JLG 680S, Service And Maintenance Manual

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SECTION 3 - CHASSIS & TURNTABLE
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Additional Checks
Cuts Out, Misses
DEFINITION: A surging or jerking that follows engine speed, usually more pronounced as the engine load increases which is not normally felt above 1500 RPM. The exhaust has a steady spit-
ting sound at idle, low speed, or hard acceleration for the fuel starvation that can cause the engine to cut-out.
Preliminary Checks Refer to Important Preliminary Checks.
Ignition System Checks Start the engine.
Wet down the secondary ignition system with water from a spray bottle, and look/listen for arcing or misfiring as you apply water.
Check for proper ignition output voltage with spark tester J 26792.
Check for a cylinder misfire.
Verify that the spark plugs are correct for use with LPG (R42LTS)
Remove the spark plugs in these cylinders and check for the following conditions:
Insulation cracks
Wear
Improper gap
Burned electrodes
Heavy deposits
Visually/Physically inspect the secondary ignition for the following:
Ignition wires for arcing, cross-firing and proper routing
Ignition coils for cracks or carbon tracking
Engine Mechanical Checks Perform a cylinder compression check.
Check the engine for the following:
- Improper valve timing
- Bent pushrods
- Worn rocker arms
- Worn camshaft lobes
- Broken or weak valve springs
Check the intake and exhaust manifold passages for casting flash.
Fuel System Checks Check the fuel system - plugged fuel filter, low fuel pressure, etc. Refer to LPG Fuel System Diagnosis.
Check the condition of the wiring to the low pressure lock-off solenoid.
Additional Check Check for Electromagnetic Interference (EMI).
EMI on the reference circuit can cause a missing condition.
Monitoring the engine RPM with a scan tool can detect an EMI.
A sudden increase in the RPM with little change in the actual engine RPM, indicates EMI is present.
If the problem exists, check the routing of the secondary wires and the ground circuit.
Hesitation, Sag, Stumble
DEFINITION: The vehicle has a momentary lack of response when depressing the accelerator. The condition can occur at any vehicle speed. The condition may cause the engine to stall if it's
severe enough.
Preliminary Checks Refer to Important Preliminary Checks.
Fuel System Checks Check the fuel pressure. Refer to LPG Fuel System Diagnosis.
Check for low fuel pressure during a moderate or full throttle acceleration. If the fuel pressure drops below specification, there is possibly a
faulty low pressure regulator or a restriction in the fuel system.
Check the Manifold Absolute Pressure (MAP) sensor response and accuracy.
Check LPL electrical connection.
Check the mixer air valve for sticking or binding.
Check the mixer module assembly for proper installation and leakage.
Check the EPR electrical connections.
Table 3-13. Symptom Diagnosis
Checks Action