Download Servicing The Engine Management System

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Servicing the Engine Management System
Safety Concerns and Cautions
Electronic components are easily damaged if proper precautions are not taken.
• Static electricity can damage solid-state components. It is always good practice to
wear a grounded static strap when handling these components.
•
Never use a test lamp or an analog meter when testing solid-state components.
•
Be sure that spark plug wires and any other circuits developing a high voltage are
not routed in the same path as the solid-state component.
•
Do not apply a current or ground to any portion of a solid-state component unless
you are sure of its operation.
•
Jump starting a vehicle with the jumper cables reversed can destroy a solid-state
circuit. Remember: Red to Red and Black to Black
•
Be careful when handling sensors as they can lose their calibration if dropped or
mishandled.
•
Refrain from wearing jewelry when working on electronic components.
•
Before removing any Electronic Control Module, disconnect the battery ground
lead.
•
Never disconnect the battery from the on-board electrical system while the engine
is running.
•
Never subject the ECM/PCM to temperatures above 80 degrees Centigrade.
•
Ensure all cable harnesses are solidly connected and battery terminals are
thoroughly clean.
•
When charging the battery, disconnect it from the vehicles electrical system.
•
Never disconnect or connect the cable harness plug at the ECM/PCM when the
ignition is switched on.
•
Before attempting any electrical arc welding, disconnect the battery leads and the
ECM/PCM connectors.
•
When steam cleaning engines, do not direct the steam cleaning nozzle at the
ECM/PCM components.
•
Use only test equipment specified by the vehicle manufacturer and observe the
recommended test procedures for that test equipment.
•
When using a digital multimeter for voltage tests, ensure a digital meter with an
internal impedance rating of at least 10 million ohms per volt (10 mega-ohms/volt) is
used.
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Servicing the Engine Management System (continued)
Handling and Electrostatic Discharge
Static electricity can harm delicate components
inside your computer. To prevent static damage,
discharge static electricity from your body before
you touch any of your computer or any of its
electronic components, such as the PROM. You
can do so by touching an unpainted metal surface
or wear a static wrist strap. You can also take the
following steps to prevent damage from
electrostatic discharge (ESD):
•
When unpacking a computer or component from its shipping carton, do not remove
the component from the anti-static packing material until you are ready to install the
component. Just before unwrapping the anti-static packaging, be sure to discharge
static electricity from your body.
•
Handle all sensitive components in a static-safe area. If possible, use workbench
pads.
ECM/PCM Electrical Testing
Grounds
Ground in the automobile serves a two-fold purpose. First of all, ground provides a path back
to the battery so that current has a path to flow back to its source - the battery. Secondly,
ground provides a reference point that establishes all voltage levels within the vehicle’s
electrical system.
Characteristically, there are two types of ground that an ECM/PCM requires - power grounds
and signal grounds. Power grounds provide a path back to the battery for the high-current
components controlled by the module. Signal grounds are used to provide dedicated ground
paths for low-current inputs to the module. Commonly, these two types of grounds are
isolated from each other so that “noise” from high-current components does not affect lowcurrent signal grounds.
Even though the vehicle’s charging system is capable of generating adequate power (voltage
and current), ground circuit faults that are directly related to ECM/PCM circuits can affect
input signals, the computing (calculation) processes within the module and how well output
components function. The two most common electrical failures that affect ECM/PCM grounds
are opens and excessive resistance.
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Servicing the Engine Management System (continued)
ECM/PCM Electrical Testing (continued)
Grounds (continued)
A reliable way to verify the integrity of ECM/PCM grounds is to perform voltage drop checks.
When a voltage drop test is performed on a ground circuit, the value displayed on the meter
indicates the voltage consumed by resistance in the ground circuit between the ECM/PCM
and the ground point. Voltage drop checks are performed when circuits are powered and
current flows through the circuit being tested. To perform a voltage drop test on a ground
circuit:
• Access the ECM/PCM connector that houses the ground circuit to be tested.
•
Remove any cover that may be used to protect the wiring at the connector. Try not
to disconnect the electrical connector.
