This document is a Cummins lift pump circuit troubleshooting file for Fault Code 278 on ISB CM550 engines. It explains the ECM-controlled lift pump operation, connector inspections, resistance checks, harness tests, voltage measurement, and final diagnostic-tool verification. The file is a 6-page PDF used for DTC work on the lift pump circuit.
Official Cummins lift pump circuit document details
| Brand | Cummins |
|---|---|
| Model coverage | ISB CM550 (1990-94) |
| System/Group | DTCs; Lift Pump Circuit |
| Publication no. | Not shown in the extract |
| Year/Revision | Unknown; document copywrite 2017 shown |
| Pages | 6 |
| File size | 0.4 MB |
| Format | |
| Language | English |
Document table of contents
- FAULT CODE: 278
- PID: P073
- SPN: 1075
- FMI: 11
- LIFT PUMP CIRCUIT
- DESCRIPTION
- COMPONENT LOCATION
- TROUBLESHOOTING STEPS
- Step 1A: Inspect the engine harness and lift pump connectors
- Step 1B: Check the resistance of the lift pump
- Step 1C: Check for a short-to-ground in the lift pump
- Step 2A: Inspect the engine harness and the ECM connector pins
- Step 2B: Check for an open circuit in the lift pump supply pin
- Step 2B-1: Check for an open circuit in the OEM harness
- Step 2C: Check for a short circuit to ground
- Step 2D: Check for short circuit from pin-to-pin
- Step 2E: Perform lift pump override test and measure voltage at the lift pump
- Step 3A: Disable the fault code and verify Fault Code 278 is inactive
- Step 3B: Clear the inactive fault codes
Applicable truck models
This file applies to Cummins ISB CM550 engines listed under ISB CM550 (1990-94) / DTCs. The extract does not identify a specific truck make or chassis model; it covers the engine-mounted lift pump on the intake side, toward the rear of the engine, and its ECM, engine harness, OEM harness, 16-pin connector, and 23-pin connector test points. For official product and service information, visit Cummins official website.
Who needs this Cummins lift pump circuit manual
Mechanics, diesel electricians, and workshop owners need this lift pump circuit manual when diagnosing Fault Code 278, PID P073, SPN 1075, FMI 11. It is useful when the lift pump does not pass connector, resistance, short-to-ground, open-circuit, pin-to-pin, override, or voltage checks.
The full PDF is available in the DiagTechPro manuals library, free account required to download.
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