Buzzer of doom

[vwhobo]

I know this has been well-covered but am losing it trying to fully understand it - maybe someone can help? The Wiki says:

"The flashing oil pressure light and 'buzzer of doom' are operated by the low-low pressure switch, hence at lower engine speeds" but also:

"if the oil pressure is low ... you will get the light and buzzer when the revs go above about 2000" - which seems contradictory??

Haven't used my 1990 1.9petrol much last couple of years, just start it up once a week. Total mileage is about 95K. Changed oil about 18months ago plus MANN filter but only done about 500 miles since then. Yesterday, took it out for a run - noisy tappets (as usual when not used regularly) and ran badly so used carb cleaner to clear out carb debris (usually works). Still ran badly. Another treatment of carb cleaner and it ran almost normally so drove it hardish for a bit to try to clear it through. Just as I got near home (conveniently) and slowed down, flashing oil light and buzzer came on (real scare!). Came on and off for last bit of drive. Turned off and left it for an hour. Started again - no oil light or buzzer. But dont like to take it out again until I'm clearer about what the issue might be. Was it driving it too hard and I've done some damage? Or could it be because I haven't used it much in last couple of years?? But then I'd expect sludgier oil rather than thinner oil?? Any thoughts appreciated.

[MidLifeCrisis]

Yeah - I'd have to agree that the Wiki is a bit confusing ....
i also found this ..... which was I think a little clearer ....
http://vincewaldon.com/index.php?option ... &Itemid=28

So, to summarise .....
a) There is a 'very low' pressure switch that operates all the time and will light up the oil pressure light when it senses 'very low' oil pressure
b) There is a 'slightly low' pressure switch that only operates when rpm is above 2000rpm and will light up the oil pressure light and sound the buzzer when it senses a 'slightly low' oil pressure.

[vwhobo]

Thanks for that, really helpful and a great article - I think it's clearer now - but seems to contradict the wiki. So as I understand it, then, the flashing oil light and buzzer of doom are operated by the high rpm/low pressure switch (as opposed to the low rpm/low pressure switch as outlined in the wiki ) - ie, at revs above 2000? And would you expect the light and buzzer to stop as soon as the revs drop below 2000?

[MidLifeCrisis]

Yes - that is what I would "expect"
But there is a lot of contradictory information out there....
My description is what makes sense to me (otherwise when you turned the key before starting the engine you'd get the buzzer sounding, as obviously oil pressure is zero when engine is stopped, but all you get is the red light on)
I was hoping that someone would come along with definitive knowledge but this is what makes sense to me ...

[silverbullet]

If the buzzer goes off above 2k it will stay on until you switch off the ignition.
Best to check all earths inc. Dash loom (crowns under dash left side) engine (lhs head on wbx) and transmission (braid on front mounting, in middle) before assuming imminent engine grenade.
PS had the same myself after van had stood a while, knew it was bad earths so drove on and it settled down. Found corroded earth later.

[AdrianC]

Virtually every vehicle out there has a low oil pressure light, that comes on when the pressure is almost non-existent (about 0.25bar). Our engines are no different. When the ignition's first switched on, the lack of pressure (before the pump starts to pump) causes the light to come on, but it should go off as soon as the engine starts.

Healthy oil pressure, hot, is about 1bar per 1,000rpm. So, if the light only comes on below 0.25bar, it's basically telling you your engine is dead if it comes on whilst revs are anywhere above idle. So our engines have a second switch. Because that switch warns when pressure drops below a much higher figure (about 1.5bar), it's ignored under 2k rpm. To keep life simple for the driver, it lights the same light. Because it'd be a PITA having a buzzer come on every time you turned the ignition on, but you really do want an urgent eyes-free warning when you're driving along, this switch (only) also fires an irritating buzzer.

If the light comes on with the engine running, with or without buzzer, INVESTIGATE as a matter of urgency, even if it's just to rule out a wiring problem! If it's coming on with the engine hot and idling, it's not quite the same "OHMIGAWDTHEWORLDISENDING" panic, but it _is_ a sign that your engine's getting warn. You could use thicker oil, but it's masking the problem, not fixing it.

[silverbullet]

The wbx oil pressure switches are unusually low-rated compared to most other engines, what passes for acceptable running pressure would be a joke in any other context.
The switch spec is probably to stop the light/buzzer coming on when the engine and oil are hot, because VW couldnt be arsed to fit a proper oil cooler.

[vwhobo]

Thank you all for your help and info. Have now removed, cleaned and replaced all earthing points - none were bad tbh, so dont think this is likely to be the problem. I can see the wires going towards the oil pressure switches but haven't yet caught a glimpse of the actual switches! Not sure where to go from here:

a) might the fact that its been doing very little the last 18 months result in sludge in oil filter (even though its only done about 500 miles since last change) causing some blockage. If this is likely/possible, then I guess its worth dropping the oil and replacing with new oil and filter and then taking it out and see what happens?

b) If sludge/blockage is unlikely, maybe I should take it to garage for a pressure test? (Or if I buy a cheap oil pressure gauge off ebay, is it easy to connect up?)

c) Ignore it for now and see how it goes?

Incidentally, when the buzzer came on, it didn't stay on. Went off and came back on, went off, came on and then went off before I cut the ignition.

[AdrianC]

I can see the wires going towards the oil pressure switches but haven't yet caught a glimpse of the actual switches!

One's behind the pulley for the water pump, on the back of the engine. The other is between the pushrod tubes on that same side, hidden behind the tin cover piece.

[vwhobo]

Thanks Adrian. Eventually found the one by the water pump - but can only see it using a mirror. If I remove the fan belt, I may be able to get a socket to it but am not convinced!
Also removed bottom bolts from aluminium protective plate and can just see pressure switch but looks like I need to remove plate completely to get at switch?? Scary, though, as the other bolts are on exhaust fixings. Have WD'd them for now.

Anybody confirm that it is the switch by the water pump that I'd need to remove to connect an oil pressure tester?