Article on The Internet Mini Encyclopædia
The Metro Turbo had the fuel pump controlled indirectly by the oil pressure switch.
The engine runs for approximately 30 seconds after oil pressure is lost.
Pete Kay tries to describe it without a diagram but with reference to the Haynes Metro to 1990 Manual Page 12.30 ("Diagram 12 - Wiring diagram for 1987-on models with dim-dip lighting and Turbo Ignition"):
When the ignition key is turned to 'Start', a direct supply is fed to the Fuel Pump from the Auxilliary (White/Purple) contact of the Pre-engaged type Starter Motor, by-passing the ballast resistor normally in circuit when the pump is running. The pump runs whilst the switch is held in the 'Start' position and thus fills the carb bowl.
When the key is released, battery supply is instead now routed to one end of the coil of the Fuel Pump Relay, located behind the dash, which has its other end connected to the normally open contact of the Oil Pressure Switch. Until oil pressure builds up, the oil pressure switch remains open.
When oil pressure reaches approximately 5psi, the switch closes, energising the Fuel Pump Relay, whose single contact feeds battery positive to the Fuel Pump via the Pump Ballast Resistor, which limits the current to the pump when running continuously.
This latter supply to the Fuel Pump Relay contact is fused by an inline fuse from the fuse box (White/Green wire). The Oil Pressure Warning Lamp is connected between the switched ignition supply and the battery feed from the Fuel Pump Relay contact to the Pump Ballast Resistor thus, until the Fuel Pump Relay energises, the lamp has one side connected to Ignition Positive and its other side to ground via the ballast resistor and fuel pump motor and will therefore light.
When the Fuel Pump Relay energises, the lamp has a positive voltage at both sides and will therefore extinguish. Therefore, if the supply from the Fuel Pump Relay contact is not present, the Oil Pressure Lamp will stay lit, even with good oil pressure on the gauge! This occurs when the inline fuse, mentioned previously, gets corroded. The engine starts from the fuel in the carb bowl and the engine then runs until the bowl is empty and then stops.
Turning the key to 'Start' again fills the bowl and the engine again runs for approximately 30 seconds. This can be repeated ad infinitum and without an understanding of the circuit causes much frustration!
The ERA Mini Turbo uses an identical circuit and all transplants with electrical fuel pumps should, for safety, be wired similarly! It means that if the engine stops, the opening of the oil pressure switch drops out the Pump Relay and stops fuel being pumped.
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