DTC 78

Data Trouble Code No. 78 - Fuel Pump Control Signal

Detecting Condition

Open or short in fuel pump circuit for 1 second or more with engine speed 1,000 rpm or less (two trip detection).
or
Open in input circuit of Fuel Pump Electronic Control Unit with engine speed 1,000 rpm or less (two trip detection).
or
Open or short in diagnostic signal line (DI) of Fuel Pump Electronic Control Unit with engine speed of 1,000 rpm or less (two trip detection).

Trouble Area

- Open or short in Fuel Pump circuit.
- Fuel Pump Electronic Control Unit.
- Fuel Pump.
- Engine Control Module power source circuit.
- Engine Control Module.

Malfunction Indicator Lamp

Normal Mode Off
Test Mode On

Code 78 Description:

Circuit Description
The fuel pump speed is controlled at two steps (high speed, low speed) by the condition of the engine (starting, light load, heavy load), when the engine starts (STA ON), the engine control module sends a Hi signal to the fuel pump electronic control unit (FPC terminal).

The fuel pump electronic control unit then outputs Hi battery positive voltage to the fuel pump so that the fuel pump operates at high speed.

After the engine starts, during idling or light loads, the engine control module outputs a High Low signal to the fuel pump electronic control unit, the fuel pump electronic control unit outputs Lo battery positive voltage (about 9V) to the fuel pump and causes the fuel pump to operate at low speed.

If the intake air volume increases (high engine load), the engine control module sends a Hi signal to the fuel pump electronic control unit and causes the fuel pump to operate at high speed.

Code 78 Diagnostic Chart:

Diagnostic Chart

Code 78 Circuit Diagram:

Circuit Diagram

Code 78 Inspection Procedure:

Code 78 Inspection Procedure Page 2:

Inspection Procedure