Morris Minor - oil pressure gauge

Symptoms of problem :  Help - oil warning light comes on !  Stop IMMEDIATELY to investigate !

Vehicle : 1957 Morris Minor 1000 Pick-up (LCV) - a CKD MM built in Petone, New Zealand. (part Series II, part Minor 1000 I think!)

Time :  1 hour   Difficulty : 4/10   Parts required :  New (or secondhand) oil pressure gauge.

Special tools required :  Drill + bits, spanners, wire crimping tool.

The original MM 'oil pressure warning light' (coloured green for some strange reason!) is switched by an 'oil pressure switch' mounted in a threaded hole at the back right hand side of the engine block - just behind the distributor. It has a large hexagonal nut, but is actually quite a small thread - the same thread is still in use on many modern cars. There are two types of oil pressure gauge which you can buy to fit to the MM engine - electrical and mechanical. The electrical type involves replacing the existing oil pressure sender switch with a sensor unit, then running a wire to the dashboard-mounted oil pressure gauge and another wire to a suitable +12v supply. The mechanical type involves connecting a special fitting to replace the existing pressure switch, then running a thin pipe to the dashboard-mounted oil pressure gauge. This thin pipe is capillary tubing, which fills with pressurised oil when the engine is running. Modern gauges are supplied with hard plastic tubing (which has the advantage you can see through it and see the oil) whereas older gauges used thin metal tubing.

oil pressure sensor

One problem with fitting either type of gauge is that the existing 'oil pressure warning light' will now remain on all the time - the solution is to use a brass T-piece screwed into the engine block, which allows both the new gauge and the existing oil pressure switch to be connected in parallel - so you have two warning devices if things start to go wrong. You will find that these T-pieces have tapered threads, as do the oil pressure switch and most of these oil pressure fittings. Alternatively, just unplug the warning lamp and remove the bulb - insulate the disconnected terminal at the engine block end.

One problem with using a modern 'electrical' oil pressure gauge is that it is designed for negative earth vehicles - if your MM is still positive earth you will have to mount the gauge on an insulated panel, connect the gauge wire to earth and connect the sender wire to the body of the gauge, which is all a bit fiddly. 

Under normal conditions, a good A-series engine should have oil-pressure of 60 P.S.I. when running at 2000rpm (the oil pressure relief valve should prevent the pressure going above about 60 P.S.I.) but may drop as low as 20-25 P.S.I when warmed up and at idling speed. A lower idling reading than this may indicate some oil-pressure problem or excess engine wear.

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