The logic module uses the manifold absolute pressure (MAP) sensor to determine the absolute pressure (not the relative to atmospheric pressure) of the air inside the intake manifold and the atmosphere (barometric pressure). This information is used to determine the density of the air entering the combustion chamber (in conjunction with the charge temperature sensor), which is used when calculating the proper air/fuel mixture for the engine, especially at WOT since the system is not in closed-loop at this time. It is also used to help adjust the AIS motor during idle.
The sensor is located above the logic module on 1984 models, in the logic module housing on 1985 and 1986 models, and under the hood on the right strut tower on later models. It uses a silicon wafer that is thinner in the center (0.001") than around the edges (0.045"), which causes it to act as a diaphragm. It is mounted with a perfect vacuum beneath the chip so that the air pressure from the other side flexes the chip. This flexing causes a change in resistance and the circuitry inside the sensor converts this to a voltage ranging from 0.02V to 4.94V when the output is pulled up towards 5V by the logic module.
The chart below describes the pressure-to-voltage relationship of the stock Chrysler Turbo MAP sensor (2 bar):
| Manifold Pressure (relative sea level / absolute) |
2 Bar MAP Output |
| 14.31psi / 29.01psi | 4.90V |
| 14.00psi / 28.70psi | 4.84V |
| 13.00psi / 27.70psi | 4.67V |
| 12.00psi / 26.70psi | 4.50V |
| 11.00psi / 25.70psi | 4.33V |
| 10.00psi / 24.70psi | 4.16V |
| 9.00psi / 23.70psi | 3.99V |
| 8.00psi / 22.70psi | 3.82V |
| 6.00psi / 20.70psi | 3.47V |
| 4.00psi / 18.70psi | 3.13V |
| 2.00psi / 16.70psi | 2.79V |
| 0.00psi / 14.70psi | 2.45V |
| 5.00inHg / 12.24psi | 2.03V |
| 10.00inHg / 9.78psi | 1.61V |
| 15.00inHg / 7.33psi | 1.19V |
| 20.00inHg / 4.87psi | 0.77V |
| 25.00inHg / 2.41psi | 0.35V |
| 29.10inHg / 0.40psi | 0.02V |
The logic module will monitor the MAP sensor output as soon as the engine starts. If the voltage does not decrease by at least 0.04V, a fault code 13 is stored, the power loss light is turned on, and the system will enter limp-in mode. The AIS motor will then be completely closed and the logic module will use the TPS and the ignition reference sensor to guess what the manifold pressure is. Also, the engine speed will be limited to 2,000 rpm on turbo engines when the throttle plate is opened more than 22 degrees to prevent overboost.
If the MAP sensor voltage is below 0.02V or above 4.94V, a fault code 14 is stored, the power loss lamp is turned on, and the system enters limp-in mode. The AIS motor will then be completely closed and the logic module will use the TPS and the ignition reference sensor to guess what the manifold pressure is. Also, the engine speed will be limited to 2,000 rpm on turbo engines when the throttle plate is opened more than 22 degrees to prevent overboost.
If the MAP sensor voltage indicates a boost pressure that is above the overboost cut-off point, a fault code 45 is stored, the power loss light is turned on, and the fuel injectors and ignition coil are shut off by the ASD relay. Once the boost has fallen to 3 psi to 5 psi, the ASD relay is turned back on and the power loss light is turned off. The overboost cut-off point for a particular vehicle depends on the year, engine, and transmission. Here is all the information I have regarding overboost cut-off points:
| Year | Engine | Manual | Auto |
| 1984 | Turbo I | 4.30V | 4.30V |
| 1985 | Turbo I | 4.54V | 4.65V |
| 1986 | Turbo I | 4.65V | 4.88V |
| 1987 | Turbo I | 4.65V | 4.88V |
| 1987 | Turbo II | 4.65V | ---- |
Following years are probably the same as 1987, but I don't have any hard data to back that up. Some have said that 4.74V was the max for Turbo I engines of later years. Turbo II was not available with an automatic transmission.
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Updated 12/19/2003.
Copyright © 1996-2003 Russ W. Knize.