How can I tell, with the PS Pump still in place, whether I have a Vickers or ZF. I need to order another pump to replace the one I have. It makes a groaning/whining noise around between 15 and 40 mph and it's getting worse. I tried flushing the old ATF (PO had used ATF) out and putting clean, fresh, M-B PS fluid in and it didn't change a thing, still groaning. I pulled the accessory belt and checked the idler pulley, it seems okay. Am I missing something? Could it be the steering gear and not the pump. You can, if you pay very close attention, feel a slight pulsing or vibration, sorta a "buzzing" feel, in the steering wheel when the noise occurs. The noise is intermittent, not continuous. I haven't tried to replicate the noise in the garage due to lack of someone to operate the accelerator (and we all know what happens when you grab the accelerator cable from inside the engine compartment...)!