Powertrain Controls - ECM/PCM

DTC 34 MAP Sensor Diagnosis (Voltage Low - Vacuum High):

Manifold Absolute Pressure Sensor Wiring Circuit:

CIRCUIT DESCRIPTION:
The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). The PCM receives this information as a signal voltage that will vary from about 1-1.5 volts at idle to 4-4.5 volts at Wide Open Throttle (WOT). Signals will vary depending on altitude.

If the MAP sensor fails the PCM will substitute a fixed MAP value and use the Throttle Position (TP) sensor to control fuel delivery.

DTC 34 WILL SET WHEN:
No current DTC 21 is present, engine speed is less than 1200 RPM, MAP is less than 14 kPa or no current DTC 21 present, engine speed greater than 1200 RPM, MAP is less than 14 kPa, TP angle is greater than 15%.

ACTION TAKEN (PCM WILL DEFAULT TO):
A default table of MAP values will be used by the PCM to calculate air flow. The MIL will become illuminated and MAP values on the Tech 1 scan tool will become minimum (11 kPa).

DTC 34 WILL CLEAR WHEN:
A current DTC 34 will clear when the MAP value exceeds the minimum low MAP threshold value. A history DTC 34 will clear after 50 consecutive ignition key cycles without a current DTC 34 being stored.

DTC CHART TEST DESCRIPTION:
Number(s) below refer to circled number(s) on the diagnostic chart.
1. This test is to see if the sensor is at fault for the low voltage or if there is a PCM or wiring problem.
2. If the PCM recognizes the high MAP signal, the PCM and wiring are OK.
3. The scan tool may not display 12 volts. The important thing is that the PCM recognizes the voltage as more than 4 volts, indicating that the PCM and CKT 432 are OK.

DIAGNOSTIC AIDS:
An intermittent open in CKT 432 or CKT 416 will result in a DTC 34.

With the ignition "ON," and the engine stopped, the manifold pressure is equal to atmospheric pressure and the signal voltage will be high. This information is used by the PCM as an indication of vehicle altitude and is referred to as BARO. Comparison of this BARO reading with a known good vehicle with the same sensor is a good way to check accuracy of a "suspect" sensor. Reading should be the same +/- 0.4 volt.

Also CHART C-1D can be used to test the MAP sensor. If MAP sensor is suspected to be faulty, refer to CHART C-1D. Chart C-1D Manifold Absolute Pressure (MAP) Output Check

Refer to "Intermittents" in "Symptoms." - Intermittent Malfunctions