With 4L60 Automatic Transmission

Code 34 Manifold Absolute Pressure (MAP) Sensor Circuit (Signal Voltage - High Vacuum)






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 carry from about 1-1.5 volts at idle to 4-4.6 volts at Wide Open Throttle (WOT). A scan tool displays manifold pressure in volts. Low pressure (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.

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:
- Engine is less than 1200 rpm.
- Manifold pressure reading is less than 14 kPa, conditions met for 1 second.
OR
- Engine speed is greater than 1200 rpm.
- Throttle angle over 21%.
- Manifold pressure less than 14 kPa, conditions met for 1 second.
2. Jumping 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 recognized the voltage as more than 4 volts indicating that the ECM and CKT 432 are OK.

Diagnostic Aids

An intermittent open in CKT 432 or CKT 416 will result in a code 34.

With the ignition ON and the engine OFF the manifold pressure is equal to atmospheric pressure and the signal voltage will be high. This information is used by the ECM as an indication of vehicle altitude.

Comparison of this 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 plus or minus .4 volt. Also, MAP output check can be used to test the MAP sensor.

- Disconnect sensor from bracket and twist sensor by hand (only) to check for intermittent connections. Output changes greater than .1 volt indicates a bad connector or connection. If OK, replace sensor.

Note: Make sure electrical connector remains securely fastened.