Connecting Dwell Meter

1. A dwell meter set on "6-cylinder" scale for all carbureted engines is used to measure ECM output and M/C dwell (duty cycle). A green connector located in wiring harness near carburetor is the dwell meter connection point.
2. When dwell meter is connected, do not allow terminal to touch any ground source, including hoses. This could damage ECM. On a normal operating engine, dwell at both idle and part throttle should read between 10-50° and will be varying. "Varying" means needle will continually move up and down the scale. The amount of variation does not matter, only fact that it does move.
3. This variation of needle indicates system is in closed loop and ECM is responding to signals from oxygen sensor. At wide open throttle or cold engine operation, dwell will be fixed (needle steady). This indicates open loop and that ECM is ignoring oxygen sensor signals.
4. If there is doubt whether engine is in closed loop or open loop, richen or lean mixture (choke carburetor or cause air leak). This action will cause dwell to change if engine is in closed loop.

DRIVER COMPLAINT SHEET ALL EXC. EFI 

Engine performance problem (odor, surge, fuel economy, etc.) Emission problem.

If "CHECK ENGINE" lamp is not on, normal checks, that would be performed on cars without CCC should be done first.

If generator or coolant light is on with the "CHECK ENGINE" lamp, they should be diagnosed first.

Inspect for poor connections at coolant sensor, M/C solenoid, etc., and poor or loose vacuum hoses and connections. Repair as necessary.

• Intermittent "CHECK ENGINE" lamp, but no trouble code stored:
  • Check for intermittent connection in circuit from:
    • Ignition coil to ground and arcing at spark plugs or wires (all engines).
    • Battery to ECM terminals "C" and "R" (exc Fed. "T" car without 5-speed).
    • Battery to ECM terminal "11" (Fed. "T" car without 5-speed).
    • ECM terminal "1" to engine ground (Fed. "T" car without 5-speed).
    • ECM terminal "19" to distributor, including tach filter (Fed. "T" car without 5-speed).
  • Loss of long term memory (exc. Fed. "T" car without 5-speed).
    • Grounding dwell lead for 10 seconds with "test" lead ungrounded should give code 23. It should be retained after engine is stopped and ignition turned to run position. If it is not, ECM is defective.
  • EST wires (exc. Fed. "T" car without 5-speed).
    • EST wires should be kept away from spark plug wires, distributor housing, coil and generator. Wires from ECM terminal "13" to distributor and shield around EST wires should be a good ground.
• Stalling, Rough Idle or Improper Idle Speed, (exc. Fed. "T" car without 5-speed).
  • See Idle Speed Control.
• Detonation - Spark Knock (exc. Fed. "T" car without 5-speed).
  • Check ESC performance (if equipped).
  • Check MAP or vacuum sensor output.
  • Check EGR operation.
  • Check TPS enrichment operation.
  • Check HEI operation.
• Poor performance and/or Fuel Economy (exc. Fed. "T" car without 5-speed).
  • Perform EST diagnosis.
  • Perform ESC diagnosis (if equipped).
• Poor Full-Throttle Performance.
  • Perform TPS diagnosis (if equipped).
• All Other Complaints.
  • Make System Performance Check on warm engine (upper radiator hose hot).
    Fig 4: System Performance Check (All Exc. EFI & Fed. "T" Car Without 5-Speed)

    Courtesy of GENERAL MOTORS COMPANY
    Fig 5: System Performance Check (Fed. "T" Car Without 5-Speed)

    Courtesy of GENERAL MOTORS COMPANY

    DRIVER COMPLAINT SHEET (EFI MODELS) 

    Intermittent "CHECK ENGINE" lamp or stored codes.

    NOTE: Do not use diagnostic charts for intermittent problems. Fault must be present to locate the problem. If fault is intermittent, use of charts may result in replacement of non-defective parts.
  • Most intermittent problems are caused by faulty electrical connectors or wiring. Diagnosis must include a careful visual and physical inspection of the following circuit wiring and connectors:
    • Poor mating of connector halves or terminals not fully seated in connector body (backed out "terminals").
    • Improperly formed or damaged terminals. All connector terminals in problem circuit should be carefully reformed to increase contact tension.
    • HEI distributor EST wires should be routed away from distributor, ignition coil, secondary wiring and generator.
    • Circuit 419 - "CHECK ENGINE" lamp to ECM - short to ground.
    • Circuit 451 - Diagnostic connector to ECM - short to ground.
    • Circuit 450 and 450R - Check ECM ground at engine block.
    • Improper installation of electrical options.
    • Open air conditioning clutch diode.
    • Electrical system interference caused by a defective relay. ECM driven solenoid, or a switch causing a sharp electrical surge. Normally, the problem will occur when defective component is operated.
• Stalling, Rough or Improper Idle Speed.
  • See EFI Idle Air Control Chart.
• Engine Cranks But Will Not Run.
  • Perform injector test.
• Hard Starting, Poor Performance, Driveability or Fuel Economy.
  • Service fuel system.
• Detonation - Spark Knock.
  • Diagnosis ESC system.
  • Diagnosis EGR system.

  Following any repairs or adjustments, clear codes and confirm closed loop operation and no "CHECK ENGINE" lamp.

TROUBLE CODE 51 ALL MODELS 

Check that all pins are fully inserted in the socket. If OK, replace PROM, clear memory, and recheck. If code 51 reappears, replace ECM.

TROUBLE CODE 55 EFI MODELS 

Replace Electronic Control Module (ECM) 

Clear codes and confirm "Closed Loop" operation and no "Check Engine" light.