Manifold Destiny

Can you reliably check out important sensors with a digital multimeter? Sure! This tech tip from Wells Manufacturing Corporation (www.wellsmfgcorp.com) shows what can be done with a good ‘meter, a couple of jumpers and a little experience.

Follow These Directions To Find Problems With MAP Sensors

MAP stands for “Manifold Absolute Pressure”, which is the pressure inside the engine’s intake manifold. Pressure is low when intake vacuum is high (as at idle), and pressure is high when vacuum is low (as at wide-open throttle). It’s called an “absolute” pressure reading because it depends solely on pressure inside the manifold, though some types of MAP sensors are actually “differential” pressure sensors that measure the difference between intake vacuum and atmospheric (barometric) pressure. A MAP sensor reads engine vacuum through a hose connected to the intake manifold. A pressure sensitive ceramic or silicon element and electronic circuit inside the sensor generates a voltage signal that changes with intake vacuum. The sensor itself may be located in the engine compartment or under the dashboard.

Most MAP sensors have three terminals: a ground terminal, a voltage reference (VRef) supply terminal (typically 5 volts, which is provided by the computer), and an output terminal for sending data back to the computer. As engine load changes, so does the MAP sensor’s output. The sensor’s output voltage will vary depending on the application, but a typical GM sensor reading might be 1.25 volts at idle and just under 5 volts at wide-open throttle. Voltage reads low when vacuum is high, and increases as vacuum drops. Output generally changes about 0.7 to 1.0 volts for every 5 inches of change in vacuum. Ford MAP sensors work somewhat differently in that they produce a digital frequency signal rather than an analog D.C. voltage signal. These sensors output a square wave signal that increases in frequency as vacuum drops. A typical reading at idle might be 95 Hertz (Hz or cycles per second) when vacuum is high, and 150 Hz at wide-open throttle when vacuum is low. The MAP sensor’s signal is used by the powertrain control module (PCM) to adjust the air/fuel mixture and spark timing. Under low load, high-vacuum conditions, the PCM typically leans the air/fuel mixture and advances spark timing for better fuel economy. Under high-load low-vacuum conditions, the PCM richens the air/fuel mixture for more power and retards spark timing to prevent detonation (spark knock). These control functions are programmed into the computer and require accurate sensor inputs. So if the MAP sensor is defective or out of calibration, driveability and performance problems can occur. Engines with a “speed-density” type of electronic fuel injection system (no airflow sensor) are especially dependent on the MAP sensor’s signal because the PCM uses it along with engine rpm, throttle-position and ambient air temperature to calculate airflow. Engines that do not have a MAP sensor estimate engine load using input from the airflow and throttle-position sensors.

Driveability symptoms that can be caused by a bad MAP sensor, grounds or opens in the sensor’s wiring circuit, vacuum leaks in the sensor hose or intake manifold include hard starting, hesitation, engine misfires, stalling, rough or erratic idle, pinging, black exhaust smoke (rich fuel condition resulting in high hydrocarbon emissions), poor fuel economy and generally poor engine performance.

MAP Sensor Checks

There are numerous ways to check a MAP sensor, including using a scan tool to check for MAP sensor trouble codes, observing the sensor’s output as a waveform on a digital storage oscilloscope, and/or comparing the sensor’s output voltage or frequency (Ford) to specifications in a manual. You can test General Motors and Chrysler MAP sensors on the vehicle using a digital voltmeter (DVOM) and two jumper wires.

An idling engine will produce a large amount of intake vacuum, which should pull the MAP sensor’s voltage down to a low reading of 1 to 2 volts (note: readings will vary with altitude). You can also do this test with the key on, engine off by applying vacuum to the MAP sensor’s hose with a hand-held vacuum pump. But do NOT apply more than 20 inches of vacuum (excessive vacuum may damage the sensor). This test verifies that the MAP sensor is responding to changes in engine vacuum. If the reading does not change, it means the sensor is faulty or the vacuum hose is plugged or leaking.

On Ford applications, a multimeter that can read frequency is normally required to check the sensor’s output. But you can also use an ordinary tachometer because a tach can display a frequency signal. Here’s the procedure.

What about turbos?

Turbocharged engines are a special challenge for manifold absolute pressure sensors, because pressures swing positive under turbo boost. Chrysler turbo 2.2L engines, for example, operate with a lens-like silicon wafer that alters circuit resistance, much like the diaphragm in a telephone handset microphone. The circuit feeds the ECU a voltage that’s proportional to the pressure reading. Here’s a typical 2.2 L Turbo MAP sensor pressure/voltage chart:

On startup, the ECU looks at the MAP sensor output for a signal between the 0.02V and 4.90V extremes, and if a discrepancy is detected, sets a code. The AIS motor will be disabled and the ECU will look to the TPS and ignition reference sensor to estimate manifold pressure indirectly. The ECU will also limit revs to guard against overboost with a dead MAP. The ECU also uses the MAP sensor to detect overboost under normal operating conditions. Here, a voltage that’s above a preset cutoff value (which varies by year and model) causes the system to kill the injectors and ignition coil. When boost falls, spark and fuel flow resume. MAP sensors are simple devices, but they’re especially crucial in turbocharged applications.

GM, Chrysler MAP Testing

1. Disconnect the MAP sensor’s electrical connector.

2. Connect one jumper wire between the connector and the MAP sensor’s terminal “A”.

3. Connect another jumper wire from the connector to terminal “C”.

4. Connect the positive lead on the DVOM to terminal “B” (the sensor’s output terminal) and the negative DVOM test lead to a good engine ground.

5. Turn the ignition key ON and observe the voltage. If the reading falls in the voltage range of 4 to 5 volts (2 to 3 volts for turbocharged engines) at sea level the sensor is functioning properly at this point.

6. Be sure the vacuum hose between the MAP sensor and engine is in good condition and does not leak. Then start the engine and let it idle.

Ford MAP Testing

1. Set the tachometer to the four-cylinder scale (regardless of how many cylinders the engine has).

2. Connect one tachometer lead to the middle terminal on the MAP sensor and the other tachometer test lead to ground.

3. Connect the two jumper cables the same as before, attaching each end terminal on the sensor to its respective wire in the wiring connector.

4. If you want to measure engine vacuum so you can correlate it to a specific frequency reading, connect a vacuum gauge to a source of manifold vacuum on the engine, or tee the gauge into the MAP sensor hose.

5. Turn the ignition ON and note the initial reading. The reading on the tachometer should be about 454 to 464 at sea level, which corresponds to a frequency output of 152 to 155 Hz.

6. Start the engine and check the reading again. If the MAP sensor is functioning properly, the reading should drop to about 290 to 330 on the tachometer, which corresponds to a frequency output of about 93 to 98 Hz. No change would indicate a defective sensor or leaky or plugged vacuum hose.