Thermactor Air By-pass
Components controlled by the MCU system in this test are the thermactor air by-pass (TAB) valve, thermactor air valve, and MCU module. Components which are not controlled by the MCU system, but could cause code fault are: broken or loose belts, plugged restrictor, faulty hoses, and/or faulty vacuum delay valve (VDV). Anything which inhibits thermactor system from switching air "downstream" to catalytic converter should be investigated as possible cause.
- Remove vacuum hose at TAB valve and connect gauge to hose. Start engine and raise speed above 2500 RPM to start "Functional Test". Observe gauge during initial pulses. If vacuum pulses are above and below 5 in. Hg, MCU system is operating properly, check thermactor system. If vacuum pulses are always below 5 in. Hg, go to next step. If vacuum pulse are always above 5 in. Hg, go to step 4 .
- Check at TAB solenoid output to be sure vacuum is present when solenoid is energized. Disconnect harness connector at TAB solenoid and insert jumper wire from circuit 190 to ground. Start engine and run at idle. If vacuum is present, go to next step. If no vacuum is present, replace solenoid and retest.
- Disconnect harness connectors at MCU and TAB solenoid. Measure continuity of wire from MCU pin 9 to TAB solenoid connector, then measure resistance. If resistance is more than 10 ohms, repair harness and retest. If resistance is 10 ohms or less, replace MCU and retest.
- Check at TAB solenoid output to be sure vacuum is not present when solenoid is not energized. Disconnect harness connector at TAB solenoid. Start and run engine at idle. If vacuum is not present, go to next step. If vacuum is present, replace TAB solenoid and retest.
- Disconnect harness connectors at MCU and TAB solenoid. Measure resistance between ground and MCU pin 9. If resistance is 1000 ohms or less, repair short to ground and retest. If resistance is greater than 1000 ohms, replace MCU and retest.