•
With a suitable electrical probe, carefully back probe the ground terminal to be
tested. Take care so that the wiring insulation is not pierced and that the terminal is
not spread.
•
Connect the positive lead of a Digital Multi-meter (DMM) set to read DC voltage to
the probe inserted into the ground terminal cavity of the ECM/PCM.
•
Connect the negative lead of the DMM to the negative terminal of the battery.
•
Turn on the circuit to be tested.
NOTE: In most cases, it is difficult to determine in the field which ground circuit provides
ground for specific components controlled by the ECM/PCM. In some cases ground circuits
can be identified as to which ones are used for signal ground or power ground sources.
•
Note and record the voltage displayed on the DMM.
NOTE: In cases where the voltage drop displayed on the DMM is equal to system voltage
(e.g. 14.2 V DC) an open circuit is indicated. Any voltage drop less than system voltage is an
indication that excessive resistance is present in the ground circuit being tested.
Any voltage drop greater than 30 mV DC can adversely affect ECM/PCM operation. Voltage
drops occurring in low-current (voltage) circuits (e.g. sensor, switched input and logic) have a
greater affect on circuit operation when compared to high-current output controlled
component circuits. Voltage drops that occur in circuits that provide ground for the logic
circuits within the ECM/PCM can prevent the logic transistors from switching as designed. If
these logic transistors do not switch, computing functions can be impaired.
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Servicing the Engine Management System (continued)
ECM/PCM Electrical Testing (continued)
Power
Power sources related to the ECM/PCM can be grouped into two categories - input power
sources and output power sources. Input power sources can originate directly from the
battery or be routed through the ignition switch. Battery power sources are used to provide
power so that information stored in non-volatile (“keep alive”) memory can be retained by the
ECM/PCM and as a system voltage reference. It is also possible for battery power sources to
be switched internally in the module to provide power to output components.
Ignition switched power sources can be used to provide current to power output controlled
components and also to provide switched input signals. Typical ignition switched voltage
sources have power applied to them when the ignition switch is in the ON, ON/START or
START position. Regardless of the power source, it is possible for multiple components to be
powered from any one power source to the ECM/PCM. It is possible that a defect in one
power source can affect the operation of many aspects of the ECM/PCM.
ECM/PCM output power sources are used to provide module controlled system voltage as
well as regulated voltage power sources to power sensors. The most common regulated
power source is the five-volt regulated source that provided power to sensors such as the
throttle position sensor, fuel tank pressure sensor and the manifold pressure sensor.
Controlled system voltage is used to provide power to Hall Effect sensors such as the
crankshaft position sensor, the camshaft position sensor as well as the vehicle speed sensor
in some applications.
Commonly, open circuits and excessive resistance are two leading causes of power circuit
malfunctions. A short to ground is indicated when open circuits are encountered where a fuse
is open. Fuses “blow” because of excessive current flow - quite possibly from a short to
ground before the load. To determine the location of the short to ground, install a test light in
place of the fuse. So long as the short to ground is present the light will remain illuminated.
Systematically inspect the power source circuit beginning at the ECM/PCM and work back
towards the power source. The circuit may be grounded directly to the vehicle’s chassis or to
a ground circuit within a harness loom. When the short to ground is located and isolated, the
test light will go out.
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Servicing the Engine Management System (continued)
ECM/PCM Electrical Testing (continued)
Power (continued)
As with ground circuit testing, performing voltage drop tests is a reliable way to verify the
integrity of ECM/PCM power source circuits. To perform a voltage drop test on an ECM/PCM
power source circuit:
•
Access the ECM/PCM connector that houses the power source circuit to be tested
•
Remove any cover that may be used to protect the wiring at the connector. Try not
to disconnect the electrical connector.
•
With a suitable electrical probe, carefully back probe the power source terminal to
be tested. Take care so that the wiring insulation does not get pierced and that the
terminal does not get spread.
•
Connect the negative lead of a Digital Multi-meter (DMM) set to read DC voltage to
the probe inserted into the power terminal cavity of the ECM/PCM.
•
Connect the positive lead of the DMM to either the battery or the fuse that protects
the power source.
•
Turn on the circuit to be tested.
