Understanding Baro Sensors: Key Insights

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Purpose

The barometric pressure (BARO) sensor monitors changes in atmospheric pressure (air density) associated with changes in elevation or ambient weather conditions while driving. The downward pressure exerted by the atmosphere varies according to elevation; air is less “dense” at higher altitudes. As a result, the amount of fuel needed to maintain a constant air/fuel ratio also decreases as elevation increases.

If fuel supply remains constant while elevation increases, the air/fuel ratio will become progressively “richer.” Conversely, if a vehicle is calibrated for “high altitude” operation, and the vehicle is subsequently driven at lower elevation, the air/fuel ratio will become “leaner.” Both conditions can result in vehicle performance problems.

The vehicle’s computer uses information from the BARO sensor to adjust the air/fuel ratio and ignition timing to compensate for changes in atmospheric pressure. On some Ford vehicles, the computer also uses the signal from the BARO sensor to adjust engine idle speed, and to control actuation of the Exhaust Gas recirculation (EGR) valve.

NOTE: Similar functions may be performed by the Manifold Absolute Pressure (MAP) sensor.

Location

The BARO sensor is generally located in the engine compartment, and may be mounted on the air cleaner, firewall, inner fender or other location. In some applications, the BARO sensor may be located in the driver’s compartment, under the dashboard.

Operation

In most vehicle applications, operation of the BARO sensor is similar to that of the Manifold Absolute Pressure (MAP) sensor. The sensor measures atmospheric pressure (air density) electronically, and generates a voltage output proportional to the ambient pressure; as atmospheric pressure decreases at higher elevations, the voltage output of the BARO sensor decreases accordingly. The vehicle’s computer continuously monitors the status of the BARO sensor signal to dynamically adjust air/fuel ratio and ignition timing during vehicle operation. Some exceptions to this functionality include:

NOTE: Refer to the vehicle’s service manual for information on testing and troubleshooting these types of BARO sensors.

Ford

On some models equipped with 2.3-liter engines, the BARO sensor generates a frequency (rather than a voltage) output.

Some Ford models are equipped with a combination barometric pressure/manifold absolute pressure (BMAP) sensor that combines the functionality of the two individual sensors.

General Motors

On some C3 systems, the vehicle’s computer checks the status of the BARO sensor only when the ignition is initially turned on. If extreme changes in elevation are made while driving, it is sometimes necessary to stop the vehicle, turn the ignition “off” then back “on” to reset the vehicle’s computer.

“J”-type vehicles are equipped with an “altitude compensator” rather than a BARO sensor. The altitude compensator is essentially an “on-off” switch that actuates when the vehicle reaches a preset elevation.

Drivability Symptoms

The following symptoms may indicate a damaged or defective BARO sensor, or trouble in a related system:

• Idle speed problems
  
• Degraded performance at high elevation
  
• Audible pining or knocking while accelerating or driving under load
  

Testing and Troubleshooting

Use a digital voltmeter and hand vacuum pump to verify proper operation of the BARO sensor, as follows. Refer to the vehicle’s service manual for BARO sensor specifications.

NOTE: It may be necessary to disconnect the wiring harness from the BARO sensor and jumper the POWER and GROUND pins from the harness to the sensor in order to facilitate access to the SIGNAL pin for testing. Be sure to jumper the pins properly.

• Set the voltmeter to the 20V scale.
• Connect the black lead of the voltmeter to the battery negative terminal (ground). Connect the red lead of the voltmeter to the BARO sensor’s SIGNAL pin.
• Turn the ignition key on. DO NOT start the engine.
  
  Verify BARO sensor SIGNAL voltage is within specification for the current altitude (elevation).
  
• Disconnect the vacuum line from the BARO sensor and connect a hand pump to the vacuum fitting.
• Observe the voltmeter, and slowly apply vacuum (approximately 18 in./Hg) to the BARO sensor.
  
  Verify BARO sensor SIGNAL voltage decreases as vacuum is applied.
  
• Disconnect the hand pump and voltmeter from the BARO sensor. Reconnect the vacuum line to the BARO sensor vacuum fitting.

The following test results may indicate a defective BARO sensor:

• If sensor SIGNAL voltage is higher than specified at no vacuum, check for a short circuit in the sensor wiring. If no short is present, the sensor may be defective. Refer to the vehicle’s service manual for additional testing and troubleshooting procedures.
• If sensor SIGNAL voltage is lower than specified at no vacuum, check for a short or open circuit in the sensor wiring. If the wiring is in good condition, a defective sensor or a malfunction in the vehicle’s computer may be at fault. Refer to the vehicle’s service manual for additional testing and troubleshooting procedures.
• If sensor SIGNAL voltage is within specifications at no vacuum, but fails to change (decrease) when vacuum is applied, the sensor is defective.

Repair/Replacement

The BARO sensor is a sealed unit. If you determine it is defective, it must be replaced. No repair or adjustment is possible.

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