Aftermarket December/January 2023

BY Frank Massey V ehicle diagnosis is not dissimilar to navigating a minefield; Tread carefully. How’s that for an opening line? Not exactly an encouraging start I know, but bear with me. Within this topic I am going to explain Audi variable valve lift operation, design, servicing, and diagnosis. In order to underscore the risks – I did just say it’s a minefield – I have an actual fault from our workshop. Principles of operation are remarkably similar across all engine groups. They are some differences though, operating on either inlet, exhaust notwithstanding, or cylinder select variants. The camshaft responsible for shift and lift control has a spline. On this is a sleeve containing the lobes for operating two valves independently, which slide laterally. Two helical grooves provide for the actuator’s engagement and disengagement. The lateral movement of approximately 7mm is locked by a spring-loaded ident ball, like that controlling gearbox selection rods. Please refer to Fig.1, which shows a variable lift control sleeve. Different engine types vary in the approach, but this should broadly explain how the system works. Inlet valve operation is asymmetrical, i.e., each pair of valves operating from a common sleeve share different lift and closure profiles, but they open together. The closure event is offset however. This improves cylinder charge and creates a tumbling motion with the fresh air intake. Intake swirl flaps are no longer required complimented by Audi TFSI piston crown profile. Due to physical limitations, the valve rocker arms on the variable lift camshaft are required to be narrower than 24 AFTERMARKET DECEMBER/JANUARY 2023 TECHNICAL www.aftermarketonline.net ROCKER ON TOMMY? Frank tiptoes through Audi variable valve lift, focusing on operation and points towards diagnosis and how to make repairs their fixed-opening counterparts. This has been complemented by larger roller bearings. The lift control actuator is not a simple solenoid, controlled by the engine PCM. A permanent power supply is switched to ground via PCM command. This is achieved by a saturated control pulse, identical to early generation 15ohm injectors. The actuator pin is extended by a 3amp current flow with 100 G acceleration, requiring a permanent magnet and percussion damper to prevent the pin bouncing out of the helical groove. The back EMF pulse is used by the PCM to confirm actuation. The actuation pin return is provided by the helix ramp, Right Fig. 1