Date:         Wed, 21 Jul 1999 19:47:20 MDT
Reply-To:     d t <tinkerman@USA.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         d t <tinkerman@USA.NET>
Subject:      followup to post regarding oil pressure false alarms
Content-Type: text/plain; charset=US-ASCII

Hello fellow list members,

This is a followup to a previous post I made to the list regarding erratic
false alarms of the oil pressure LED and buzzer in my 1990, 2.1L Vanagon GL.
After once losing a motor (and about $2800) due to a coolant warning system
failure, I decided I couldn't take a chance with these false oil pressure

alarms and charged head-on at the problem...
Thank's to a few kind souls (I wish to thank them all), I managed to get a few
relevant pages from the Bentley manual (I still need an electrical schematic
of all the oil pressure warning system though). Here is what I found:

1. I think I've found an error in the Bentley manual: In page 90.23, Fig.2
seems to be a mix of wrong picture and text: the text talks about removing the
control unit for dynamic oil pressure system (and mentions some screw to be
removed), while the picture (#90-662) shows the voltage stabilizer (and no
removable screw in sight). This same picture appears in the right conotation
on page 90.25, were instructions tell you to connect test clips to the voltage
stabilizer. Am I wrong?

2. Anyway, I've managed to remove the dynamic oil pressure system control unit
("L-board"), and as far as I could see it's not corroded or broken, nor does
the printed circuit seem to me damaged (although I couldn't test it due to
missing current-flow diagram and difficult access to Oil warnig LED area, as
mentioned in the Bentley). Since I have just replaced the oil pressure sender
units with original ones, it seemed like either a bad L-board or more probably
broken wire from the sender units.

3. So I connected some test points to the instrument panel L-board connector
(thank's to the pin function list Fig.3 in page 90.23) and re-installed it in
the car. I checked the sender unit inputs (by measuring voltages at the
L-board inputs, assuming zero volts when shorted to ground and some positive
voltage when not). It must be remembered that the low and high pressure sender
units operate in an opposite fashion: the low pressure sender OPENS when it's
pressure threshold is exceeded, while the high pressure sender CLOSES when
it's pressure threshold is exceeded. The rest is logic done by the L-board.

4. It seems that the low pressure (0.3 bar) sender is functioning properly
(short to ground with ignition switched on but motor not running and oil LED
blinking, and open when motor runs and LED goes off). I've checked this by
measuring the voltage across the sender: 0V when motor isn't running, and 12V
when it is.

5. On the other hand, the high pressure (1.8 bar) sender behaves suspiciously:
the voltage across it (as measured at the control unit) is around 0.8V and
varies a bit with motor speed. It doesn't go down to zero (short to ground) at
>2000rpm as it should, so this might hint a broken wire to the sender. On the
other hand, when open it should have provided a higher voltage (around 12V,
like the other one. It should have behaved the oposite of the low pressure
sender: 12V at low (or no) rpm, and 0V at >2000rpm. So this is weird. Does
anyone know (or can measure) the voltage across this sender to help me debug
the problem? I think this is the one at the rear (near the oil pump), easiest
to access.

Of course, it could be a multiple fault situation (both broken wires AND
defective L-board).

I have other questions too:

6. What happens when 0.3bar opens at > 2000rpm? This is not mentioned in the
Bentley, and in real life results in turning the LED on (but not the buzzer.
Weird. If the low pressure sender indicates loss of pressure, then this is
even worse than loss of high pressure).

7. Does anyone have more info regarding the internal circuit of the L-board?
From what I could see, it contains a custom chip that measures the RPM (it has
an internal crystal frequency reference and is probably quite accurate) and
reads the oil pressure sender units, and turns the LED and/or the buzzer on
based on it's internal logic.

8. Is there any info about timings of the L-board tachometer input?

9. During removal of the speedometer, I broke by mistake a tab located at the
back of the speedometer, exposing the speedometer "flywheel". It seems that
this is a (purposedly) REMOVABLE tab and that something can be attached to the
back of the speedometer there (since there are two mounting screws there too).
Anyone got info about this?

10. What is the conversion function from Bar to PSI? I'd like to test the old
pressure sender units I removed to see if they were faulty or not (in case of
a multiple failure situation, so that at least I know what were the reasons
for the problems).

That's all folk's...

bye,
        Amnon


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