Powertrain Controls - ECM/PCM

DTC 34 Chart:

CIRCUIT DESCRIPTION
The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold vacuum. The PCM provides the MAP sensor with a 5 volt reference and a sensor ground. The MAP sensor varies the signal on CKT 432 to PCM between 0 and 5 volts depending on manifold vacuum. As vacuum increases, the MAP signal voltage will decrease indicating low engine load. As vacuum decreases, the MAP signal voltage will increase indicating higher engine load. When the ignition is first turned "ON" (prior to cranking) the PCM calculates Barometric Pressure (BARO) from the MAP signal voltage. If the difference between BARO and MAP at part throttle is less than expected. DTC 34 will set.

DTC 34 WILL SET WHEN
- DTC(s) 21, 22, 26, 27, 31 or 32 are not set.
- BARO is greater than 75 kPa.
- Throttle Position (TP) switch is closed.
- Throttle Position (TP) sensor is less than 18 degrees.
- Engine RPM is greater than 400 RPM.
- Difference between BARO and MAP is 11 kPa or less.
- All above conditions are present for 15 or more seconds.

ACTION TAKEN (PCM WILL DEFAULT TO)
- With a current DTC 34 set, the PCM will illuminate the Malfunction Indicator Lamp (MIL) (Service Engine Soon).
- The PCM will disable the long term fuel correction and substitute a default MAP value based on RPM and throttle position switch status.

DTC 34 WILL CLEAR WHEN
The fault no longer exists.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. MAP at idle should be 30-50 kPa depending on engine load. BARO should be 85 to depending on altitude. The difference between the two should always be greater than 11 kPa with the engine running at or near idle.
2. Check for vacuum at the throttle body with a vacuum gage. At idle, typical vacuum readings are 4"-2"Hg, depending on engine load.
3. Checking 5 volt reference and ground circuits for an open or short.
4. Checking 5 volt reference and MAP sensor signal for an open or short.
5. Checking CKT 432 for an open or short to ground

DIAGNOSTIC AIDS
If idle is rough or unstable, refer to Rough, Unstable or Incorrect Idle, Stalling in Diagnosis by Symptom. Rough, Unstable, or Incorrect Idle, Stalling
An open in CKT 452 or the connection will result
With the ignition "ON" and engine stopped, the manifold pressure is equal to atmospheric pressure and the signal voltage will be high. This information is used by the PCM 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. Readings should be the same ±0.4 volt.