Aidan wrote:does the switch for the buzzer of doom switch the wrong way by any chance, ie it should go closed circuit to earth, maybe it's switching like the low pressure switch does open circuit, (closed when engine off, opens at 3psi) or does it give a resistive load like the gauge sender does ?
Didn't read every word but didn't see this mentioned: yes, the 2nd hi-OP switch for the VW DOPWS controller acts in the opposite manner of the low-OP idiot light switch.
The idiot light switch is just like every such switch in use since day one, it is a normally-closed (NC) switch that grounds the idiot light circuit, causing the bulb to glow, and then opens when OP exceeds the setpoint range (usually around 0.3 bar to avoid nuisance warnings at idle).
The hi-OP switch is a normally-open (NO) switch, at rest it is open-circuit to ground but grounds the sending wire when pressure exceeds the setpoint. So if the DOPWS controller has an open circuit to ground on the hi-OP input, and also counts engine rpms at over 2k, it triggers the warning. That's why if you wish to disable the DOPWS you only need to ground out the hi-OP input and it will always look to the controller like there is adequate OP.
All the switches are plain on-off, no resistance is introduced. To me that's why it's important to distinguish "switch" from "sensor" or "sender". And I personally use "sender" when it feeds a passive gauge, reserving "sensor" for one that an active system such as the FI uses. But that's just me.
All of these switches have a pressure setpoint range, and it is normally indicated on the switch body. The switching event can occur anywhere within the range of pressures.
A VDO OP combi sender/switch will also have the warning circuit switchpoint range printed or stamped somewhere on the body, usually on the base hex, so if your dealer can't tell you what that value is, ask him to look on the unit itself, it should be there. Other manufacturers likely do the same. If you use that type, the warning circuit switch will be NC, unless you manage to find one that's the opposite but I doubt such an animal exists, so you can't use that one for the DOPWS hi-OP input because it would cause the warning to go off every time revs exceed 2k, as the poster above discovered. That circuit can only be used for the lo-OP idiot light circuit. In the US we mostly discourage using the VDO combi unit because it's switchpoint is around 0.7 bar, whereas the low-OP switch is only around 0.3 bar, so many wbx's will have nuisance warnings at idle or very low rpms when in fact their OP is just fine (any OP at all at idle is basically OK, it just better climb quickly as the revs get into the working range, but this is totally common to see). So we typically use the simple sender and mount it on a tee with one or both OP switches. I make a little pipe extension/mount that has become pretty popular with my engine jobs, it relocates all the OP monitoring stuff above the left cyl head on one little manifold, keeping this gear and its wiring in a more sheltered place and easily accessible.
An OT sender can't be mounted this way,. though, it has to be in the active oil flow to measure temp, but since the hi-OP switch has been removed from the rear main galley location, that's the perfect place to pick up OT on a wbx. The galley is threaded M16x1.5 and VDO OT senders can be bought in that thread size, or the more common M10x1.0 sender can be fitted in the existing steel reducer bushing, but usually requires coring out the bushing a bit so the sender will thread in far enough that it's tip is well exposed in the galley. That problem will depend on the exact sender you use, there is a lot of variance.
The correct hi-OP switchpoint for a wbx is 0.9 bar, but some of the literature (like the Bentley service manual here in the US) shows a 1.8bar value there on some pages, which is in error at least for the wbx. That high a switchpoint almost guarantees frequent nuisance warnings in normal driving, and making that mistake has caused a lot of hair to be torn and garments to be rent thinking there was a genuine engine problem.
I cut mine into the pipe that connects to the t/stat assembly on the advice of futbus, seems to work quite well there.
