Oil pressure problems

When the oil pressure light comes on, it’s time to switch off and diagnose. Alisdair Cusick shows how to find the causes

Switch off and diagnose

TOOLS NEEDED: Multimeter, 22 mm spanner, screwdriver and jumper wire

TIME: 30 mins DIFFICULTY:

MODELS: Most models

COST: £4.60 Oil pressure switch (part no. 90519864 for this SI)

WHICHEVER Land Rover you own, it will feature one warning light that we all have to pay attention to: an illuminated oil pressure light means we should stop the engine because it suggests there isn’t sufficient engine oil pressure. Low oil pressure means insufficient lubrication, and no engine will run for long without the friction-easing vital fluid being pumped around its moving parts at the correct pressure and flow rate.

The oil light is operated by a pressure switch attached to the oil circuit. When the ignition is turned on, an ignition feed is sent to one side of the warning light bulb. The other feed comes from the pressure switch mounted on the engine and is grounded to vehicle earth. As soon as the pressure drops below a specified threshold, the switch makes the circuit and sends an earth feed to the warning bulb. That is why, when we start up, the light comes on as a system check; the ignition sends power to the bulb holder, and because there is no oil pressure the switch sends a feed, illuminating the light. On cranking the engine, enough pressure builds, and the switch opens, cutting the earth feed, so the light goes out.

There are a number of reasons why the oil pressure light will come on, some serious, some simple. All that’s needed to diagnose a problem with the oil pressure light is some basic understanding, and half an hour. The important thing here is, if you suspect you have an oil pressure problem, never just run the engine to diagnose it because, if there really is no pressure, it can cause severe damage to the engine.

The most accurate way to check oil pressure is to measure it by fitting a gauge in place of the pressure switch (or in addition using a T-piece) and running the engine to get a reading. That way, if you see low pressure you will instantly know if the problem is mechanical. But the electrical circuit can also cause issues, and this is the easiest place to start.

I’m showing this work on my Series I because the simplicity of this vehicle makes it easy to follow what we’re doing. The basic workflow of diagnostics comes into play, simply checking each part of the system in turn, and confirming each is doing what it should.