In '00–'15 Allison® 1000/2000/2400 units, wear in the control main relief valve bore can cause or contribute to reduced control main pressure to the shift solenoids and reduced A & B trim pressures. The control main relief valve limits peak control main pressure and is always active when the vehicle engine is operating. In action, the valve reacts against a stiff spring outboard using small diameter spools inboard to limit contol main circuit pressure. The combination of a stiff spring and small valve spools creates significant valve side loads which can wear the valve body bore. As bore wear becomes excessive ATF is allowed to escape into the exhaust backfill circuit or directly to the sump and critical pressure is reduced to the shift solenoids and A & B trim valves causing shift issues. Reconditioning the bore and installing Sonnax oversized control main relief valve S121741LB-1 eliminates escaping oil and restores hydraulic control to the control main relief circuit. Hardcoat anodized aluminum valve combats premature wear; Added annular grooves to reduce wear rate
Control Main Relief Valve, Oversize
