Petrol engines low oil pressure
Low oil pressure warning (and Dynamic Oil Pressure Warning System - DOPWS)
NB. Read this description of the DOPWS first:-
If the oil light comes on whilst you are driving stop immediately it is safe to do so. There is a buzzer which may go off and give you a heart attack as well, sometimes the buzzer is disconnected by a previous owner as they can be unreliable and annoying if pressure system checks out OK.
To test oil pressure you need an oil pressure tester that screws into the hole that your oil presure low oil switch fits, between the pushrods on 3 and 4 (thats the low rpm/low pressure sender on petrols)
NB. It is more usual to check the oil pressure at the high rpm/low pressure sender beneath the water-pump pulley, or for diesels on the filter housing - HM)
HarryMann: There are two (sensors), that are called high rpm/low pressure and low rpm/low pressure, the normal higher speed one (~ .9 bar) is in a really awkward spot somewhere down between the water pump and pulley area. The low one (~ 0.3 bar) is under the left hand cylinder head, measuring the supply pressure to the top end oilways... they work in harmony connected in series ... one is open circuit and one closed circuit by default and the circuit continuity checked against an approx 2000 rpm break-point. The way it's wired, one is being monitored above that speed and the other below it.
The wires come in from the front nearside corner of engine bay... your problem may well simply be bad contacts at the instrument cluster, the multi-ribbon connector may have corroded a bit - careful around there, take out ribbon, clean and refit, test etc.
Qu: Can you remove the high pressure sender in-situ without removing the water pump? I had a look once and thought I Hope I never have to get to that!
GHOST123: Yep, When I first looked at it I thought it was going to be awkward and that at least the water pump would have to come off. But it is actually not at all hard.
I fitted an old oil switch off god knows what, just to plug the hole until the proper replacement arrived from GSF. I did not wire this temp switch up, just earthed the brown wire to stop the buzzer. (Mine was not a buzzer problem, it was a leak from a crack in the plastic bit of the switch). So I did it twice in a week.
The trick is to get underneath with your legs to the front and head a bit to the nearside, with a decent torch. You can then get a spanner on it without too much trouble. Do not forget the torch, it really helps
HSB: Oil warning light should come on full, not dimly lit as the switch is either on or of, digital not analog. If it is coming on dimly then suspect electrics - damp in the dash pod and/or coroded contacts are a good starting point. The main connector, the big rectangular one that connects to the circuit board in the dash - pop it off and use a scouring pad to clean up what's there. Bad earths can also be a culprit. Check behind the fusebox where there is a crown of spade terminals; get some emery paper on them (remove spades first!) earth strap from battery check both ends clean them up well, including the point of contact with the bottom of the battery tray. 2.1 engine - check the bundle of wires that go from the engine block to the left hand side of the engine bay. Remake the heavy gear-box braided earth. But before you do all of that get the oil pressure checked.
A typical scenario
AndyRB: I would appreciate any advice with this problem.
I've looked at other threads on this issue and am trying to identify as precisely as possible what's going on with mine.
Basically, I have a T25 1.9 petrol Komet camper, 1989, since March. 90k miles. Seems in generally good order, FSH, no leaks etc. For the past 3-4 months the oil pressure buzzer and lights have started to go off when driving.
Specifics: Buzzer and light always come on together, only when engine is warmed up, and not continuously - I can make it go off by revving i.e. it tends to be on at low revs. If I'm approaching traffic lights and the buzzer comes on, I can drop down a gear, accelerate briefly, then shift into neutral to get rid of the buzzer. It is consistent - I know know exactly when it will come on and go off - i.e. at what revs.
What I've tried already: I'm not good technically, so the van has been to three diferent garages. It has high and low pressure sensors - these have been replaced, new oil, filter etc. The pressure has also been tested by the garage (i.e. hooked up to a pressure monitor/gauge and driven around) and is fine, so the pump isn't suspect.
