Cummins ISB CM550 (1990-94) Fault Code: 391 PID: SID 017 SPN: 632 FMI: 11 Fuel Pump Relay Coil Supply Circuit

Circuit Description
The fuel pump relay is used to supply voltage to the fuel pump. The engine control module
(ECM) can shut down the engine by cutting off the power to the fuel shutoff relay circuit.
Component Location
This component is supplied by the equipment manufacturer. Refer to the original equipment
manufacturer (OEM) service manual for component location.

Shop Talk
1) Inspect the fuel pump relay circuit for external wires that are spliced in to power another
device. Remove any extra wires that are found in the circuit.
2) If there is an external shutdown system on the vehicle that uses the fuel shutoff relay
circuit for engine shutdown, make sure it has not failed and pulled down the voltage on
the fuel shutoff circuit.
3) Inspect the engine block to chassis ground wire to make sure it is securely fastened to a
clean, unpainted, dry surface.
4) Check the starter solenoid (+) terminal for a loose connector or accessory wiring with
damaged insulation.
5) High resistance in the fuel pump power supply circuit can result in active Fault Code 364
and an engine that will not start. The troubleshooting steps for Fault Code 391 apply to
other symptoms and fault codes that are voltage-dependent.
6) Fault Code 364 may be active in conjunction with this fault. Follow Fault Code 391tree
first.