JLG 680S, Service And Maintenance Manual, Page: 257 / 530

SECTION 3 - CHASSIS & TURNTABLE
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Ignition System Checks Note: LPG being a gaseous fuel requires higher secondary ignition system voltages for the equivalent gasoline operating conditions. If a prob-
lem is reported on LPG and not gasoline, do not discount the possibility of a LPG only ignition system failure and test the system accordingly.
Check for the proper ignition voltage output with J 26792 or the equivalent.
Verify that the spark plugs are correct for use with LPG (R42LTS).
Check for faulty spark plug wires.
Check for fouled spark plugs.
Additional Check Check for manifold vacuum or air induction system leaks.
Check the generator output voltage.
Backfire
DEFINITION: The fuel ignites in the intake manifold, or in the exhaust system, making a loud popping noise.
Preliminary Check Refer to Important Preliminary Checks.
Ignition System Checks Important! LPG, being a gaseous fuel, requires higher secondary ignition system voltages for the equivalent gasoline operat-
ing conditions. The ignition system must be maintained in peak condition to prevent backfire.
Check for the proper ignition coil output voltage using the spark tester J26792 or the equivalent.
Check the spark plug wires by connecting an ohmmeter to the ends of each wire in question. If the meter reads over 30,000 ohms, replace the
wires.
Check the connection at each ignition coil.
Check for deteriorated spark plug wire insulation.
Check the spark plugs. The correct spark plugs for LPG are (R42LTS).
Remove the plugs and inspect them for the following conditions:
- Wet plugs
- Cracks
- Wear
- Improper gap
- Burned electrodes
- Heavy deposits
Engine Mechanical Check Important! The LPG Fuel system works on a fumigation principle of fuel introduction and is more sensitive to intake manifold leakage than a
gasoline fuel supply system.
Check the engine for the following:
- Improper valve timing
- Engine compression
- Manifold vacuum leaks
- Intake manifold gaskets
- Sticking or leaking valves
- Exhaust system leakage
Check the intake and exhaust system for casting flash or other restrictions.
Fuel System Checks Perform a fuel system diagnosis. Refer to LPG Fuel System Diagnosis.
Lack of Power, Sluggishness, or Sponginess
DEFINITION: The engine delivers less than expected power. There is little or no increase in speed when partially applying the accelerator pedal.
Preliminary Checks Refer to Important Preliminary Checks.
Refer to the LPG Fuel system OBD System Check.
Compare the customer's vehicle with a similar unit. Make sure the customer has an actual problem. Do not compare the power output of the
vehicle operating on LPG to a vehicle operating on gasoline as the fuels do have different drive feel characteristics.
Remove the air filter and check for dirt or restriction.
Check the vehicle transmission Refer to the OEM transmission diagnostics.
Table 3-13. Symptom Diagnosis
Checks Action