Pressure gauge for Volkswagen Corrado

What a pressure gauge reveals that the warning light does not

The oil pressure warning light is not an indicator, it is a threshold alarm: it comes on when the pressure drops below a very low level, in other words, when the damage has already been done. Between normal pressure and the alarm threshold lies the entire margin within which the health of an engine is determined, and the warning light reveals nothing about this.

A pressure gauge shows this range. It allows you to see that the pressure when the engine is hot drops year on year, that it plummets when the engine is idling whilst hot, or that it takes too long to build up when starting. Each of these signs indicates wear and tear and gives you time to take action. On an older engine, this is the instrument that prevents breakdowns, which is why they were routinely fitted to tuned cars.

The pressure gauge is useless without its sensor

An electric pressure gauge is simply a graduated galvanometer: it is the sensor screwed onto the engine that takes the reading by varying a resistance. The two form a pair and must be matched.

• The measurement range, expressed in bars for pressure, must be identical on both the pressure gauge and the sensor. A sensor with a different range will give a false reading across the entire scale, with no indication that this is the case.
• The resistance curve must match: two manufacturers do not necessarily use the same one, and mixing brands is a common cause of the needle reading all sorts of things.
• The probe’sthread must match the hole in the engine block or sump, and there are several thread pitches depending on the installation. Adapters are available, but you’ll need to make provision for them.
• Number of terminals: a two-terminal sensor also powers the warning light, allowing both systems to be retained.

Which parameters to monitor

• Oil pressure first and foremost. This is the parameter that provides the earliest warning of engine wear and the one where any deviation is most clearly visible.
• Next,oil temperature, particularly on an air-cooled engine, where it is the only available thermal indicator. On a water-cooled engine, it usefully complements the coolant temperature, as the oil reaches higher temperatures and cools more slowly.
• Battery voltage, which immediately reveals a faulty charge, well before starting becomes difficult.
• Fuel level, for which vintage gauges are often unreliable and can be recalibrated using a dial and a voltage stabiliser.

If you’re only fitting one gauge, go for the oil pressure gauge. If you’re fitting two, add the oil temperature gauge.

Fitting without drilling into the dashboard

On a classic car, drilling into the dashboard is irreversible and reduces the vehicle’s value. There are several ways to avoid this: brackets that attach to an existing air vent, brackets integrated into a car radio cover, under-dash consoles, or instrument slots left empty as standard on basic trim levels.

On the electrical side, ensure you have a dedicated earth, a fuse-protected power supply that activates when the ignition is on, and the dial lighting connected to the sidelights so that it comes on with the rest. Route the sensor wire away from the ignition cables, as these can interfere with the reading. Finally, ensure the dials complement one another: a modern instrument next to a vintage speedometer stands out far more than a dial chosen in the same style.