Diagnostic Aids

Perform this test when directed by QUICK TEST  or CIRCUIT TEST S. To prevent replacement of good components, be aware the following non-EEC related areas may be cause of problem:

• Unusually high or low atmospheric barometric pressure.
• Kinked or blocked vacuum lines.
• Engine mechanical condition (compression, timing, etc.).

This test is intended to diagnose the following:

• MAP sensor.
• Wiring harness circuits (VREF, MAP SIG and SIG RTN).
• MAP vacuum line.
• Powertrain Control Module (PCM).

Fig 1: MAP Sensor Circuits, Connector Terminals & Tester Hookup

1. Step 1) Code 126: Check Power To MAP Sensor  Code 126 indicates MAP sensor is out of self-test voltage range (1.4-1.6 volts). Following are possible causes of this code:
   • Vacuum trapped at MAP sensor.
   • MAP signal circuit open between sensor and PCM.
   • MAP signal circuit shorted to VREF, SIG RTN or GND.
   • VREF circuit open at sensor.
   • SIG RTN circuit open at sensor.
   • Faulty MAP sensor.
   • Faulty PCM.

   Turn ignition off. Disconnect MAP sensor from wiring harness. Connect MAP tester between wiring harness and MAP sensor. Connect banana plugs of tester into DVOM. Set DVOM on 20-volt scale. Turn ignition on. If Red light or no light is on, VREF is out of range. Go to next step. If Green light is on, VREF is okay. Go to step 3).

2. Step 2) Check Power At Sensor Wiring Harness Connector  Disconnect MAP sensor. Turn ignition on. If Green light on tester is lit, replace MAP sensor and repeat QUICK TEST  . If Green light is not on, remove MAP tester. Connect MAP sensor, and go to CIRCUIT TEST C.
3. Step 3) Check MAP Sensor Output  With MAP tester connected and ignition on, measure sensor output voltage. If output voltage is within range for altitude in which vehicle is being tested, remove MAP tester and go to next step. See MAP SENSOR VOLTAGE OUTPUT  table. If output reading is not within range, remove MAP tester and go to step 5).

If possible, measure output voltage of several known-good MAP sensors on available vehicles. Average voltage reading will be typical for location and time of testing.

1. Step 4) Check MAP SIG Circuit Continuity  Turn ignition off. Disconnect MAP sensor wiring harness connector. Disconnect PCM 60-pin connector, and inspect for damaged pins, corrosion and loose wires. Repair as necessary. Install Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Measure resistance between MAP SIG terminal at sensor wiring harness connector and test pin No. 45. If resistance is less than 5 ohms, replace PCM and repeat QUICK TEST  . If resistance is 5 ohms or more, repair open MAP SIG circuit and repeat QUICK TEST.
2. Step 5) Check MAP SIG Circuit For Short To VREF, SIG RTN & Ground  Turn ignition off. Disconnect PCM 60-pin connector, and inspect for damaged pins, corrosion and loose wires. Repair as necessary. Install Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Disconnect wiring harness at MAP sensor. Measure resistance between test pin No. 45 and test pins No. 26, 40, 46 and 60 at breakout box. If any reading is less than 10,000 ohms, repair short circuit. Repeat QUICK TEST  . If all readings are 10,000 ohms or more, replace MAP sensor. Repeat QUICK TEST.
   NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 5) to step 7). No test procedures have been omitted.
3. Step 7) KOER Code 126: Check For EGR-Related Codes  Code 126 indicates MAP signal is out of self-test range during KOER SELF-TEST  . Possible causes for this code are:
   • Excess EGR flow.
   • Damaged or mis-routed MAP sensor vacuum hose.
   • Faulty MAP sensor.

   If Code 326, 327, 328, 332, 334, 336 or 337 is present, perform applicable CIRCUIT TEST. See DIAGNOSTIC TROUBLE CODE REFERENCE CHART  . If codes are not present, go to next step.

