Diagnostic Aids

WARNING: This page does not describe the selected car, but rather 26 other vehicles, including the 1999 Mercury Tracer, 1999 Mercury Sable, 1999 Mercury Mystique, 1999 Mercury Mountaineer, and 1999 Mercury Grand Marquis. However, it is still accessible from the selected car via links, so may be relevant.

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

Barometric Pressure (BARO) sensor.
Wiring harness circuits (BARO SIG, SIG RTN and VREF).
Powertrain Control Module (PCM).

1) DTC P0106, P0107, P0108 Or P0109

Turn ignition on. Using scan tool, select VREF PID from PID/DATA monitor menu. If PID voltage is 4-6 volts, go to next step. If PID voltage is not 4-6 volts, replace PCM.

2) Check BARO PID

Using scan tool, select BARO PID from PID/DATA monitor menu. Using a vacuum pump, apply vacuum to BARO vent. Compare PID readings with table. See BARO SENSOR SPECIFICATIONS table. If PID readings are not as specified, go to step 8 (for DTC P0108) or go to next step (all others).

BARO SENSOR SPECIFICATIONS

In. Hg	Volts	Frequency (Hz)
17.1	2.45	122.4
18.3	2.65	125.5
19.5	2.85	128.7
20.7	3.05	131.9
21.8	3.25	135.1
23.0	3.45	138.3
24.2	3.65	141.8
25.4	3.85	145.4
26.6	4.05	148.9
27.7	4.25	152.5
28.9	4.45	156.1
30.1	4.65	159.6
31.0	4.80	162.4

3) Check VREF Voltage To BARO Sensor

Disconnect BARO sensor connector. Measure voltage between VREF and STG RTN terminals at BARO sensor wiring harness connector. See Fig 1. If voltage is 4-6 volts, go to step 5. If voltage is not 4-6 volts, go to next step.

Fig 1: Identifying BARO Sensor Circuits & Connector Terminals
Courtesy of FORD MOTOR CO.

4) Check For Open Circuit

Turn ignition off. Measure resistance between PCM connector pin No. 63 and BARO terminal at BARO sensor wiring harness connector. Also measure resistance between PCM connector pin No. 91 and SIG RTN terminal at BARO sensor wiring harness connector. If both resistance readings are less than 5 ohms, go to next step. If any resistance reading is 5 ohms or more, repair open circuit.

5) Check For Open VREF Circuit

Measure resistance between PCM connector pin No. 90 and VREF terminal at BARO sensor wiring harness connector. If resistance is less than 5 ohms, go to next step. If resistance is 5 ohms or more, replace open circuit.

6) Check BARO Circuit For Short To Ground

Measure resistance between PCM connector pins No. 63 and 91. Also measure resistance between PCM connector pin No. 63 and negative battery terminal. If both resistance readings are more than 10,000 ohms, go to next step. If any resistance reading is 10,000 ohms or less, repair short circuit.

7) Check VREF Circuit For Short To Ground

Measure resistance between PCM connector pins No. 90 and 91. Also measure resistance between PCM connector pin No. 90 and negative battery terminal. If both resistance readings are more than 10,000 ohms, replace PCM. If any resistance reading is 10,000 ohms or less, repair short circuit.

8) DTC P0108: Induce Opposite Code

Disconnect BARO sensor connector. Turn ignition on. Check for DTCs. If DTC P0107 is present, replace BARO sensor. If DTC P0107 is not present, go to next step.