Other info: I had to have the sump plug re-tapped (threads all came away when I changed the oil) - the new plug is a homemade job, works fine, but may protrude slightly (a few mm max) into the sump - could this be a possible cause? Or perhaps it's the wiring behind the dash? I also took the van to an auto-electrician but he said the wiring was basically impossible to get at or check and a new dash pod may be the best answer (surely not?) Engine seems OK, though mpg is lower than I expected (perhaps low 20s per gallon, and the power on hills seems (I dont know) to be maybe a little on the low side. There's no indication that there's anything wrong with the engine, so i'm praying it's just wiring or something similar, but from the other threads, I suspect it may be worse.
Anyway, I'd appreciate any help/advice on this as the buzzer is driving my family mad.
sukhoi27: Have you looked here 
dingleyhythe: I had an Audi that did the same thing - the fault was in the dash - it was sensing the revs wrong so the buzzer was going off before the revs were high enough to get the pressure up. No way of fixing except to change the dash so I bypassed the high oil pressure switch and fitted a gauge.
Try (temporarily) disconnecting the high pressure switch, if the dash is working the buzzer should then always go off but at fairly high revs. If it is going off continuously or at very low revs then the dash is probably faulty.
If this works try earthing the sensor wire - it should now not buzz whatever. If the buzzer still comes on at certain revs a wiring fault is likely.
NB. This assumes the high pressure sensor switch is closed when the oil pressure is good - if it is the other way round (can't remember for sure) then these tests need to be reversed.
Low 20s MPG is normal I'm afraid, especially if yours is a high top.
Mocki: ... or perhaps it's the wiring behind the dash? I also took the van to an auto-electrician but he said the wiring was basically impossible to get at or check and a new dash pod may be the best answer (surely not??)
Rubbish, he didnt want to do the job!
If it is the foil PCB then thats a difficult job to repair, but possible. It could just be the connection plug to the dash pod, try giving it a gentle clean. If you are sure it isn't the oil pressure, it will be either the senders or the wiring from the engine bay.
HarryMann: Good idea to check the actual oil pressure too, at idle and at 2000 rpm, cold and hot (80C)... I sense that this it's the DOPWS system malfunctioning cry is sometimes the easy answer - however, it is always wise first to check the oil pressure at the specified conditions using a screw-in test gauge. All the engines VW fitted to T25s including the diesels can develop low oil pressure with time. The diesels can heat their oil up a lot, or TDs develop low oil pressure due to turbobearing wear, and the WBX's develop certain bottom-end problesm (see petrol engine specifics). Maybe assume it's 50/50 likelihood of being buzzer of doom or the engine itself.
ghost123uk: I would get the hot oil pressure checked ASAP! For your sake I hope it is an electrical fault, but sadly it is often a sign that the crank bearings and/or the oil pump is on it's way out.
andyRB: Many thanks for your insights and comments, I hope to get this checked again shortly and will report back
Hello again. I am pleased to report that it is now sorted! It was the buzzer itself which was faulty. It looked fine visually, but was faulty. Apparently, you can't simply remove it as the current for all dash components (speedo etc) flows through it - it is an integral part of the circuitry. Anyway, a new buzzer unit (PCB) costing around £50 plus fitting has done the trick. Thanks for your advice
KarlT: I've got the exact same problem, including stripping the oil drain plug!
HarryMann: Removed the complete DOPWS chip without affecting other dashboard functions... after 200 miles it was that or go totally maad! Oil pressure fine (1 bar per 1000 rpm on a diesel, probably pretty similar on petrols)
90% of oil light and buzzer problems are due to the fact that it was a stupid design.
If the buzzer goes off, stop the engine and re-start it. If the oil light goes out and stays out on tickover, you probably have enough oil pressure.
Also, if your tappets are reasonably quiet, that also proves that you have oil pressure.
There are 2 wires to the negative side of the coil. Disconnect the thinner one and drive.
Honestly! I have stripped and rebuilt engines because of the buzzer, under warranty and found, on stripping them nothing whatsoever wrong with them.
So. If the light stays out on tickover, the problem is the buzzer, not the engine.