Powertrain Controls - ECM/PCM

Code 34 Chart:

Code 34 Wiring Diagram:

***UPDATED BY TSB # 90-6-27 DATED APRIL 1990

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).

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 ECM CONNECTOR IDENTIFICATION under ELECTRICAL AND ELECTRONIC WIRING DIAGRAMS for correct color code identification of circuit.

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

1. This step determines if code 34 is the result of a hard failure or an intermittent condition. A code 34 will set when MAP signal voltage is to low and the ignition is "ON."


2. Jumpering harness terminals "B" to "C" (5 volts to signal circuit) will determine if the sensor is at fault, or if there is a problem with the ECM or wiring.

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 416 or circuit 432 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 altitude.


- Comparison of this 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


- Make sure electrical connection remains securely fastened. Check all connections. Disconnect sensor from bracket and twist sensor by hand (only) to check for intermittent connections. Output changes greater than 0.1 volt indicate a bad connector or connection. If ok, replace sensor. Refer to CHART C-1D, MAP sensor voltage output check for further diagnosis.