper is to study the failure of hydro-mechanical transmission in earth moving equipments and introducing an auxiliary system to overcome the transmission disengagement on working sites. The main aim of the auxiliary system is to get the transmission in gear in order to get the machine to move and avoid the crane cost lift the machine for maintenance. A 936F caterpillar wheel loader is used as a case study where mostly for the hydromechanical transmission under study; the speed selector spool is failure in the transmission hydraulic controls causes the loader cannot move. So an auxiliary control module to engaged No.6 clutch has been presented and have first speed forward only to rescue the equipments. In the present work, a mathematical model has been developed to simulate the electrohydraulic control system for an auxiliary control module to engage the first speed. An auxiliary module consists of an electrohydraulic directional control valve that used to engage the first speed by actuated the clutch engagement piston. Experimental setup has been conducted to study the dynamic behavior of auxiliary controller on the engage and disengage of the first speed train. The experimental test rig included an auxiliary pump as well as directional control valve where the transmission input and output speeds are monitored as well as the actuating pressure. The results of both simulation and experimental test shows that the purposed auxiliary system is able to engage the transmission into first gear in case of main system pressure leakage. Mathematical model has been proposed for the electrohydraulic control system that used to be the auxiliary engagement system.