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Code 44 - THERMACTOR System

  1. Remove vacuum hose from TAB valve and connect gauge to hose. Start engine and increase RPM to above 2500 to activate "Functional Test". Observe vacuum gauge. If vacuum pulses are above and below 5 in. Hg, proceed to next step. If pulses are always above 5 in. Hg, proceed to step 7). If pulses are below 5 in. Hg, proceed to next step.
  2. Reconnect hose to TAB. Disconnect hose at TAD valve and connect vacuum gauge. Start engine and raise speed above 2500 RPM to start "Functional Test". Observe vacuum readings. If above and below 5 in. Hg, go to next step. If always above 5 in. Hg, go to step 14). If pulses are always below 5 in. Hg, go to step 16).
  3. Reconnect hose to TAD valve. Remove upstream air hose at TAD valve. Start engine and raise RPM above 2500 to activate "Functional Test" and maintain RPM. Check for air at TAD nipple 20 seconds after initialization (air will last for about 6 seconds).
  4. Turn engine off and reconnect air hose. Disconnect harness from oxygen sensor and jumper harness to ground. Start engine and increase RPM to initiate self test (2 or 3 pulses on VOM connected to diagnostic connector). Maintain RPM until diagnostic codes are received (ignore any pulses longer than 1 second). If code 41 is read on voltmeter, check choke system, then go to next step. If code 44 is received, replace MCU module.
    CAUTION: For the next step, a digital VOM must be used which has an input impedance of at least 10 megohms.
  5. Place DVOM selector in lowest voltage position and connect it between oxygen sensor and ground. Start engine and run at 2000 RPM for 1 minute to warm up sensor. Turn engine off and immediately check DVOM. If voltage is less than 0.4 volts, check carburetor (too rich). If voltage is greater than 0.4 volts, go to next step.
  6. Immediately purge exhaust system. Disconnect coil "horseshoe" connector and crank engine for 10 seconds with throttle wide open. If voltage is greater than 0.4 volts, replace oxygen sensor. If voltage is less than 0.4 volts, check carburetor (too rich).
  7. Disconnect MCU connector, then connectors at TAD and TAB solenoids. Check continuity between MCU connector pin 20 and TAD solenoid, then between pin 9 and TAB solenoid. If less than 5 ohms resistance, go to next step. If greater than 5 ohms resistance is measured, repair wiring.
  8. Measure resistance of TAB solenoid. If between 50-110 ohms, go to next step. If not within 50-110 ohms, replace TAB solenoid.
  9. Check at TAB solenoid output to be sure vacuum is not present when solenoid is energized (12 volts). If vacuum is present, replace TAB solenoid. If no vacuum, replace MCU module.
  10. Check vacuum hose between TAD and TAB solenoid, then between source and TAB solenoid. Repair as necessary. If hoses are okay, go to next step.
  11. Check vacuum switch (TVS) and retard delay valve (RDV) for proper installation and operation. Check vacuum schematic for usage and location. Service valves if necessary, otherwise go to next step.
  12. Check at TAB solenoid output to be sure vacuum is present when the solenoid is energized (12 volts). If vacuum is not present, replace TAB solenoid. If vacuum is present, go to next step.
  13. Disconnect TAB solenoid connectors and MCU connector. Measure resistance between pin 20 and TAB connector, then pin 9 and TAB connector. If resistance is less than 5 ohms, replace MCU module. If higher than 5 ohms, repair circuits.
  14. Disconnect MCU connector. Measure resistance from pin 10 to ground. If greater than 1000 ohms, go to next step. If less than 1000 ohms, repair short to ground.
  15. Check at TAD solenoid to be sure vacuum is not present when solenoid is deactivated. If vacuum is present, replace TAD solenoid. If vacuum is not present, replace MCU module.
  16. Check vacuum hoses between TAD valve and TAD solenoid, then between TAD solenoid and vacuum source. Repair if necessary. If vacuum source and hoses are okay, go to next step.
  17. Check retard delay valve (RDV) for proper installation and operation. Check thermal vacuum switch (TVS) for proper installation and operation (if used on vehicle). Replace if necessary. If okay, go to next step.
  18. Measure resistance of TAD solenoid. If not between 50-110 ohms, replace solenoid. If resistance is okay, go to next step.
  19. Check at TAD solenoid to be sure vacuum is present when solenoid is energized (12 volts). If vacuum is not present, replace solenoid. If vacuum is present, go to next step.
  20. Disconnect wiring at TAD solenoid and MCU. Measure between MCU pin 20 and TAD solenoid, then between pin 10 and solenoid. If resistance is greater than 5 ohms, repair wiring. If less than 5 ohms, replace MCU module.
Fig 1: Code 44 Test Wiring Diagram
G189
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