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Unread 03-19-2008, 08:42 PM   #1
Stargazer79
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2008 MK Compass 
 
Join Date: Feb 2008
Location: Dallas
Posts: 12
Mysteries of the Jeep Speed Control

Just wondering if you fine folks have any insight into the machinations of the factory installed speed control in the Compass. Specifically, what mechanism it uses to regulate vehicle speed. (RPM/Gearing/Braking, how much of each) And what sort of impact is has on fuel economy for long hauls..

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Unread 03-19-2008, 08:59 PM   #2
Gramps
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2007 MK Compass 
 
Join Date: Aug 2006
Location: Madison/Janesville, Wisconsin
Posts: 5,624
Never having made the same trip twice so that I could do a with/without comparison on economy, I can't say. But it sre lets you streach your legs.

DESCRIPTION

The speed control system is fully and electronically controlled by the Powertrain Control Module (see below PCM). A cable and a vacuum controlled servo are not used. This is a servo-less system. The switch is labeled: ON/OFF, RES/ACCEL, SET, COAST, and CANCEL.

The system is designed to operate at speeds above 25 mph (40 km/h).

WARNING: the use of speed control is not recommended when driving conditions do not permit maintaining a constant speed, such as in heavy traffic or on roads that are winding, icy, snow covered, or slippery.

PCM

POWERTRAIN CONTROL MODULE

The PCM receives input signals from various switches and sensors that are referred to as PCM Inputs. Based on these inputs, the PCM adjusts various engine and vehicle operations through devices that are referred to as PCM Outputs .

PCM Inputs:

Air Conditioning Controls
Battery Voltage
Brake Switch
Camshaft Position Sensor
Clutch Upstop Switch
Clutch Interlock
Crankshaft Position Sensor
Engine Coolant Temperature Sensor
Fuel Level Sensor (Bus message)
Ignition Switch
Intake Air Temperature Sensor
Knock Sensor
Evaporative System Integrity Monitor
Manifold Absolute Pressure (MAP) Sensor
Oil Pressure Switch
Oxygen Sensors
Power Steering Pressure Switch
Speed Control Switches
Vehicle Speed Sensor (MTX-equipped models)

PCM Outputs:

Air Conditioning Clutch Relay
Charging Indicator Lamp (Bus Message)
Proportional Purge Solenoid
Fuel Injectors
Generator Field
Ignition Coils
Malfunction Indicator (Check Engine) Lamp (Bus Message)
Manifold Flow Valve
Oxygen Sensors Heater Controls
Variable Valve Timing
Vehicle Speed (Manual Transmission)

Based on inputs it receives, the PCM adjusts fuel injector pulse width, idle speed, ignition spark advance, ignition coil dwell and EVAP canister purge operation. The PCM also determines the appropriate transmission shift schedule and shift points, depending on the present operating conditions and driver demand. The PCM regulates the cooling fan, air conditioning and speed control systems. The PCM changes generator charge rate by adjusting the generator field. The PCM also performs diagnostics.

The PCM adjusts injector pulse width (air-fuel ratio) based on the following inputs.

Battery voltage
Coolant temperature
Exhaust gas content (oxygen sensor)
Engine speed (crankshaft position sensor)
Intake air temperature
Manifold absolute pressure
Throttle position

The PCM adjusts ignition timing based on the following inputs.

Coolant temperature
Engine speed (crankshaft position sensor)
Knock sensor
Manifold absolute pressure
Throttle position
Transmission gear selection (park/neutral switch)
Intake air temperature

The PCM also adjusts engine idle speed through the idle air control motor based on the following inputs.

Air conditioning sense
Battery voltage
Brake switch
Coolant temperature
Engine speed (crankshaft position sensor)
Engine run time
Manifold absolute pressure
Power steering pressure switch
Throttle position
Transmission gear selection (park/neutral switch)
Vehicle distance (speed)

The camshaft position sensor and crankshaft position sensor signals are sent to the PCM. If the PCM does not receive the signal within approximately 1 second of engine cranking, it deactivates the fuel pump. When these are deactivated, power is shut off to the fuel injectors, ignition coils, oxygen sensor heating elements and fuel pump.

The PCM contains a voltage converter that changes battery voltage to a regulated 5 volts direct current to power the camshaft position sensor, crankshaft position sensor, manifold absolute pressure sensor, throttle position sensor, A/C pressure switch, A/C pressure transducer, and vehicle speed sensor.
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Unread 03-19-2008, 09:29 PM   #3
Stargazer79
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2008 MK Compass 
 
Join Date: Feb 2008
Location: Dallas
Posts: 12
Thanks, Gramps
__________________
"It's always suicide-mission-this and save-the-planet-that. No one ever stops by just to say 'Hi' anymore.." - Jack O'Neill

~No Longer Breathlessly anticipating my '08 Rallye, now just enjoying the [Expletive Deleted] out of it.~
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