Diagnostic Aids

WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 1995 Ford RV Cutaway, 1995 Ford Pickup, 1995 Ford F-Super Duty, 1995 Ford Econoline, and 1995 Ford Cutaway. However, it is still accessible from the selected car via links, so may be relevant.

Perform this test when directed by QUICK TEST or CIRCUIT TEST S. Barometric pressure sensor output is digital and must be measured using an oscilloscope or MAP/BARO tester. 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/BARO sensor.
Wiring harness circuits (VREF, MAP/BARO SIG and SIG RTN).
MAP vacuum line.
Powertrain Control Module (PCM).

Fig 1: MAP/BARO Sensor Circuit, Connector Terminal & Tester Hookup ID (7.5L)

TEST PIN NO. 26 (VREF) WIRE COLOR IDENTIFICATION

Application	Wire Color
All Models	Brown/White

TEST PIN NO. 45 (MAP/BARO SIG) WIRE COLOR IDENTIFICATION

Application	Wire Color
All Models	Light Green/Black

TEST PIN NO. 46 (SIG RTN) WIRE COLOR IDENTIFICATION

Application	Wire Color
All Models	Gray/Red

MAP SENSOR SPECIFICATIONS

Manifold Vacuum: In. Hg	Frequency: Hz
0	159
3	150
6	141
9	133
12	125
15	117
18	109
21	102
24	95
27	88
30	80

MAP/BARO SENSOR SPECIFICATIONS

Barometric Pressure: In. Hg	Frequency: Hz
17.1	122.4
18.3	125.5
19.5	128.7
20.7	131.9
21.8	135.1
23.0	138.3
24.2	141.8
25.4	145.4
26.6	148.9
27.7	152.5
28.9	156.1
30.1	159.6
31.0	162.4

1) Code 126: Check Power To MAP/BARO Sensor

Code 126 indicates MAP/BARO sensor is out of self-test voltage range (1.4-1.6 volts). Following are possible causes of this code:

Vacuum trapped at MAP/BARO sensor.
High atmospheric pressure.
MAP/BARO signal circuit open between sensor and PCM.
MAP/BARO signal circuit shorted to VREF, SIG RTN or GND.
VREF circuit open at sensor.
SIG RTN circuit open at sensor.
Faulty MAP/BARO sensor.
Faulty PCM.

Turn ignition off. Disconnect MAP/BARO sensor from wiring harness.Connect MAP/BARO tester between wiring harness and MAP/BARO 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) Check Power At Sensor Wiring Harness Connector

Disconnect MAP/BARO sensor. Turn ignition on. If Green light on tester is lit, replace MAP/BARO sensor and repeat QUICK TEST. If Green light is not on, remove MAP/BARO tester. Connect MAP/BARO sensor, and go to CIRCUIT TEST C.

3) Check MAP/BARO Sensor Output

With MAP/BARO 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/BARO tester and go to next step. See MAP SENSOR VOLTAGE OUTPUT table. If output reading is not within range, remove MAP/BARO tester and go to step 5).

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

MAP SENSOR VOLTAGE OUTPUT

Elevation (Feet)	Volts
0	1.59
1000	1.56
2000	1.53
3000	1.50
4000	1.47
5000	1.44
6000	1.41
7000	1.39

4) Check MAP/BARO SIG Circuit Continuity

Turn ignition off. Disconnect MAP/BARO sensor wiring harness connector. Disconnect PCM 60-pin connector. Inspect it for damaged wiring, and repair as necessary. Install Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Measure resistance between MAP/BARO 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/BARO SIG circuit and repeat QUICK TEST.

5) Check MAP/BARO SIG Circuit For Short To VREF, SIG RTN & Ground. Turn ignition off, and wait 10 seconds. Disconnect PCM 60-pin connector. Inspect it for damaged wiring, and repair as necessary. Install Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Disconnect wiring harness at MAP/BARO sensor. Measure resistance between test pin No. 45 and test pins No. 26, 40, 46 and 60. If any resistance is less than 10,000 ohms, repair short circuit. Repeat QUICK TEST. If all readings are 10,000 ohms or more, replace MAP/BARO 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.

7) KOER Code 126: Check For EGR-Related Codes

Code 126 indicates MAP/BARO 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/BARO sensor vacuum hose.
Faulty MAP/BARO sensor.

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

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 KOEO 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.

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/BARO sensor vacuum hose.
Faulty MAP sensor.

Repeat KOEO 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.

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.

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.

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 KOEO 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.

90) Continuous Memory Code 126

Code 126 indicates MAP/BARO sensor was out of self-test range (1.4-1.6 volts) during normal driving conditions. Possible causes for this code are:

Faulty MAP/BARO sensor.
Faulty wiring harness or connectors.
Unusually high or low barometric pressure.

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

Connect a vacuum pump to MAP/BARO 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/BARO sensor. Repeat QUICK TEST. If a fault is not indicated, go to next 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.

92) Check PCM & Wiring Harness Connectors

Turn ignition off. Disconnect PCM 60-pin connector. Inspect connectors and terminals for damage. Repair if necessary. If connectors and terminals are okay, fault cannot be duplicated at this time. Testing is complete. Refer to INTERMITTENTS in TESTS W/O CODES (EEC-IV) article in the ENGINE PERFORMANCE section.