Relative Engine Compression Test

Engine Controls and Fuel - 3.0L LM2 Diesel | Diagnostic Information and Procedures | Document ID: 5591866 Diagnostic Instructions Perform the Diagnostic System Check prior to using this diagnostic procedure: Diagnostic System Check - Vehicle Review the description of Strategy Based Diagnosis: Strategy Based Diagnosis An overview of each diagnostic category can be found here: Diagnostic Procedure Instructions Circuit/System Description The Relative Engine Compression test monitors battery voltage drop during engine cranking to determine the relative compression of all cylinders. Each compression stroke causes mechanical resistance, increasing the current draw on the battery. Starter current and battery voltage are inversely proportionate. Diagnostic Aids An engine with little to no compression variance between cylinders will have waveform valleys that are even. If the engine compression is equally low across all cylinders, the oscilloscope pattern will appear good. The total amplitude (range from highest to lowest point of the waveform) is NOT an indicator of good compression. Reference Information Component View Reference Master Electrical Component List Special Tools CH-51450-A Oscilloscope Diagnostic Kit CH-51450-LEAD-A Oscilloscope Basic Lead Kit Equivalent regional tools: Special Tools Circuit/System Testing

1. Ignition/Vehicle » Off
2. Apply » Parking Brake
3. Install » CH-51450-A Oscilloscope Diagnostic Kit & CH-51450-LEAD-A Oscilloscope Basic Lead Kit 3.1. Channel A Lead: Blue @ Oscilloscope 3.2. Channel A Lead: Blue @ B+ & Ground

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Hover the mouse cursor over an icon button to display the button’s description.

4. Coupling » DC
5. Select: ConnectDetect Verify the following information is correctly displayed: Connected Channels = Green ⇒ If the information is not correctly displayed Repair the poor connection & Repeat this step ⇓ If the information is correctly displayed
6. Setup » CH-51450-A Oscilloscope Diagnostic Kit & CH-51450-LEAD-A Oscilloscope Basic Lead Kit 6.1. Select: Automotive 6.2. Select: Charging and Starting 6.3. Select: Relative Compression 6.4. Select: Voltage
7. Disconnect the electrical connector: X160 @ Fuel Injector Wiring Harness
8. Select: Start Capturing
9. Perform the following action: 9.1. { Automatic Transmission } Apply & Hold » Brake Pedal { Manual Transmission } Apply & Hold » Clutch Pedal 9.2. Engine » Cranking — For 5–10 s 9.3. Select: Stop Capturing Verify the following information is correctly displayed: The waveform valleys are all even. Refer to: Oscilloscope Waveform Library ⇒ If the information is not correctly displayed Verify the condition does not exist with the component and repair as necessary:

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The following are examples of what could cause compression issues. Inspect all mechanical components for the applicable cylinder. The list is not all encompassing Refer to: Symptoms - Engine Mechanical Camshaft Lobe(s) — Visible Damage / Worn Exhaust Valve or Intake Valve — Stuck Open / Visible Damage Exhaust Valve Seat or Intake Valve Seat — Visible Damage Piston — Cracked / Visible Damage Piston Ring — Seized / Visible Damage / Worn Valve Lifter — Visible Damage / Worn Valve Spring — Visible Damage / Worn Valve Rocker Arm — Visible Damage / Worn ⇓ If the information is correctly displayed

10. All OK. Repair Instructions Perform the Diagnostic Repair Verification after completing the repair: Diagnostic Repair Verification