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Home >> Lexus >> 1998 >> GS 400 >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Tests W/Codes >> Diagnostic Tests >> DTC P1200: Fuel Pump Relay/Ecu Circuit Malfunction >> Circuit Description
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Circuit Description

This system is equipped with a variable speed fuel pump. Fuel pump speed is controlled at 3 speeds (high, medium and low) depending on engine load (starting, idling, light load and heavy load).

During engine starting, heavy engine loads and high engine speed, ECM sends a hi (about 3.8 volts) signal voltage to fuel pump ECU. The fuel pump ECU then sends a hi (battery voltage) signal voltage to fuel pump.

During heavy engine loads and low engine speed, ECM sends a medium (about 2.5 volts) signal voltage to fuel pump ECU. The fuel pump ECU then sends a medium (about 10 volts) signal voltage to fuel pump.

During idling or light engine loads, ECM sends a low (about 1.3 volts) signal voltage to fuel pump ECU. The fuel pump ECU then sends a low (about 8.5 volts) signal voltage to fuel pump. DTC P1200 is set if PCM detects open or short in fuel pump circuit with engine speed of 1000 RPM or less for one second. DTC P1200 is also set if PCM detects open in fuel pump ECU input circuit (FPC) with engine speed of 1000 RPM or less. DTC P1200 is also set if PCM detects open or short in fuel pump ECU diagnostic signal circuit (DI) with engine speed of 1000 RPM or less. Possible causes are:

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