You need to test the oil pressure and maybe even the oil temperature. How long-fast were you driving before the light-buzzer turned on? Did you replace the rear oil pressure switch while doing the water pump? Is this really a new problem? IF you changed the switch try changing it again. At this point I only get them from the dealer ($$$). The aftermarket ones are all over the place.  If it was a wiring problem the buzzer would go off anytime the engine goes over 2000 rpm. If it waits for the engine to get warm/hot then it is due to a drop in oil pressure. The only next point is if the switch is activating at the correct pressure. 

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of John Rodgers
Sent: Wednesday, October 14, 2015 10:14 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Oil pressure light- '88 GL

Driving down the road the oil pressure light came on , the buzzer sounded and I near had a heart attack! Didn't last but seconds, but scare the heck out of me. Did this numerous times, though not consistantly, and only around 2000 rpm. I just finished that onerous water pump swap along with hoses all around, added new coolant and bled the system, reinstalled the oil cooler with a new seal, put on a new Mann oil filter, added 4-1/2 quarts of 20-50wt oil. Now this! Any suggestions? Engine has 150,000 since major overhaul and is running strong. Don't have an oil pressure nor oil temp guage on this one. Cleaned the rearmost connection for oil pressue thoroughly down to bright copper  before reconnecting it.

Open to suggestions on this one.

John