The Porsche 959 was another first to use a tire pressure monitoring system (TPMS) because it was equipped with the world’s first self-supporting run-flat tires made by Bridgestone. This TPMS setup was again used on the 928 for model years 1990 to 1994.
The early system used two sensors mounted 180 degrees apart in special holes in the valley of the wheel. The sensors looked like a stack of poker chips. A diaphragm inside the sensors connected to switches that were set to a reference pressure. The receiver was mounted on the knuckle and was an inductive coil that would pass current through the sensors. Depending on the position of the diaphragm and which switches were open or closed, the amount of current flowing through the receiver’s coil would change.