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Home >> Ford >> 1988 >> LTD Crown Victoria Base, 4D Wagon >> Repair and Diagnosis >> Engine Performance >> System >> V8 MCU Control System >> Diagnosis & Testing >> Non-Code Tests >> No Self-Test Output
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No Self-Test Output

Disconnect jumper wire from Self-Test Trigger in Self-Test connector.

  1. Disconnect harness connector at MCU. Connect a voltmeter between Pin No.s 14 and 20 in harness connector. With ignition on, 10.5 volts or more should be present. If so, go to step 3). If not, go to next step.
  2. Connect voltmeter between Pin No. 20 and ground. If voltage is more than 10.5 volts, repair circuit from Pin No. 14 to ground, and re-test. If not 10.5 volts, repair circuit from Pin No. 20 to battery, and re-test.
  3. With ignition off, check continuity of circuit from Self-Test connector (Blk/Lt. Grn) to ground. If resistance is less than 1K ohms, go to next step. If greater, repair circuit.
  4. Check continuity between MCU harness connector Pin No. 19 and Self-Test Trigger socket (Tan/Red). If resistance is less than 5 ohms, go to next step. If greater than 1K ohms, repair circuit, and re-test.
  5. Check continuity between harness connector Pin No. 8 and "Tach Test" lead at coil "horseshoe" connector (remove connector from coil). If resistance is 5 ohms or less, go to next step. If greater than 5 ohms, repair circuit, and re-test.
  6. Disconnect harness connector at canister purge solenoid. Check continuity between MCU connector Pin No. 9 and Gry/Yel wire at canister purge connector. Also check between Pin No. 20 and Gry/Yel Stripe wire. If resistance of each circuit is less than 5 ohms, go to next step. If greater than 5 ohms, repair circuit, and re-test.
  7. Connect ohmmeter across canister purge solenoid. Resistance should be 50-100 ohms. If not, replace solenoid. Retest. If no service code is present, replace MCU. If resistance meets specifications, go to next step.
  8. Connect ohmmeter between engine ground and Gry/Yel harness wire at canister purge solenoid. If resistance is less than 1K ohms, go to next step. If greater than 1K ohms, repair short circuit to ground, and re-test.
  9. Check to ensure canister purge solenoid passes vacuum when energized with 12 volts, and blocks vacuum when de-energized. Check housing for leaks. If solenoid is okay, replace MCU, and re-test. If not, replace solenoid and re-test.
Fig 1: No Self-Test Output Test Connections
G5987
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