BASIC TESTING - GASOLINE -1996 Chevrolet Tahoe
Page 1 of 3
C-4, BASIC IGNITION SYSTEM CHECKS
CIRCUIT DESCRIPTION
System includes a distributor, ignition coil, ignition control module, secondary wires, spark plugs, VCM and crank sensor. Ignition system is controlled by Vehicle Control Module (VCM). The VCM monitors the information from various engine sensors, computes the desired spark timing and controls the dwell and firing of the ignition coil via an (IC) line to the ignition control module.
ENHANCED IGNITION SYSTEM CHECK
NOTE: Before clearing DTCs, use scan tool Capture Info to save Freeze Frame and Failure Records for reference. Control module's data is deleted once Clear Info function is used.
1.Perform Powertrain On-Board Diagnostic (OBD) System Check. 2.Check spark plug wires for open circuits, cracks or improper seating of terminals at spark plugs, distributor and ignition coil before proceeding with test. Check spark at plug with J-26792 spark tester or equivalent while cranking. (If there is no spark on one wire, check a second wire). A few sparks then nothing is considered no spark. If adequate spark is present, go to HARD START diagnosis. If adequate spark is not present, go to step 3). 3.Remove coil wire from distributor cap. Insert J-26792 spark tester into the coil wire and clamp the tester onto a ground. Crank engine. If adequate spark is present, go to step 13). If adequate spark is not present, go to step 4). 4.Measure coil wire resistance using a DVOM. The resistance should be approximately 1000 ohms/inch. If resistance is within specified value, go to step 5). If resistance is not within specified value, go to step 32). 5.Disconnect ignition coil harness connector. Probe ignition coil connector terminal C with a test lamp connected to battery positive terminal. Crank engine. If test lamp is flashing while cranking engine, go to step 7). If test lamp is not flashing while cranking engine, go to step 6). 6.Turn OFF ignition. Probe ignition coil harness connector terminal C with a test lamp connected to battery positive terminal. If test lamp is illuminated, go to step 2
. If not, go to step 9). 7.Probe ignition coil harness connector terminal B with a test lamp connected to battery positive. If test lamp is illuminated, go to step 29). If not, go to step
. 8.Ignition ON, engine OFF. Measure terminal A voltage with a DVOM connected to a ground. If DVOM measures above 10V, go to step 21). If not, go to step 1
. 9.Turn OFF ignition. Disconnect ignition control module harness connector. Check for an open circuit between ignition coil harness connector terminal C and ignition control module harness connector terminal D. If circuit is open, go to step 27). If circuit is not open, go to next step. 10.Disconnect ignition control module harness connector. Ignition ON, engine OFF. Measure voltage on terminal A at ignition control module harness connector with DVOM connected to ground. If DVOM measures above 10V, go to next step. If not, go to step 19). 11.Probe ignition control module harness connector terminal "C" with a test lamp connected to battery positive terminal. If test lamp is illuminated, go to next step. If not, go to step 23). 12.Disconnect ignition control module harness connector. Probe ignition control module harness
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BASIC TESTING - GASOLINE -1996 Chevrolet Tahoe
Page 2 of 3
connector terminal B with DVOM set to AC scale connected to ground. Crank engine. Observe voltage while engine is being cranked. Voltage should be between 1-4 VAC. If voltage is as specified, go to step 20). If voltage is not as specified, go to step 15). 13.Remove distributor cap. Check cap for cracks, moisture, carbon tracks or physical damage. If any of these conditions are present, go to step 34). If cap is okay, go to next step. 14.Crank engine. Observe distributor rotor while engine is being cranked. If rotor did not turn, refer to appropriate REMOVE/INSTALL/OVERHAUL article. If rotor did turn, go to step 33). 15.Turn OFF ignition. Disconnect VCM C3 connector. Check ignition timing signal circuit for an open between the VCM C3 harness connector and ignition control module harness connector terminal B. If circuit is open, go to step 24). If circuit is okay, go to next step. 16.Probe Ignition Control (IC) circuit at VCM C3 connector with a test lamp connected to battery positive terminal. If test lamp is illuminated, go to step 25). If not, go to next step. 17.Ignition ON, engine OFF. Probe Ignition Control (IC) circuit at VCM C3 connector with a test lamp connected to ground. If test lamp is illuminated, go to step 26). If not, go to step 22). 18.Check for an open or shorted to ground ignition positive voltage circuit at terminal A of ignition coil. Repair as necessary. Go to step 37). 19.Check for an open or shorted to ground ignition positive voltage circuit at terminal A of the ignition control module. Repair as necessary. Go to step 37). 20.Check for poor ignition control module connection. If problem was found, go to step 30). If problem was not found, go to step 35). 21.Check for a poor coil connection. If problem was found, go to step 30). If problem was not found, go to step 31). 22.Check for a poor VCM connection. If problem was found, go to step 30). If problem was not found, go to step 36). 23.Repair open ignition control module ground circuit. Go to step 37). 24.Repair open IC circuit between VCM and ignition control module. Go to step 37). 25.Repair grounded IC circuit between VCM and ignition control module. Go to step 37). 26.Repair short to voltage in the IC circuit between the VCM and the ignition control module. Go to step 37). 27.Repair open in tachometer signal circuit between ignition coil and ignition control module. Go to step 37). 28.Repair short to ground in tachometer signal circuit between ignition coil and ignition control module. Go to step 37). 29.Repair short to ground in tachometer input circuit between ignition coil and instrument cluster. Go to step 37). 30.Repair circuit as necessary. Go to step 37). 31.Replace ignition coil. Refer to appropriate REMOVE/INSTALL/OVERHAUL article. Go to step 37). 32.Replace coil wire. Go to step 37). 33.Replace distributor rotor. Go to step 37). 34.Replace distributor cap. Go to step 37). 35.Replace ignition control module. Refer to appropriate REMOVE/INSTALL/OVERHAUL article. Go to step 37). 36.Replace VCM. Go to step 37).
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BASIC TESTING - GASOLINE -1996 Chevrolet Tahoe
Page 3 of 3
37.Operate vehicle within conditions under which original symptom was noted. If system is not functioning properly, repeat testing from step 1).
NOTE: Numbers below refer to the step numbers in diagnostic test.
NOTE: Battery should be fully charged prior to any tests.
Diagnostic Aids
1.Perform the OBD system check before proceeding unless it was already performed. 2.Checks for proper output from the enhanced ignition system. The spark tester requires a minimum of 25,000 volts to operate. This check can be used in case of an ignition miss, because the system may provide enough voltage to run the engine but not enough to operate a spark plug under heavy load. 3.This test separates distributor cap, rotor and ignition wires from ignition coil in order to help identify a secondary ignition system problem. 4.This test checks ignition control module, connections, and wiring. 5.This test begins to determine if the VCM is providing a signal to the ignition control module. If the VCM is not providing a signal to the ignition control module, the problem exists between the ignition control module and the VCM. 6.This test checks for a basic engine mechanical problem.
© 2007 Mitchell Repair Information Co., LLC.
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8/23/2008
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