Oil Warnig

[LCSCHADT@aol.com]

Chris,

I don't have a manual for the Audi 4 cylinder, but for the Jetta 4's, the
High Pressure switch is the one on the oil filter bracket, the other (the low
pressure one) is located on the the block/head at the transmission (back)
end.  Your car's symptoms make it sound like it is either the high pressure
switch, or the circuit board.  The switch should be open (infinite
resistance) engine not running, closed (low resistance) once pressure above ~
1.8 bar (~30 psi).

You ask about disabling the high pressure switch...Since that switch is
supposed to close when things are right, to "disable" it you would have to
ground the wire (ok as far as I know, thats what the switch does...).  Your
other switch would still warn you in the event of a catastrophic oil pressure
failure.  Wouldn't advise it for long term, might be ok for short term...

Some of my following comments have already been covered by other responses,
but some may be relevant to your circumstances...
 
In your recent post you ask why the oil sender is wired to the tach ...

There is a computer which monitors the input from the tachometer, and two
different pressue switches.  One switch opens once a minimal oil pressure of
.3 bar is reached, the other remains open until the pressure reaches 1.8 bar
WHICH MIGHT NOT QUITE BE REACHED AT IDLE, and closes once that pressure is
exceded.  So this little chip ignores the second one until RPM are above
2050, by then oil pressure should be above 1.8 bar (1.8 atmospheres or ~30
psi - not sure of the conversion off of the top of my head).  Could be a
problem with that little chip, or could be that you're not reaching 1.8 bar
until just above 2050 RPM (or, you are, but the switch is a little stiff, and
not closing until a slightly higher pressure). Thus, below 2050 RPM the
switch is simpley ignored, slightly above 2050 RPM your pressure is finally
high enough that the switch closes and tells the chip all is right, but at
2050 RPM the switch is being listened to, but your pressure just a hair too
low (or the switch a little to stiff).

The 1.8 bar switch might not be closing until a slightly higher pressure, and
again, your oil pressure not quite high enough to close the switch until just
above 2050 RPM.  Replacing the board is difficult (pulling the instrument
cluster +++) and expensive (>$100 I suspect).  If the problem persists,
before diving into the instrument cluster, you might consider replacing the
1.8 bar sender (not sure where that is located on the 4 cylinder, on the 5
cylinder it is located on the driver's side of the block down from the space
between 2nd/3rd cylinder, has the white wire connector).  It is easier (big
wrench) and less expensive (about $10, and I believe it is a generic VW/Audi
part).

Why does it happen only after a "hard run"?....  The engine is probably a
little warmer, the oil a little thinner, and the pressure a little bit
lower...

The described test sequence for my 5kSQ are as follows:

Vehicles without Auto-Check system:

-Ignition on
 > oil check lamp must light up
-start engine and let idle
> oil check lamp must go out
-disconnect wire to low pressure switch (brown insulator)
-hold brown wire to ground
> oil check lamp must light up
-reconnect wire to low pressure switch
[> oil check lamp must go out - my comment]
-disconnect wire from high pressure switch (white insulation)
-run engine rpm to 2500 for longer than 1 second
> oil check lamp must light up and buzzer must sound
-reconnect wire to high pressure switch


For vehicles with the Auto-Check System

-Ignition on
-disconnect wire to low pressure switch (brown insulator)
-hold brown wire to ground
> symbol "oil pressure" on instrment cluster must flash
-remove wire from ground
> after approx. 3 seconds symbol "oil pressure" must go out
-reconnect wire to low pressure switch
-disconnect wire from high pressure switch (white insulation)
-start engine, and increase engine rpm to 2500 for longer than 1 second
> oil check lamp must light up and buzzer must sound
-reconnect wire to high pressure switch 


My comments are somewhat generic, intermittant electrical problems can be
especially difficult to sort out.....  On our '82 jetta, alas, it was the
circuit board - $115 in '87.

Good Luck, Let me or the digest know how things turn out...

If interested, I have some anecdotal back ground why the 2 different pressure
switches, and the chip...

Larry 5kSQ (have also played nursemaid to '82 & '85 jettas)