This fault code can only be caused by an internal ECM problem. Repairs are not possible for the ECM. Possible repairs: 1)Replace the ECM. Cummins ISB CM550 (1990-94) Fault Code: 2559 PID: SPN: 1845 FMI: 4 Transmission ECU to ECM Communication – Abnormal Update Rate Cummins ISB CM550 (1990-94) Fault Code: 779 PID: SID 051 SPN: 703 FMI: 11 OEM Input – Engine Protection Cummins ISB CM550 (1990-94) Fault Code: 768 PID: SID 009 SPN: 923 FMI: 11 Output Device Driver (Trransmission Shift Modulation) Cummins ISB CM550 (1990-94) Fault Code: 599 PID: SID 025 SPN: 640 FMI: 14 Auxiliary Commanded...

Circuit DescriptionThe engine speed sensor provides engine speed information to the engine control module (ECM). The sensor must be powered up by (+) 5 VDC to operate. The sensor generates its signals bysensing the movement of target teeth machined into a tone wheel that is mounted to the crankshaft. The tone wheel has 35 teeth and a gap where the 36th tooth would be placed. This missing tooth indicates cylinder 1 and 6 are at top dead center. Component LocationThe combination sensor is located in the back of the gear housing behind the injection pump on QSBengines with the Bosch...

Cummins ISB CM550 (1990-94) Fault Code: 115VP44 PID: 190 SPN: 190 FMI: 2 Engine Speed Sensor (ESS) Circuit Circuit DescriptionThe engine speed sensor provides engine speed information to the engine control module (ECM).The sensor must be powered up by (+) 5 VDC to operate. The sensor generates its signals bysensing the movement of target teeth machined into a tone wheel that is mounted to the crankshaft. The tone wheel has 35 teeth and a gap where the 36th tooth would be placed. This missing tooth indicates cylinder 1 and 6 are at top dead center. Component LocationThe engine speed sensor...

Cummins ISB CM550 (1990-94) Fault Code: 122 PID: 102 SPN: 102 FMI: 3 Intake Manifold Pressure Sensor Circuit Circuit DescriptionThe intake manifold pressure sensor monitors intake manifold pressure and passesinformation to the electronic control module (ECM) through the sensor harness. If the intakemanifold pressure becomes too high, it will cause a derate condition. Component LocationThe intake manifold pressure sensor is located on the rear of the intake air manifold in thesecond port on the side of the head to the right of the fuel filter. Shop Talk1) Determine if engine is being overfueled.2) Confirm that the correct intake manifold pressure...

Circuit DescriptionThe intake manifold pressure sensor monitors intake manifold pressure and passesinformation to the electronic control module (ECM) through the sensor harness. If the intakemanifold pressure becomes too high, it will cause a derate condition. Component LocationThe intake manifold pressure sensor is located on the rear of the intake air manifold in thesecond port on the side of the head to the right of the fuel filter. Shop Talk1) Determine if engine is being overfueled.2) Confirm that the correct intake manifold pressure sensor part number is being used.3) Confirm that the correct turbocharger is being used.4) If it is suspected...