Volkswagen: Oil Pressure warning light/buzzer, vw passat glx, oil pressure gauge

Question
Year/Model:  1999 VW Passat GLX 2.8L V6
            5Speed Auto (Tiptronic)

The oil perssure warning light/buzzer comes on (intermitent) at apprx 1500-1800rpm (45-50mph). Dealer ran diags and recommended new sending unit. I had them replace it (they also reported good oil pressure). Light still came on.  I installed new oil pump on my own and still get the light. Pressure mesured 40psi (cold) and never dropped below 33 while I had a mechanical oil pressure gauge plugged in (manual says 2.0 bar (30psi)at 2000 rpm so pressure looks good. I also checked bearings while replacing oil pump and they looked fine. Engine is very quiet with no valve ticking or knocking from con rods. The dealer is suggesting an electrical prolblem but the cost for them to diagnose it is prohibitive.

Can you help in the diagnostics of the problem?
Is there anything else that may be sending the idiot light a false signal? If so how do I find it? I also thought about disconnecting the sending unit and plugging in an aftermarket mechanical gauge (VDO) but the dealer says the light will probably still come on. Is this possible?  If so how can I make this thing go out?

Thanks,
Harold

Answer
if you unplug the oil pressure sender and put the wire straight to earth, the light should not come on again, if it does, try to isolate the wire (open circuit) and try it. failing that, seek advice or help from a reputible auto electrician as they should be cheaper than main agents and probably be able to locate problem quicker.