---------- Forwarded message ---------- Date: Tue, 19 Aug 1997 13:00:57 -0500 From: bachmanb@smtpgw1.plk.af.mil To: Multiple recipients of list 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!