4. Step 8) Check MAP Sensor Operation  Turn ignition off. Disconnect vacuum hose from MAP sensor. Connect vacuum pump to MAP sensor, and apply 18 in. Hg. If MAP sensor does not hold vacuum, replace sensor and repeat QUICK TEST  . If MAP sensor holds vacuum, release vacuum and go to next step.
5. Step 9) Attempt To Eliminate Code 126  Plug MAP sensor vacuum supply hose. Start engine, and run it at 1400-1600 RPM. Slowly apply 15 in. Hg to MAP sensor. Perform KOER SELF-TEST  . Check for Code 126. Disregard any other codes at this time. If Code 126 is still present, replace MAP sensor. If Code 126 is not present, inspect MAP sensor vacuum supply hose. If hose is okay, service other codes at this time. If no other code is present, check engine mechanical condition for cause of low vacuum.
6. Step 10) Code 129: Repeat Dynamic Response Test  Code 129 indicates MAP sensor output did not change enough during dynamic response test. Possible causes for this code are:
   • System failed to detect Wide Open Throttle (WOT).
   • Damaged or mis-routed MAP sensor vacuum hose.
   • Faulty MAP sensor.

   Repeat KOER SELF-TEST  . Ensure complete WOT is performed during dynamic response portion of test. If KOER Code 129 is still present, go to next step. If code is not present, service any other codes as necessary.

7. Step 11) Continuous Memory Code 128  Continuous Memory Code 128 indicates MAP sensor vacuum has not changed more than 2 in. Hg during normal vehicle operation. Possible causes for this code are:
   • MAP sensor vacuum hose is improperly routed, blocked or is leaking.
   • MAP sensor is leaking.

   Check vacuum lines for correct routing. Refer to Vehicle Emission Control Identification (VECI) decal. Check for loose connections, kinks and blockage. Repair vacuum lines as necessary. If vacuum lines are okay, go to next step.

8. Step 12) Check MAP Sensor Operation  Turn ignition off. Disconnect vacuum supply hose from MAP sensor. Attach vacuum pump to MAP sensor. Apply 18 in. Hg to MAP sensor. If MAP sensor holds vacuum, release vacuum and go to next step. If sensor does not hold vacuum, replace MAP sensor and repeat QUICK TEST  .
9. Step 13) Verify Vacuum To MAP Sensor Decreases During Dynamic Response Test  Turn ignition off. Use a "T" fitting to install a vacuum gauge in MAP sensor vacuum hose. Perform KOER SELF-TEST  while observing vacuum gauge. If vacuum decreases by more than 10 in. Hg during dynamic response test, replace MAP sensor and repeat QUICK TEST  . If vacuum does not decrease by more than 10 in. Hg, EEC-IV system is okay. Check for engine mechanical problems affecting engine vacuum.
   NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 13) to step 90). No test procedures have been omitted.
10. Step 90) Continuous Memory Code 126  Code 126 indicates MAP sensor was out of self-test range (1.4-1.6 volts) during normal driving conditions. Possible causes for this code are:
    • Faulty MAP sensor.
    • Faulty wiring harness or connectors.

    Enter CONTINUOUS MONITOR MODE (WIGGLE TEST)  . Observe test equipment while performing following:

    • Connect a vacuum pump to MAP sensor.
    • Slowly apply 25 in. Hg to sensor.
    • Slowly bleed off vacuum from sensor.
    • Lightly tap on sensor (to simulate road shock).
    • Wiggle sensor connector.

    If a fault is indicated, disconnect sensor and inspect connectors. Repair if necessary. If connectors are okay, replace MAP sensor. Repeat QUICK TEST  . If a fault is not indicated, go to next step.

11. Step 91) Check EEC-IV Wiring Harness  Stay in wiggle test. Observe analog VOM or scan tester for indication of fault while bending or wiggling small sections of harness from sensor connector to firewall, and from firewall to PCM. If fault is indicated, isolate fault in harness and repair as necessary. Repeat QUICK TEST  . If no fault is indicated, go to next step.
12. Step 92) Check PCM & Wiring Harness Connectors  Turn ignition off. Disconnect PCM 60-pin connector, and inspect for damaged pins, corrosion and loose wires. Repair as necessary. If connectors and terminals are okay, fault cannot be duplicated at this time. Testing is complete. Refer to INTERMITTENTS in the TESTS W/O CODES - 7.5L EEC-IV article.