Date: Tue, 19 Aug 97 12:06:12 -0700
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: bachmanb@smtpgw1.plk.af.mil
Subject: Possible FAQ? (warning: kinda long)
Ron-
Here's an idea for another FAQ. I've taken the basic information
that Ken Lewis (kdlewis@juno.com) posted on Aug 18, and added a
detailed explanation based on my experience (and early-June post)
troubleshooting MY system.
******************************************************************
How works the "Dynamic Oil Pressure Sensing" System
by Blaine Bachman (blaineoftrains@earthling.net)
and Ken Lewis (kdlewis@juno.com)
PREFACE
This is not intended to be a complete guide to troubleshooting the
Dynamic Oil Pressure Sensing System. Rather, it is intended to
familiarize the reader with the essential pieces of, and logical
operation of the system. The idea is that once you understand what's
happening (or supposed to happen), you can get out the Vanagon Bentley
Service Manual and follow the troubleshooting steps which span several
pages.
BASICALLY...
The Wasserboxer engine is similar to most other VW watercooled engines
in that it has two oil pressure switches electrically connected to a
circuit board (behind the tach or clock - often referred to as the "L"
board) which controls the idiot light on the dash.
The low pressure switch is located on the left side of the Wasserboxer
engine case, between and below cylinder 3 and 4 and hidden by some
engine tin. This switch operates similarly to the single switch many
of us are used to from our air-cooled days: when the pressure is low
or zero, the circuit is "made" (closed). When this happens in the old
Beetle, the ground circuit is made to the idiot light, causing it to
light if the ignition is on. When the pressure rises above the preset
level, the switch opens, extinguishing the light.
On the Wasserboxer, this switch, when functioning properly, opens
between 2.2 and 6.5 psi. Rather than go directly to an idiot light,
this switch connects to the "L" board.
The high pressure switch looks pretty much like the low pressure one
(except for having a different color of plastic end) and is located at
the rear of the engine, above and slightly to the left of the oil
pump. It's pretty hard to see, and almost impossible to get at, but
it can be replaced with a little work. This switch works just the
opposite of the typical oil pressure switch in that it is normally
open (does not make contact) until the pressure rises above a certain
level. On the Wasserboxer, this switch must CLOSE between 10.9 and
14.8 psi. It also connects to the "L" board.
The "L" board takes the input status of the two sensors, plus the
"count" from the tach input and causes the LED to flash and the buzzer
to sound based on an internal "truth table". The various combinations
are pretty well illustrated in the Bentley Manual, but here's a brief
synopsis:
1. When the key is on but the engine not running, the LED should
flash. If it does not, the circuit to the low pressure switch
(including the switch itself) should be checked. Problems with the
LED, "L" board, or voltage stabilizer are also a possibility.
2. Shortly after the engine is started, the LED should stop flashing
and turn off. If the pressure at the low pressure switch drops below
the switch's limit (closing the circuit) at any time while the engine
is running below ~ 2000 RPM, the LED should flash.
3. Whenever the RPMs pass above ~2000, the high pressure switch is the
focus of the system. When the "L" board senses the revs above ~2000
and the high pressure switch shut off, it believes that the oil
pressure has dropped below the limit needed to guarantee safe
operation. To get your attention, it flashes the LED and sounds the
buzzer.
If you're having problems with condition 1 or 2 above, probably the
easiest thing to do is to change the oil and filter (reams have been
written on problems with U.S. made filters on the Wasserboxer - go
with the Mann, Mahle, or VW filter) and the switch. While you're at
it, do a simple test on the wire by grounding it to the engine case
and checking to see that the LED flashes while the key is turned on,
and that it stops flashing when the wire is ungrounded.
Problems with condition 3 are a bit trickier, and can be compounded by
problems with conditions 1 and/or 2. The first step, is to probably
do the oil/filter change recommended above. The problem may go away,
but maybe it won't. Either way, since the high pressure system is so
critical, you should complete the remainder of the troubleshooting
process found in the Bentley Manual. At any rate, if your LED/buzzer
come on immediately and consistently once the engine revs over ~2000
RPM, you either have a serious oil pressure problem, a faulty high
pressure switch, a bad connection, or a bad "L" board. If it only
comes on when the engine is hot, you MAY have an overheating problem,
either alone, or in combination with others.
PRESSURE SWITCH NOTES
In theory at least, all points of a pressurized system should have the
same pressure. This is generally true in the Wasserboxer, however it
doesn't take a total failure to render a pressure switch faulty.
Weakening of the spring loaded contacts and clogging can also occur.
Both situations are bad for either pressure switch and can cause
erroneous readings at the "L" board and subsequent "false" alarms.
As already noted, replacing the low pressure switch is a "piece of
cake". The high pressure switch is another story. You pretty much
have to have a 24mm socket. (Sears' deep socket is too long to fit in
the cramped space, and the normal one is too short to fit on the
switch when the ratchet is inserted - a "half long" socket would be
perfect, but we'll show how you can use the short one). Further
"icing on the cake" is provided by Bentley's rather cryptic comment to
slide some piece of tin aside to get at the switch - this didn't bear
any relation to reality on Blaine's '91, but may with earlier models.
Best way to start is to first remove all the V-belts. Disconnect the
wire from the switch. Put the shallow socket on the switch. Put the
3/8" to 1/2" adaptor on the 3/8" drive ratchet; Grab a hold of
"killer" (the big, long Craftsman screwdriver that can double as a
pry bar) and use it to push the socket as far out on the switch as
possible but still allow the socket to catch the switch. Stick the
ratchet/adaptor on the drive end (again, just part way on, because the
terminal is sticking into the drive hole); turn.
With any luck it won't fight you on the way out. The insert may come
out with the switch, but that's a quick fix; just remove it carefully
and thread it on the new switch. Get the whole thing rethreaded and
tight. Wire on and belts tight and you're done!