Date: Sun, 8 Nov 2015 14:09:51 -0500
Reply-To: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Subject: Re: Oil pressure light and buzzer
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Mark,
Your explanation is great. I'll just add one more point. The higher pressure switch logic is reversed as a fail safe. The switch closes when pressure is sufficient. As such a broken or disconnected wire will make the warning active. If the warning is intermittent and based somehow on engine temperature or run time then most likely the switch is bad or the incorrect one or there is a true oil pressure problem.
Dennis,
From my phone.
________________________________
From: vw_van_fan_Mark<mailto:madvws@COX.NET>
Sent: 11/7/2015 11:38 PM
To: vanagon@GERRY.VANAGON.COM<mailto:vanagon@GERRY.VANAGON.COM>
Subject: Re: Oil pressure light and buzzer
Oil pressure is provided by the mechanical oil pump that turns with the
engine. The faster the engine turns the faster the pump turns. The
faster the pump turns the more oil pressure it makes.
In a basic system there is 1 oil pressure switch, with a low pressure
setting because it has to trigger off at idle speed even with a hot
engine when the oil is thinner.
In the dynamic system there are 2 oil pressure switches. The first
switch is just the same as the basic system has. Then there is a second
switch that only triggers off at a much higher pressure. To keep the
second switch from turning on the oil warning at idle there is a circuit
board that it connects through. This circuit board also watches the
engine rpm and the circuit board holds back the second switch signal
unless the engine rpms are above about 2000 rpm. Any time the rpms are
above 2000 the circuit board lets the second switch also control the oil
warnings, including the buzzer. Below 2000 it only lets the lower
pressure switch control the warning led but not the buzzer. Any time you
hear the buzzer it means the higher pressure second switch is the one
that triggered the oil warning.
The buzzer is only triggered by the second switch.
The second switch is ignored unless the rpms are above 2000.
Once the buzzer triggers it keeps buzzing until the pressure is high
enough to trigger the second switch off. If you are driving down the
road and the oil is hot and thinner it means the pressure will drop more
as you slow down. If before the engine slows below 2000 the pressure
drops low enough to trigger the second switch to on then the buzzer will
sound and the led will start flashing. If you continue to slow down
these alarms will continue to sound. You must rev the engine up to make
enough pressure to clear the alarms.
Hopefully the above is clear enough. The seemingly odd buzzer behavior
isn't really that odd. It just depends on whether the alarm happens when
you are speeding up or slowing down. If it happens as you are speeding
up it will clear itself as you go faster and the pump makes more
pressure. If it happens as you are slowing down it can't clear itself
and the driver must rev the engine to clear it. Easy with a manual trans
but a little trickier with an automatic.
Mark
John Rodgers wrote:
> I'm trying to get it in my head how the dynamic oil pressure warning system
> is supposed to work. I have an intermittent fault where the oil warning
> light and buzzer come on at odd times and I need to understand this
> monster. Comes on at different speeds/rpm. Oil level is fine. But it's a
> bit scarey!
>
> Anybody?
>
> John
>