JLG 450A_AJ Series II Service Manual - Manual (Page 186)

SECTION 3 - CHASSIS & TURNTABLE
3-136 – JLG Lift – 3121180
Intermittent Malfunction Indicator
Lamp (MIL) The following components can cause intermittent MIL and no DTC(s):
1. A defective relay, Control Module driven solenoid, or a switch that can cause electrical system interference. Normally,
the problem will occur when the faulty component is operating.
2. Improper installation of electrical devices, such as lights, 2-way radios, electric motors, etc.
3. Ignition secondary voltage shorted to a ground.
4. Malfunction Indicator Lamp (MIL) circuit or the Diagnostic Test Terminal intermittently shorted to ground.
5. Control Module grounds.
Loss of DTC Memory To check for the loss of DTC Memory:
1. Disconnect TMAP sensor.
2. Idle engine until Malfunction Indicator Lamp illuminates.
The ECM should store a TMAP DTC. The TMAP DTC should remain in memory when ignition is turned OFF. If the TMAP
DTC does not store and remain, the ECM is faulty
Additional Checks
No Start
DEFINITION: Engine cranks OK, but does not start.
Preliminary Checks Refer to Important Preliminary Checks.
Control Module Checks If a scan tool is available:
1. Check for proper communication with both the ECM
2. Check the fuse in the ECM battery power circuit. Refer to Engine Controls Schematics.
3. Check battery power, ignition power and ground circuits to the ECM. Refer to Engine Control Schematics. Verify volt-
age and/or continuity for each circuit.
Sensor Checks 1. Check TMAP sensor.
2. Check magnetic pickup sensor (RPM).
Fuel System Checks Important: A closed LPG manual fuel shut off valve will create a no start condition.
1. Check for air intake system leakage between mixer and throttle body.
2. Verify proper operation of low pressure lock-off solenoids.
3. Check fuel system pressures. Refer to LPG Fuel System Diagnosis.
4. Check for proper mixer air valve operation.
Ignition System Checks Note: LPG being a gaseous fuel requires higher secondary ignition system voltages for the equivalent gasoline operating
conditions.
1. Check proper ignition voltage output with J 26792 or equivalent.
2. Verify spark plugs are correct for use with LPG (R42LTS)
3. Check spark plugs for the following conditions:
- Wet plugs
- Cracks
- Wear
- Improper gap
- Burned electrodes
- Heavy deposits
4. Check for bare or shorted ignition wires.
5. Check for loose ignition coil connections at the coil.
Engine Mechanical Checks Important: The LPG Fuel system works on a fumigation principle of fuel introduction and is more sensitive to intake man-
ifold leakage than a gasoline fuel supply system.
Check for the following:
- Vacuum leaks
- Improper valve timing
- Low compression
- Bent pushrods
- Worn rocker arms
- Broken or weak valve springs
- Worn camshaft lobes.
Table 3-15. Symptom Diagnosis
Checks Action