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	Fault Code 13 - No change in MAP from start to run
	OBD1 DTC's for 1994-1997 Rams with Gas Engines
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The MAP sensor uses a variable potentiometer to convert the intake manifold absolute pressure to an input voltage to the PCM.  The complete circuit contains a 5V supply voltage and a ground from the PCM to feed the MAP sensor. A pullup resistor internal to the PCM is connected from the 5V supply to the MAP

When the key is rotated to the ON position, the computer samples the MAP sensor to obtain a reference atmospheric pressure. After the engine starts, the computer again samples the MAP sensor and compares it to the first sample. If the two samples are not significantly different, the computer sets code 13. A code 13 with a code 14 indicates that the samples were not different and they were outside of the normal voltage range - probably due to a wiring short, open or ground problem. A code 13 by itself is usually due to a loss of the manifold pressure to the sensor or a bad sensor.

The MAP sensor is a piezoelectric device which changes a 5-volt reference voltage in response to changes in manifold pressure. As manifold pressure increases, the voltage from the MAP sensor also increases. NOTE: while you may think there is a vacuum in the manifold, there is pressure - it is just lower than atmospheric pressure. Vacuum is a measurement relative to 14.7psi +/- atmospheric pressure.

With the key ON and the engine not running, Map Sensor output should be 3.5 to 4.8 volts. With the engine idling, sensor output will be 1.2 to 1.9 volts, usually near 1.5 volts at low altitude.

With the engine idling, snap the throttle open and then allow it to close. The voltage should jump up as the engine speed increases, then drop down as the throttle is closed and the engine decelerates. If the voltage seems to hang, jump wildly, or not follow this pattern, remove the sensor and make sure the vacuum port is not clogged. If cleaning the port does not improve the sensor output, the sensor is probably bad. Further testing with a controllable pressure source applied directly to the sensor might be necessary.

This page was edited on: May 3, 2004