1986 Models

Code 34 indicates the ECM has seen the following:

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

NOTE: The following step numbers refer to the numbers in the accompanying flow chart.

This tests output of sensor at idle to determine if sensor is within specification. Normal sensor will read .59-.64 volts with key in the "ON" position and engine not running.
Signal voltage should read over 2 volts with engine idling with a minimum of 15 in. Hg vacuum.
If voltage reads between 4-6 volts, VAC sensor is faulty.
Low voltage indicates a faulty circuit 416.

Fig 1: Code 34 - Diff. Press. (VAC) Sensor Flow Chart - 1986 Models

Fig 2: Code 34 - Diff. Press. (VAC) Sensor Flow Chart - 1986 Models