The VDO type sender in my Patrol is a resistive type sender, and as the name says their resistance varies according to the oil pressure. These sender units are VERY sensitive to bad connections (the wire going to the sender itself) or bad earthing (where the sendr screws into the block, or the Madman's earth). They will typically show very high readings or wild fluctuations when this is the case. If it used to work properly, it may simply be dirty. Pull the lug off the sender, clean it well (carb cleaner, etc.), maybe give it a light crimp again and fit it tightly back onto the sender. If you're lucky (like I was), this is all that's required.
If not, you'll need to follow that same wire for joints that have gone bad with age, etc. Also check the Madman's earth position and condition. Here are two important quotes from the installation docs:
- "Apply Loctite thread sealer to pressure and temperature fittings – thread tape may cause electrical connectivity problems."
- "When doing the wiring for the unit it is critical that you fit the black earth wire to the engine block – failure to do so will possibly result in erratic readings as other loads are switched on such as lights, fans, indicators etc."