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by Bob O’Neill and Bob Lincoln
The air inside the intake manifold and atmospheric or barometric pressure is sensed by the Manifold Absolute Pressure sensor, which sends a signal to the ECU. The ECU uses this information along with data on the air’s temperature to infer the density of the air entering the combustion chamber, when calculating how much fuel to add to this mixture. This information is especially important at Wide Open Throttle (WOT) because the system does not use feedback from the oxygen sensor at that time. At idle, information from the MAP is used to regulate the AIS motor (setting the idle speed).
John Dickinson wrote: ‘“I had a 1986 Dodge Aries I bought new, and Chrysler replaced 6 MAP sensors under warranty before they figured out it was a moisture problem and built a trap for it. I put over 225,000 miles on this car with the original engine.”
The MAP sensor is a strain gauge which changes its output voltage based on the amount of vacuum or boost it is connected to. This vacuum/boost signal moves a silicon diaphragm inside the MAP sensor. This diaphragm is thinner in the center than it is around the edges and is mounted with a vacuum below the chip. A steady 5v supply is provided by the ECU, so the voltage from the MAP shows the relative pressure of the manifold.
When vacuum or boost moves the diaphragm the resistance inside the strain gauge changes, altering the voltage it produces (within a range of .02 volts to 4.94 volts). The ECU uses this voltage to regulate fuel delivery.
This chart describes the pressure-to-voltage relationship of the stock Mopar Turbo II MAP two-bar sensor:
(relative sea level / absolute)
2 BarMAP Output
14.31psi / 29.01psi
14.00psi / 28.70psi
13.00psi / 27.70psi
12.00psi / 26.70psi
11.00psi / 25.70psi
10.00psi / 24.70psi
9.00psi / 23.70psi
8.00psi / 22.70psi
6.00psi / 20.70psi
4.00psi / 18.70psi
2.00psi / 16.70psi
0.00psi / 14.70psi
5.00inHg / 12.24psi
10.00inHg / 9.78psi
15.00inHg / 7.33psi
20.00inHg / 4.87psi
25.00inHg / 2.41psi
29.10inHg / 0.40psi
A problem with another system may set a MAP sensor code in the computer. For example, suppose the code is "MAP pneumatic circuit problem." This indicates that the powertrain control module did not see a drop in manifold pressure after the engine started. The most likely culprit is a cracked/loose vacuum hose running to the MAP sensor (though it could be the sensor itself). The computer now sees a wide-open throttle power demand and proceeds to cheerfully flood the engine while you're idling.
Dead MAP sensors are fairly common. When the MAP sensor goes, the result may be stalling, the engine stopping even at highway speed, or failure to stop. We were told that there is “an available air bleeder valve kit to prevent intermittent operation and premature failure of the MAP sensor. The valve allows a small amount of dry air into the MAP vacuum line to prevent a condensation buildup. It’s a low cost add-on, inquire at the dealer for the part. I used up six MAP sensors before finding out about this - haven't lost one since.”
The MAP sensor input from the manifold to the atmosphere is switched by the Barometric Pressure Read solenoid. This solenoid is controlled by the logic module. It takes a reading of the barometric pressure to calculate incoming air density and uses this to control boost in turbo engines. At closed throttle the solenoid is momentarily activated. This happens one time per closure but not more frequently than every three minutes and this happens only within a certain engine speed band.
At idle, some engines may ‘miss’. This is because the computer continues to sample the outside barometric pressure using the MAP sensor. If this happens it can be ignored as it is a glitch in the software of the logic module.
While the logic module activates and deactivates the solenoid, it monitors the output of the solenoid driver. If it doesn’t see near 0 volts DC when turning the solenoid on or 12 volts when turning the solenoid off, there is either an open or shorted circuit. If this happens, it triggers a code 37.
LINKS: Stalling problems | Idle fixes and MAP sensor diagnostics | Driveability
Sensors, Switches, and Other Systems | Main Repairs Page | EEKs
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