What Does The Turbo/APC Gauge In My Classic Saab 900 Turbo Measure?

This gauge measures the pressure in the inlet manifold. When the engine is only lightly loaded and during engine overrun (engine braking) a depression will be present in the inlet manifold and the needle on the gauge will be within the white zone.

At higher engine speeds and when the engine is under a greater load, the turbocharger will boost the pressure in the inlet manifold and the needle will enter the yellow zone.

Under normal conditions, the turbo/APC gauge will not stray into the red zone as a safety cut-out system limits the charging pressure in order to protect the engine. Under certain atmospheric conditions, though, the needle may just enter into the red zone. This is normal and does not indicate a malfunction.

If, however, the needle repeatedly enters the red zone and is accompanied by a loss of power, due to the actions of the safety cut-out system, this is bad news. Your next stop should be a Saab garage.