Powertrain Controls - ECM/PCM

Code 34 Chart:

Code 34 Wiring Diagram:

CIRCUIT DESCRIPTION:

The manifold absolute pressure (MAP) sensor responds to changes in manifold pressure (vacuum). The ECM receives this information as a signal voltage that will vary from about 1 to 1.5 volts at idle, when manifold pressure is low (high vacuum), to 4.5 - 4.8 volts at wide open throttle (low vacuum or high pressure).

A "Scan" tool displays information in volts. Low presure (high vacuum) reads a low voltage while a high pressure (low vacuum) reads a high voltage.

If the MAP sensor fails, the ECM will substitute a fixed MAP value and use the throttle position sensor (TPS) to control fuel delivery.

NOTE: Because of all the possible color code combinations used on electrical wiring diagrams, always refer to SCHEMATIC DIAGRAMS/ELECTRICAL AND ELECTRONIC WIRING DIAGRAMS/ECM CONNECTOR IDENTIFICATION for correct color code identification of circuit.


TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.

1. Code 34 will set when:

- Signal is too low, (less than 14 kPa or greater than 28" hg, and engine running less than 1200 rpm.
OR
- Engine speed is greater than 1200 rpm.
- Manifold pressure less than 21% kPa (over 1.5 volts).

2. If the ECM recognizes the high MAP signal, the ECM and wiring are OK.
3. The "Scan" tool may not display 5 volts. The important thing is that the ECM recognizes the voltage as more than 4 volts, indicating that the ECM and circuit 432 are ok.


DIAGNOSTIC AIDS:

- An intermittent on circuit 432 or circuit 474 can cause a Code 34. With the ignition "ON" and the engine "OFF," the manifold pressure is equal to the atmospheric pressure and the signal voltage will be high. This information is used by the ECM 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 the accuracy of a suspect sensor. Readings should be the same +/- 0.4 volts. Also CHART C-1D can be used to check the MAP sensor

- Refer to DIAGNOSIS BY SYMPTOM/INTERMITTENTS.