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. This CIRCUIT TEST is intended to diagnose the following:

MAF sensor.
Wiring harness circuits (VPWR, PWR GND, MAF and MAF RTN).
Powertrain Control Module (PCM).

To prevent replacement of good components, be aware the following non-EEC related areas may be cause of problem:

Air cleaner element.
Inlet air duct.
Throttle body.

Code 159, retrieved during KOEO SELF-TEST, indicates voltage exceeds .7-volt test range. Code 159, retrieved during KOER SELF-TEST, indicates voltage is not within .2-1.5 volts operating range. Possible causes are faulty MAF sensor or PCM.

Fig 1: MAF Sensor Circuits & Connector Terminal ID (Black Connector)

Fig 2: MAF Sensor Circuits & Connector Terminal ID (Gray Connector)

TEST PIN NO. 9 OR 15 (MAF RTN) WIRE COLOR IDENTIFICATION

Application	Wire Color
All Models	Tan/Light Blue

TEST PIN NO. 14 OR 50 (MAF) WIRE COLOR IDENTIFICATION

Application	Wire Color
All Models	Light Blue/Red

TEST PINS NO. 37 & 57 (VPWR) WIRE COLOR IDENTIFICATION

Application	Wire Color
All Models	Red

TEST PINS NO. 40 & 60 (PWR GND) WIRE COLOR IDENTIFICATION

Application	Wire Color
7.5L	Black/White

MAF SIGNAL VOLTAGE ⁽¹⁾

Application

Volts

Idle

20 MPH

1.1

40 MPH

1.7

60 MPH

2.1

⁽¹⁾	MAF signal voltage is typical for normal operating temperature. Voltage signal may vary due to engine load and temperature.

NOTE: Code 159 may be caused by use of a garage exhaust ventilation system. Ensure vehicle is vented to outside atmosphere before repeating QUICK TEST.

1) Code 159: Check VPWR Circuit Voltage

Code 159 indicates that MAF sensor was out of range during KOEO or KOER SELF-TEST. Possible causes for this fault are:

Air leak before or after MAF sensor.
Faulty MAF sensor.
Faulty IAC solenoid.
Open or grounded wiring harness.
Faulty PCM.

Turn ignition off. Disconnect MAF sensor connector. Set DVOM on 20-volt scale. Turn ignition on. Measure voltage between VPWR terminal at MAF sensor wiring harness connector and negative battery terminal. If voltage is less than 10.5 volts, repair open VPWR circuit and repeat QUICK TEST. If voltage is 10.5 volts or more, go to next step.

2) Check MAF Sensor Ground

Turn ignition on. With MAF sensor disconnected, measure voltage between VPWR terminal and PWR GND terminal at MAF sensor connector. If voltage is less than 10.5 volts, repair open PWR GND circuit. Connect MAF sensor, and repeat QUICK TEST. If voltage is 10.5 volts or more, go to step 13).

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 2) to step 10). No test procedures have been omitted.

10) Continuous Memory Code 157: Check For Intermittent Sensor Failure

Continuous Memory Code 157 indicates MAF signal was less than 0.4 volt sometime during last 80 warm-up cycles. Possible causes for this fault are:

Open MAF circuit.
MAF circuit shorted to ground.
Faulty MAF sensor.
MAF sensor disconnected.

Start engine and allow to idle for 5-10 minutes. Perform KOEO SELF-TEST. If Code 157 is present, go to step 13). If Code 157 is not present, go to next step.

11) Turn ignition off. Disconnect and inspect 60-pin connector at PCM. Service if necessary. Install Breakout Box (T83L-50-EEC-IV), leaving PCM connected. Connect DVOM between MAF test pin and MAF RTN test pin. See appropriate table at beginning of this CIRCUIT TEST. DVOM should read about 0.4 volt. Start engine and allow to idle. Lightly tap on MAF sensor and wiggle wiring harness to simulate road conditions. If sudden change in DVOM reading occurs, fault is indicated. If fault is indicated, replace MAF sensor and repeat QUICK TEST. If no fault is indicated, leave DVOM connected and go to next step.