•
Note and record the voltage displayed on the DMM.
NOTE: In cases where the voltage drop displayed on the DMM is equal to system voltage
(e.g. 14.2 V DC) an open circuit is indicated. Any voltage drop less than system voltage is an
indication that excessive resistance is present in the circuit being tested.
The value displayed on the meter when a voltage drop test is performed indicates the voltage
used pushing current through the power source. Ideally, no voltage should be “used” in a
circuit conductor. In other words, there should be no voltage drop between the battery (power
source) and the load that is designed to consume power. No voltage drops should be present
between harness terminal connectors. However, for each mechanical connection (e.g. switch
or relay contact) a 100 mV voltage drop is considered acceptable.
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Servicing the Engine Management System (continued)
ECM/PCM Electrical Testing (continued)
Noise
Noise, electrically speaking can be defined as any unwanted voltage activity within a circuit.
There are three majors sources of “noise” that can disrupt voltage signals received by or
transmitted from an ECM/PCM. They include:
•
AC voltage
•
High-current and voltage sources
•
Intermittent power and grounds
AC Voltage
The one source of AC voltage that has the ability to disrupt ECM/PCM functions is the
generator, or alternator. Alternating current is generated within the generator/alternator. This
AC voltage is converted to DC current by rectifier diodes within the generator/alternator.
When these diodes fail, the AC power created within the generator/alternator is not converted
to DC power.
Commonly, the rectifier diodes will fail by either shorting or opening. When a rectifier diode
shorts, AC current is not converted and is allowed to flow through the vehicle circuitry. There
will be a decrease in DC voltage and an increase in AC voltage. When a diode fails open, no
voltage will flow through that portion of the rectifier. With an open diode, there will be a
decrease in DC voltage with no increase in A/C voltage.
Many powertrain components use control signals that rely on the pulse width modulation
(PWM) of DC voltage. AC voltage cycles positive to negative and negative to positive at
various frequencies. When excessive AC voltage is present in the powertrain management
system, these PWM control signals can be disrupted by the cycling AC voltage.
There are instances where sensors are used that generate AC voltage. Permanent magnet
generators, such as wheel speed sensors, transmission speed sensors, knock sensors, some
vehicle speed sensors and some crank or camshaft speed sensors all generate AC voltage.
Permanent magnet sensors typically generate very low voltage signals. The associated
wiring is typically isolated (insulated) from other circuits in the vehicle. This prevents noise (or
transient voltages) from affecting these low-voltage signals. AC voltage that is not converted
by the rectifier can “override” signals generated by these sensors. The ECM/PCM will detect
corrupted signals instead of the proper AC voltage signals generated by these sensors.
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Servicing the Engine Management System (continued)
ECM/PCM Electrical Testing (continued)
Noise (continued)
To measure AC voltage that may be present in the vehicle’s electrical system:
• Start the engine and allow it to run at idle for a minimum of two minutes. This two
minute period allows the battery and charging system to stabilize
•
Connect a Digital Multi-meter (DMM) set to read AC volts to the terminals of the
battery - positive to positive; negative to negative.
•
Read the AC voltage value displayed on the DMM.
•
Turn on vehicle loads such as the high-beam headlights and the rear window
defogger together.
•
Read the AC voltage value displayed on the DMM.
Alternating current voltage should not exceed 50 mV AC when voltage is measured at the
battery. Excessive AC voltage present at the battery is an indication that the
generator/alternator regulator should be diagnosed.
High-Current and Voltage Sources
Components with high current and high voltage demands have the capacity to develop
greater magnetic fields around the component and their associated wiring when they are
active. Low current and voltage components and their associated circuit can have voltage
induced into them if they were to come in close proximity to high current or voltage loads or
their wiring voltage. When the vehicle’s electrical system is designed, consideration is given
to isolate high current and voltage components and their related circuits from low voltage and
current components and their associated wiring.
Two of the most common high-current components that have dedicated circuits are the blower
motor and the rear window defogger. These components are wired as stand alone
components in series having dedicated fused and ground sources.
