Cummins ISB CM550 (1990-94) Fault Code: 779 PID: SID 051 SPN: 703 FMI: 11 OEM Input – Engine Protection

Circuit Description
The OEM device input to engine protection allows vehicle OEMs to wire up a device, such as a
hydraulic oil temperature limit switch, transmission temperature limit switch, etc., to the ECM.
Some units have been upfitted with a Fuel Pressure Switch. The ECM will monitor values from
this device the same as other engine protection inputs, such as coolant level, etc.. If a
threshold value is exceeded, engine speed derate will occur and possible engine shutdown.

Component Location
The location of the OEM device is dependent upon the OEM. If the unit has a fuel pressure
switch installed, it is located at the fuel filter outlet.

Shop Talk
The fuel pressure switch is normally closed and grounds through the fuel filter housing. The
switch actuates (opens) at 24 kPa [3.5 psi]. Engine speed must be above 500 rpm before the switch input is recognized by the ECM. The switch is wired to pin “U” of the 23-pin connector
which goes to pin 15 of the ECM connector.

1) Sterling and Ford applications use a 16-pin OEM connector instead of a 23-pin. In
this case the fuel pressure switch is wired directly to pin 15 of the ECM connector.

Fault Code 779 will log anytime pin 15 of the ECM connector has continuity to ground when
the engine is above 500 rpm.