Gm Engine Symptom Diagnosis - JLG 400S Service And Maintenance Manual

SECTION 3 - CHASSIS & TURNTABLE
Checks
Before Using This Section
LPG Fuel System Check
Visual and Physical Checks
DEFINITION: The problem may or may not turn ON the Malfunction Indicator Lamp (MIL) or store a Diagnostic Trouble Code (DTC).
Preliminary Checks
Faulty Electrical Connections or Wiring
Operational Test
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Table 3-14. GM Engine Symptom Diagnosis
Important Preliminary Checks
Before using this section, you should have performed On Board Diagnostic Check and determined that:
1. The Control Module and MIL (Malfunction Indicator Lamp) are operating correctly.
2. There are no Diagnostic Trouble Codes (DTCs) stored, or a DTC exists but without a MIL.
Several of the following symptom procedures call for a careful visual and physical check. The visual and physical checks are very important. The
checks can lead to correcting a problem without further checks that may save valuable time.
1. Verify the customer complaint.
2. Locate the correct symptom table.
3. Check the items indicated under that symptom.
4. Operate the vehicle under the conditions the symptom occurs. Verify HEGO switching between lean and rich.
IMPORTANT! Normal HEGO switching indicates the LPG fuel system is in closed loop and operating correctly at that time.
² Check all ECM system fuses and circuit breakers.
² Check the ECM ground for being clean, tight and in its proper location.
² Check the vacuum hoses for splits, kinks and proper connections.
² Check thoroughly for any type of leak or restriction.
² Check for air leaks at all the mounting areas of the intake manifold sealing surfaces.
² Check for proper installation of the mixer module assembly.
² Check for air leaks at the mixer assembly.
² Check the ignition wires for the following conditions:
- Cracking
- Hardness
- Proper routing
- Carbon tracking
² Check the wiring for the following items:
- Proper connections, pin. or cuts.
² The following symptom tables contain groups of possible causes for each symptom. The order of these procedures is not important. If the scan
tool readings do not indicate the problems, then proceed in a logical order, easiest to check or most likely to cause first.
² Refer to Important Preliminary Checks.
² Do not use the DTC tables. If a fault is an intermittent, the use of the DTC tables may result in the replacement of good parts.
² Faulty electrical connections or wiring can cause most intermittent problems.
² Check the suspected circuit for the following conditions:
- Faulty fuse or circuit breaker
- Connectors poorly mated
- Terminals not fully seated in the connector (backed out)
- Terminals not properly formed or damaged
- Terminal to wires poorly connected
- Terminal tension insufficient.
² Carefully remove all the connector terminals in the problem circuit in order to ensure the proper contact tension. If necessary, replace all the
connector terminals in the problem circuit in order to ensure the proper contact tension.
² Checking for poor terminal to wire connections requires removing the terminal from the connector body.
If a visual and physical check does not locate the cause of the problem, drive the vehicle with a scan tool. When the problem occurs, an abnormal
voltage or scan reading indicates the problem may be in that circuit.
Action
Intermittent
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