[Vwdiesel] Oil Pressure Testing on an '84 Bunny

'Bryan K Walton' bryankwalton at machlink.com
Tue Nov 28 11:59:34 EST 2006

I thought I'd send a followup to the list regarding my fix to my
oil pressure warning problem on my '84 Bunny in case anybody else ever
has a similar problem.

To bring everybody up to speed, my problem was that my oil pressure
warning light would come on while idling with the oil up to temperature.
I checked the pressure with a gauge and new that my pressure was good.
Through a process of elimination (following the Bentley instructions),
I also new that the sender on the cylinder head was good (this is the
sender that normally governs the warning light during idle).

Here is what I discovered.  The high pressure sender on the oil filter
flange is designed to trip when the pressure is between 23 and 29psi. 
What Bentley doesn't make very clear is that it trips in this range
REGARDLESS of the rpms.  In other words, if the pressure is 28psi at
the oil filter when idling with warmed up oil, it is going to trip --
even though you are idling.  It is the job of the oil pressure relay
to determine whether the car is above 2000 rpms or not when it receives
this signal from the high pressure sender.

My problem was with the relay.  Essentially it was stuck open.  It was
never reading the rpms from the tach.  So, at hot idle, when pressure
is lowest, my oil pressure was just low enough to trip the high 
pressure signal (right under 29psi) and by fault, my relay was
allowing the current to flow to the warning light -- rather than
blocking that signal like it is designed to do.

In hindsight, had I understood this design as well as I do now, the
fact that the relay's buzzer never sounded would have hinted at 
the relay as being suspect. I replaced the relay and now my light no 
longer comes on while driving. (And when testing, everything now works
at it should have -- including the warning buzzer).


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