Date: Mon, 6 Jan 1997 19:11:55 -0500
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: SyncroHead@aol.com
Subject: Re: why 2 oil switches (was: how engine oil degrades) oils)
In a message dated 97-01-02 22:54:21 EST, jscohen@m2.sprynet.com (James
Cohen) writes:
> Oil pressure that is too high is detremental also. At the very least it
> will blow out all the oil seals, which will let all the oil leak, and then
> the low pressure switch will get a chance to warn you. And if there is a
> high pressure situation it is likely there is so other major problem
> causing it.
>
> James
While there is a "high" pressure switch and a "low" pressure switch, they
both cause warnings when the oil pressure drops too LOW. Since "too low" is
a bit of a subjective thing in an engine, VW decided to implement two
standards. The higher the engine RPM, the higher the oil pressure should be.
If you have just one oil pressure switch with a setpoint low enough (say
4.4PSI, 0.3 bar) to not engage at idle, that leaves way too much slack for
when the engine is at high RPMs when the engine should have at least 13.2 PSI
(0.9 bar) . The "low" pressure switch is used to alert the driver if the oils
pressure dips too LOW while the engine is running at less than 2000 RPM and
the "high" pressure switch is used to alert the driver if the oils pressure
dips too LOW while the engine is running at more than 2000 RPM.
More specifically it works like this:
The dynamic oil pressure monitoring system gets inputs from two pressure
switches. One switch is closed with no oil pressure and opens when oil
pressure rises to about 0.3 bar, called the .3 bar switch. The .3 bar switch
is located between the left (driver's side) two cylinders.
The other switch is open with no oil pressure and closes when oil pressure
rises to about 0.9 bar, thus called the .9 bar switch. The .9 bar switch is
located below the water pump & crank pulley, near the oil pump.
The monitoring system seems to only "look at" the .3 bar switch below 2000
RPM and ignore the .9 bar switch. Above 2000 RPM the active switch is the .9
bar switch while the .3 bar switch is ignored. Below I've described a
properly operating monitoring system. Note that I say "monitoring system"
because some of the conditions describe an abnormally low oil pressure
condition.
A. Ignition ON, engine NOT running -- or -- engine running at LESS than
2000RPM
.9 bar switch is ignored
1. Low oil pressure exists (below .3 bar), keeping .3 bar switch closed
Oil light: Flashes
Buzzer: Silent
2. Normal oil pressure exists (above .3 bar), opening .3 bar switch
Oil light: Off
Buzzer: Silent
B. Engine running at MORE than 2000RPM
.3 bar switch is ignored
1. Low oil pressure exists (below .9 bar), keeping .9 bar switch open
Oil light: Flashes
Buzzer: Sounds
2. Normal oil pressure exists (above .9 bar), closing .9 bar switch
Oil light: Off
Buzzer: Silent
There does not appear to be a condition in a normally operating monitoring
system when the light should be on steady.
Regards,
Jim Davis
87 GL Syncro
88 GL Wolfsburg