Diagnostic Aids

WARNING: This page is about a different car, the 1995 Mercury Villager. However, it is still accessible from the selected car via links, so may be relevant.

Perform this test when directed by QUICK TEST procedure. This CIRCUIT TEST is intended to diagnose:

Wiring harness circuit (MAF+, MAF- and VPWR).

Fig 1: Identifying MAF Circuit & Connector Terminals

MAF CIRCUIT INFORMATION

Circuit

PCM Pin No.

⁽¹⁾ BOB Pin No.

Wire Color

MAF-

Orange/Blue

MAF+

White/Blue

VPWR

Red/Black

VPWR

Red/Black

VPWR

109

Red/Black

⁽¹⁾	Rotunda Breakout Box (007-00033).

1) Check Power To MAF Turn ignition off. Disconnect MAF wiring harness connector. Turn ignition on. Measure voltage between harness connector VPWR (Red/Black wire) terminal and ground. If voltage is less than 10 volts, go to CIRCUIT TEST VPWR. If voltage is 10 volts or more, go to next step.

2) Check MAF- Circuit Turn ignition off. Leave MAF wiring harness connector disconnected. Install Breakout Box (007-00033), leaving PCM disconnected. Measure resistance between breakout box test pin No. 39 and MAF- terminal of MAF harness connector. Resistance should be 5 ohms or less. Measure resistance between breakout box test pin No. 39 and ground. Resistance should be 10,000 ohms or more. If resistance is as specified, go to next step. If resistance is not as specified, repair or replace circuit as necessary.

3) Check MAF Sensor Turn ignition off and allow engine to cool. Disconnect MAF sensor wiring harness connector. Connect a jumper wire between VPWR terminal of harness connector and VPWR terminal at MAF sensor. Connect a jumper wire between MAF- terminal of harness connector and MAF- terminal at MAF sensor. Turn ignition on. Measure voltage between MAF+ terminal at MAF sensor and ground. Start engine and warm to operating temperature. With engine idling, repeat previous voltage measurement. Voltage should be about 0.7 volt when engine is cool (ignition on) and about 1.7 volts with engine warm and idling. If voltage is as specified, go to next step. If voltage is not as specified, replace MAF sensor.

4) Check MAF+ Circuit Turn ignition off. Leave MAF sensor and PCM wiring harness connectors disconnected. Measure resistance between breakout box test pin No. 50 and MAF+ terminal of MAF harness connector. Resistance should be 5 ohms or less. Measure resistance between breakout box test pin No. 50 and ground. Resistance should be 10,000 ohms or more. If resistance is not as specified, repair MAF+ circuit. If resistance is as specified, system is okay. Repeat QUICK TEST. If Code 12 is still present, replace PCM.