Jeep 1996 Grand Cherokee Service Manual - page 1080
operating conditions, the PCM will adjust fuel injec-
tor pulse width and ignition timing.
VEHICLE SPEED AND DISTANCE SENSOR—PCM
INPUT
The vehicle speed sensor is located on the speed-
ometer pinion gear adapter (Fig. 11). The pinion gear
adapter is located on the extension housing of the
transmission (drivers side). The sensor input is used
by the powertrain control module (PCM) to deter-
mine vehicle speed and distance traveled.
The speed sensor generates 8 pulses per sensor
revolution. These signals, in conjunction with a
closed throttle signal from the throttle position sen-
sor, indicate a closed throttle deceleration to the
PCM. When the vehicle is stopped at idle, a closed
throttle signal is received by the PCM (but a speed
sensor signal is not received).
Under deceleration conditions, the PCM adjusts
the idle air control (IAC) motor to maintain a desired
MAP value. Under idle conditions, the PCM adjusts
the IAC motor to maintain a desired engine speed.
POWER GROUND
The power ground is used to control ground cir-
cuits for the following powertrain control module
(PCM) loads:
• Generator field winding
• 8 volt (PCM) power supply
• Fuel injectors
• Ignition coil
AIR CONDITIONING (A/C) CLUTCH RELAY—PCM
OUTPUT
The A/C relay is located in the Power Distribution
Center (PDC). The PDC is located in the engine com-
partment (Fig. 12). Refer to label on PDC cover for
relay location.
The powertrain control module (PCM) activates the
A/C compressor through the A/C clutch relay. The
PCM regulates A/C compressor operation by switch-
ing the ground circuit for the A/C clutch relay on and
off.
When the PCM receives a request for A/C from A/C
evaporator switch, it will adjust idle air control (IAC)
motor position. This is done to increase idle speed.
The PCM will then activate the A/C clutch through
the A/C clutch relay. The PCM adjusts idle air control
(IAC) stepper motor position to compensate for
increased engine load from the A/C compressor.
By switching the ground path for the relay on and
off, the PCM is able to cycle the A/C compressor
clutch. This is based on changes in engine operating
conditions. If, during A/C operation, the PCM senses
low idle speeds or a wide open throttle condition, it
will de-energize the relay. This prevents A/C clutch
engagement. The relay will remain de-energized until
the idle speed increases or the wide open throttle
condition exceeds 15 seconds or no longer exists. The
PCM will also de-energize the relay if coolant tem-
perature exceeds 125°C (257°F).
Fig. 11 Vehicle Speed Sensor Location—Typical
Fig. 12 Power Distribution Center (PDC)
ZJ
FUEL SYSTEM
14 - 35
DESCRIPTION AND OPERATION (Continued)