This sensor is used to read the air pressure in the intake manifold, which is always positive (but less than atmospheric pressure) when the engine is running. The Powertrain Control Module (PCM) sends a reference voltage to the sensor, then a pressure sensitive resistor in the sensor reduces the voltage returning to the PCM. The portion of the voltage returned to the PCM is interpreted as engine load. The return signal is low (low load) when engine vacuum is high (throttle closed). As the throttle is opened and engine vacuum decreases (increased manifold air pressure), the return signal increases. While the engine is running, this type of load sensing automatically accounts for changes in altitude, so no separate altitude sensor is needed. A high pressure (14-15 psi) will allow 4-5 volts to return to the PCM while a low pressure (5-7 psi) will allow 0.5-0.9 volts. On SOHC engines with TBI, the MAP sensor is mounted on the camshaft/valve cover. On all MFI engines, the sensor is mounted directly to the end of the intake manifold.
See Figures 1, 2 and 3
- Unplug the MAP sensor connector and turn the ignition switch ON . Check for about 4.5-5 volts between connector terminal C (gray wire) and ground.
- Check for the same voltage between terminals C and A (black wire). This is a ground through the PCM.
- Reconnect the wiring, then install a Saturn diagnostic probe, or equivalent, on the green wire and connect either a voltmeter probe or a scan tool to the ALDL. Connect a vacuum gauge to the intake manifold.
- With the ignition switch ON and the engine not running, the return signal voltage should be close to the supply voltage in Step 1, if below a 1000 ft. (305 meters) altitude. If above this altitude, see the chart for the correct voltage.
- With the engine at idle, there should be at least 16 in. Hg (54 kPa) of manifold vacuum and 1-1.5 volts on the signal return wire. When the throttle is opened suddenly, the signal voltage should increase. It may only change momentarily.
- If the voltage at idle is the same as in Step 1, or if the voltage does not change when the throttle is moved suddenly, the sensor is faulty and must be replaced.
REMOVAL & INSTALLATION
See Figures 4, 5 and 6
- As an added safety precaution, you may wish to disable the air bag(s). refer to Chassis Electrical for the disabling procedure.
- Disconnect the negative battery cable.
- For SOHC TBI engines, remove the sensor attaching nuts and pull the sensor slightly from the engine, then disconnect the sensor vacuum hose.
- For all MFI engines, remove the attaching bolts and pull the sensor slightly from the engine. If the port seal remained in the intake manifold, remove the seal.
- Unplug the sensor electrical connector and remove the sensor from the vehicle.
- On TBI engines, connect the vacuum hose to the sensor.
- On MFI engines, install the port seal into the intake manifold. To ensure proper seating, the seal must be installed directly into the manifold, rather than onto the sensor.
- Attach the electrical connector to the MAP sensor.
- Position the sensor to the engine and install the fasteners. Tighten the retaining nuts to 53 inch lbs. (6 Nm) or the retaining bolts to 44 inch lbs. (5 Nm).
- Connect the negative battery cable.
- If necessary, enable the air bag(s). refer to Chassis Electrical .