Paraphrased form the workshop manual, the AFM solenoids are hot (one side of the solenoid always has 12V) and the other side is connected to the ECU. When 4 cylinder mode is required, the ECU grounds the AFM solenoids. This ECU grounding energises the solenoids which then open and thus feed high pressure oil though the solenoids to the AFM lifters. This high pressure oil enters the AFM lifters and compresses the lifter lock pin within the AFM lifters and thus frees the two halves of the lifter from each other. As such, the lower half of the lifter follows the cam and slides into the upper half of the lifter which keeps enough pressure on the pushrod to stop noise but not enough to operate the pushrod and rocker. That way the associated valve remain closed. Whether the ECU also inhibits fuel and spark, isnt stated within the workshop section but I’d expect at-least fuel is cut in the AFM cylinders when commanded. Just couldn’t be faced searching for that clarify within the workshop manual.
When 8 cylinder mode is no longer required, the ECU removes the ground circuit and the hot solenoids can no longer be energised (as current can no longer flow). That way the AFM solenoids close and the high pressure oil stops flowing to the lifters. The high pressure oil within the passages and AFM lifters bleeds out and thus the lock spring within the AFM lifters push out. That way the lock pins can again lock the two half’s of the lifter together in a non collapsed position so they can operate the pushrods, rocker and the valves.
As the AFM system required really clean oil, there is a serviceable AFM oil screen filter that sits under the oil pressure sensor at the back passenger side of the AFM valley plate. But if this filter is blocked, all it means is that high pressure oil may not flow to the AFM solenoids so tne system may not be able to get into 4 cylinder mode when commanded by the ECU so it’s not a failure mode that would cause 4 cylinder activation… In any case, the AFM system requires clean oil is circulating the engine so it’s best to ensure factory oil change intervals are adhered to. Sloppy change intervals will bite owners. Ideally, for long engine life, doing more frequent oil and filter changes is a favourite of many jc members. Oddly the AFM screen filter isn’t mentioned in the service schedule but some have said helps if checked/cleaned and/or replaced at least every now and again.
My opinion, really the AFM system is somewhat robust… However, with AFM solenoids being hot, that means that if an earthing fault on the ground wires to the ECU occurs, the AFM solenoids will do their thing and your engine will operate in 4 cylinder mode without ECU intervention. Since it’s a non commanded situation, the ECU may continue to feed fuel to the cylinder which may not be a good thing
As such, check your AFM connector plugs and particularly the earth part of the AFM wiring going to the ECU. How the wiring is routed and what colours are involved, I haven’t looked…
Good luck with it