Overboost Switch: Description and Operation

Turbo Over-Boost Pressure Switch Located At Coolant Tank:

Turbo Over-Boost Pressure Switch Located Above Brake Pedal:

The turbo over-boost pressure switch is mounted to the coolant recovery tank or above the brake pedal in the passenger compartment.
The turbo over-boost pressure switch, which uses the same electrical circuit as the suppressor relay, is grounded through the fuel injection ECU. This enables the ECU or the switch to interrupt the operation of the fuel pumps and injectors if needed.
The turbo over-boost pressure switch is connected to the intake manifold via a pressure hose. This hose will transmit the intake manifold pressure to the pressure switch at all times.
If the turbo boost pressure (measured at the intake manifold) exceeds 85 - 95 kPa (12.1 - 13.5 psi), the turbo over-boost pressure switch will interrupt the electrical circuit (as long as the ECU supplies the ground connection), therefore interrupting operation of the fuel pumps and injectors. This will limit the maximum turbo boost pressure if a mechanical failure in the turbo boost control system should occur.