12) Check Wiring Harness For Open Or Short Circuit

Turn ignition on. Shake and bend small sections of wiring harness starting at MAF sensor and working toward dash panel. Shake and bend small sections of wiring harness starting at dash panel and working toward PCM. If fault is indicated, isolate and repair as necessary. If fault is not indicated, problem is intermittent and cannot be duplicated at this time.

13) Code 129 Or 157: Check For Air Leak At MAF Sensor

Check for broken or loose air outlet tube clamps at throttle body and air cleaner assembly. Repair or replace as necessary. If no faults are found, go to next step.

14) Check Continuity Of MAF & VPWR Circuit

Turn ignition off. Disconnect MAF sensor and PCM wiring harness connector. Install Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Measure resistance between MAF terminal of MAF sensor wiring harness connector and test pins No. 37 and 57. Measure resistance between MAF terminal of MAF sensor wiring harness connector and MAF test pin (No. 14 for MFI or No. 50 for SFI) at breakout box. If readings are more than 5 ohms, repair open circuit and repeat QUICK TEST. If readings are 5 ohms or less, go to next step.

15) Check MAF Signal For Short To Ground

Turn ignition off. Ensure MAF sensor and PCM wiring harness are disconnected. Measure resistance between MAF test pin and test pins No. 40, 60 and MAF RTN test pin. If resistance is less than 10,000 ohms, repair short circuit and repeat QUICK TEST. If resistance is 10,000 ohms or more, go to next step.

16) Check PRW GND Circuit

Turn ignition off. Ensure MAF sensor and PCM wiring harness connector 6are disconnected. Measure resistance between PWR GND terminal at MAF sensor wiring harness connector and negative battery terminal. If resistance is 10 ohms or more, repair open PWR GND circuit and repeat QUICK TEST. If resistance is less than 10 ohms, go to next step.

17) Check Continuity Of MAF RTN Circuit

Ensure ignition is turned off. Measure resistance between MAF RTN terminal of MAF sensor wiring harness connector and MAF RTN test pin at breakout box. If resistance is more than 5 ohms, repair open circuit and repeat QUICK TEST. If resistance is 5 ohms or less, go to next step.

18) Check MAF Circuit For Short To Ground

Turn ignition off. Ensure MAF sensor is disconnected. Connect PCM to breakout box. Measure resistance between MAF test pin and test pins No. 40, 60 and MAF RTN test pin. If resistance is less than 10,000 ohms, replace PCM and repeat QUICK TEST. If resistance is 10,000 ohms or more, go to next step.

19) Check MAF Circuit Output

Turn ignition off. Connect MAF sensor wiring harness connector. Start engine. Measure resistance between MAF test pin and negative battery terminal. If voltage is .36-1.50 volts, go to next step. If voltage is not .36-1.50 volts, replace MAF sensor and repeat QUICK TEST.

20) With engine running, measure resistance between MAF test pin and MAF RTN test pin at breakout box. If voltage is .36-1.50 volts, go to next step. If voltage is not .36-1.50 volts, replace MAF sensor and repeat QUICK TEST.

21) Check For Other Trouble Codes

Check if Code 411 or 412 is also present with Code 129 or 159. If Code 411 is also present , go to step 15) in CIRCUIT TEST KE. If Code 412 is also present, go to step 1) in CIRCUIT TEST KE. If no other codes are present, replace PCM. Remove breakout box, reconnect all components, and repeat QUICK TEST.

22) Code 158: Repeat KOEO SELF-TEST

Turn ignition off. Disconnect MAF sensor wiring harness connector. Start engine, and allow it to idle for one 5-10 minutes. Raise engine speed to 1500 RPM for 10 seconds. Turn ignition off. Repeat KOEO SELF-TEST. If Code 157 is present, replace MAF sensor and repeat QUICK TEST. If Code 157 is not present, go to next step.

23) Check MAF Circuit For Short To VPWR

Turn ignition off. Disconnect PCM 60-pin connector. Inspect connector for damaged pins, corrosion and loose wires. Repair as necessary. Measure resistance between MAF and VPWR terminals at MAF sensor wiring harness connector. If resistance is 10,000 ohms or more, replace PCM. Remove breakout box, reconnect all components, and repeat QUICK TEST. If resistance is less than 10,000 ohms, repair short circuit. Remove breakout box, reconnect all components, and repeat QUICK TEST. Remove breakout box, reconnect all components, and repeat QUICK TEST.