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Open Loop-1 Test

NOTE: If alternate source of vacuum was used for Open Loop-1 Check, begin test with second step.
  1. Measure vacuum at hose leading to yellow vacuum switch with engine running and coolant temperature below 100°, F. A vacuum of 5 in. Hg or more should be indicated. If vacuum is below 5 in. Hg, repair vacuum leak or replace CTO switch.
  2. With ignition "OFF", disconnect 6-wire connector from MCU and measure resistance from harness side terminal 6 to ground. It should have infinite resistance. If okay, proceed to step  4. If not, continue to step 3) 
  3. Disconnect vacuum switch electrical connector and measure resistance from terminal 6 (vacuum switch side of connector) to ground. If resistance is infinite, repair short in harness. If continuity is indicated, replace shorted vacuum switch assembly and reconnect harness.
  4. Apply and hold 5 to 10 in. Hg vacuum to yellow vacuum switch. Perform continuity test from harness side terminal 6 to ground. There should be continuity. If okay, replace MCU. If not okay, continue to step 5)  .
  5. Disconnect 4-wire connector and test switch side of terminal 6 for continuity to ground. If there is continuity to ground, repair open in harness between 4-wire connector and 6-wire connector. If not, replace vacuum switch assembly and reconnect harness connectors.
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