DTC 34

Fig. 72 - Chart #34 Diagnosis:

Fig. 73 - Chart #34 Diagnosis, 80-81 C-3, 82 3.8 Only:

Fig. 74 - Chart #34 Diagnosis:

Code 34 indicates the ECM has detected the following:

a. Pressure outside a specified voltage range (seen by ECM as voltage at Term. "20").
b. Engine RPM less than a given value.
c. Engine at operating temperature.
d. All the above for a time greater than specified.

The VAC sensor is a differential pressure sensor that measures the difference in pressure between atmosphere and manifold. The voltage output is opposite the MAP sensor. The VAC sensor supplies high voltage at high vacuum. High voltage increases spark advance while low voltage reduces advance.

1. Checks output of sensor at idle to determine if sensor is within specification. Normal sensor will read less than 1 volt with key "ON", engine "OFF" and over 3 volts with engine idling with a minimum of 15" vacuum.
2. A normal sensor should drop below 1 volt when vacuum is removed. This step tests for that drop at the sensor.
3. Checks for a ground in wire from Term. 'B' of vacuum sensor to ECM. This would be the case if the voltage went up over 2 volts when the line was opened.
4. Checks to see if the fault is in the sensor, or in the ECM wiring, or in the ECM. If the voltage goes up over 2 volts with the sensor disconnected, the sensor or sensor connections are faulty.