Hombre S Regular Cab 4WD V6-4.3L (1999)
Engine Control Module: Description and Operation
Vehicle Control Module Description
Vehicle Control Module (VCM)
The Vehicle Control Module (VCM) is located in the engine compartment. The VCM is the control center for the engine component systems which
effect engine operation. The VCM constantly monitors and processes information from the various information sensors and switches. The VCM then
sends the necessary electrical responses to control the various control system components. Review the wiring diagrams in order to determine which
systems are controlled by the VCM. The VCM has the ability to perform, or run, on-board diagnostic tests on itself, the information system components,
and certain control system components. The VCM has the ability to alert the driver of a malfunction by illuminating a Malfunction Indicator Lamp
(MIL). The VCM has the ability to store certain information pertaining to a malfunction, including Diagnostic Trouble Codes (DTC)s. Using serial data
communication, a scan tool can retrieve this information in order to be viewed by the technician. The technician can use this information in order to
identify, diagnose, and verify the repair of the malfunction. The VCM is sometimes referred to as the control module.
VCM Memory
The VCM control calibrations for a particular vehicle are based on the vehicle Identification Number (VIN). The calibrations are stored in the
Electrically Erasable Programmable Read Only Memory (EEPROM). The EEPROM is part of the VCM and can not be replaced. The EEPROM can be
reprogrammed using the Techline equipment. The Electronic Ignition Control Module is located in the VCM. The Knock Sensor Module contains the
knock Sensor (KS) system calibrations. The Knock Sensor Module is not part of the VCM and is serviced separately from the VCM. The VCM has the
ability to make corrections in the fuel control system to allow for minor vehicle variations. This information is stored in the Random Access Memory
(RAM). RAM needs a constant voltage supply to be retained. Short Term Fuel Trim and Long Term Fuel Trim are stored in RAM. When the battery is
disconnected during service of the vehicle this memory is lost and the driver may notice a change in the performance of the vehicle. In order to restore
this memory perform the following steps:
1. Start the engine.
2. Allow the engine to reach normal operating temperature.
3. Drive the vehicle under idle, partial throttle and moderate acceleration conditions.
4. Continue until normal vehicle performance returns.
VCM / Speed Density System
Speed Density System Description
Three specific data sensors provide the VCM with the basic information for fuel management portion of its operation. That is three specific signals to the
VCM establish the engine speed and air density factors. The engine speed signal comes from the distributor high resolution circuit. The VCM uses this
information to determine engine speed (RPM). Air density is derived from IAT and MAP sensor inputs. The IAT sensor measures the air temperature
that is entering the engine. The IAT signal works in conjunction with the MAP sensor to determine air density. As the intake manifold pressure increases,
the air density in the intake manifold also increases and additional fuel is required. This information from the IAT and MAP sensors is used by The
VCM to control injector pulse width.
The speed density system is only needed when there is a Mass Air Flow (MAF) sensor malfunction. If the VCM detects a malfunction with the MAF
Sensor Circuit, the VCM will default to speed density fuel management.
