Manifold Absolute Pressure Sensor - DaimlerChrysler LH New Yorker 1994 Electronic Service Manual

8D - 8 IGNITION SYSTEMS
Fig. 23 Knock Sensor Locations—3.5L Engine Only

MANIFOLD ABSOLUTE PRESSURE SENSOR

The powertrain control module (PCM) supplies 5
volts to the MAP sensor. The MAP sensor converts
intake manifold pressure into voltage. The PCM
monitors the MAP sensor output voltage. As vacuum
increases, MAP sensor voltage decreases proportion-
ately. Also, as vacuum decreases, MAP sensor voltage
increases proportionately.
During cranking, before the engine starts running,
the PCM determines atmospheric air pressure from
the MAP sensor voltage. While the engine operates,
the PCM determines intake manifold pressure from
the MAP sensor voltage. Based on MAP sensor volt-
age and inputs from other sensors, the PCM adjusts
spark advance and the air/fuel mixture.
On 3.3L engines the MAP sensor mounts to the rear
of the intake manifold plenum (Fig. 24). On 3.5L
engines, the MAP sensor mounts to the drivers side of
the intake manifold plenum (Fig. 25).
Fig. 24 MAP Sensor—3.3L Engine
AUTO SHUTDOWN AND FUEL PUMP RELAYS
The PCM operates the auto shutdown (ASD) relay
and fuel pump relay through one ground path. The
PCM operates them by switching on and off the
Fig. 25 MAP Sensor—3.5L Engine
ground path for the solenoid side of the relays. Both
relays turn on and off at the same time.
The ASD relay connects battery voltage to the fuel
injectors and ignition coil. The fuel pump relay con-
nects battery voltage to the fuel pump.
The PCM turns the ground path off when the igni-
tion switch is in the Off position. Both relays are off.
When the ignition switch is in the On or Crank
position, the PCM monitors the crankshaft position
sensor and camshaft position sensor signals to deter-
mine engine speed and ignition timing (coil dwell). If
the PCM does not receive a crankshaft position sen-
sor signal and camshaft position sensor signal when
the ignition switch is in the Run position, it de-
energizes both relays. When the relays de-energizes,
battery voltage is not supplied to the fuel injectors,
ignition coil and fuel pump.
The ASD relay and fuel pump relay are located in
the power distribution center (PDC) near the battery
(Fig. 26). A decal on the underside of the PDC cover
shows the locations of each relay and fuse contained
in the PDC.
Fig. 26 Power Distribution Center