D12 Typical Engine Compartment Overview Engine Features VECTRO II Parameter Values, D7 VECTRO II Parameter Values, D12 Engine Variants, D7 Engine Variants, D12 Engine Data. Heavy-Duty Service Schedule for VHD-Series General and Check-Chart Maintenance Categories. Maintenance Engine, Maintenance Flushing Cooling System, D7 Flushing Cooling System, D12 Turbo and Charge Air Cooler. Replacing Fuel Filters, Fuel System Service Transmission and Rear Axle Maintenance.