The most common high-voltage source that can be found within a vehicle is the ignition coil
and its associated secondary ignition cable(s). It is not uncommon for OBD- II vehicles to use
multiple coils. Typically, the ignition coils themselves are not so much as an issue as are the
secondary ignition cables that connect the spark plug to the ignition coil. Secondary ignition
voltage requirements can be quite high when lean air/fuel ratios are burnt or when the engine
is subjected to high load demands. The secondary ignition system is capable of producing
upwards of 40 thousand volts to meet these demands. In order to contain this high voltage,
secondary ignition cables are heavily insulated. Over time, heat and continuous exposure to
high voltage causes secondary wire insulation to deteriorate. Worn or fouled spark plugs
exhibit more resistance that when they are new. As the spark plug ages, their resistance
increases. Coil output increases in an attempt to overcome this resistance further stressing
the secondary ignition cable(s).
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Servicing the Engine Management System (continued)
ECM/PCM Electrical Testing (continued)
Noise (continued)
Aside from replacing old secondary ignition cables to prevent transient voltage from being
induced into low voltage components and their associated wiring, ensuring that vehicle wiring
is properly routed is the most efficient way to prevent noise for this condition. Be sure that all
wiring harnesses are contained in convolute and are adequately wrapped with electrical tape
as required or necessary. Secondary ignition cables should be mounted in their looms and
secured on stanchions so that they are sufficiently isolated from other vehicle wiring.
Intermittent Power and Grounds
Loose or high resistive connections can cause intermittent connections that disrupt current
flow in circuits. The engine management system relies on accurate voltage signals (input
data) to maintain proper control of output components to meet emissions, fuel economy and
performance requirements. Loose connections and high resistance can both contribute to
disrupting signals that are received by and transmitted from the ECM/PCM. Intermittent
connections commonly occur at terminal connections and harness splice points.
Changes in voltage signals that occur to input data signals will have an effect on how output
components are controlled. Disruptions in voltage signals that can occur in output circuits will
affect how powertrain components operate. Intermittent connections can cause:
• Input signals to increase or decrease unexpectedly.
•
Input signals to “drop out” for a given period.
•
Output components that are switched on and off to not function at all or at a fraction
of their full capacity.
•
Pulse-width modulated control signals to effectively function at a different rate and
cause output components to operate differently than the way they are commanded.
Diagnosis of intermittent conditions is more difficult than diagnosing open or excessive
resistance conditions. Both a Digital Multi-meter (DMM) and Digital Storage Oscilloscope
(DSO) can be used to diagnose intermittent conditions.
The DMM can be used to diagnose intermittents related to:
• Inputs signals with linear variable voltage signals such as throttle position sensors
and engine coolant temperature signals.
•
Input signals that are switched (brake, cruise, A/C request, etc.).
•
Output control signals of switched components.
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Servicing the Engine Management System (continued)
ECM/PCM Electrical Testing (continued)
Noise (continued)
To diagnose and locate intermittents with a DMM:
• Connect a DMM set to read DC voltage to the circuit to be tested. Connect the
positive lead to the circuit being tested and the negative lead of the DMM to a
known good ground.
•
Wiggle the harness and connector terminals while monitoring the voltage displayed
on the DMM. Begin at the component and work back to the ECM/PCM.
•
Monitor the signal of the circuit to be diagnosed and look for any sudden or
unexpected change of the voltage.
Any change in voltage when a circuit is wiggled indicates that the intermittent connection is
close to the area where the harness was disturbed.
Inputs that generate frequency-based signals, such as mass airflow sensors and output
components that are controlled with pulse-width modulated signals should not be diagnosed
with a DMM. Intermittent connections will cause disruptions in the frequency of the signals
but it will be difficult to determine whether the change in frequency is part of the normal
operating strategy. For components that generate frequency-based signals the DSO is the
tool of choice.
To diagnose and locate intermittents with a DSO:
• Connect the positive lead of the scope to the signal wire to be tested. Do not
disconnect the circuit.
•
Connect the negative lead of the scope to a known good ground.
•
Start the engine and observe the pattern displayed on the scope.
•
Starting at the component and working back to the ECM/PCM, systematically
wiggle the harness and any connectors while monitoring the scope pattern.
Intermittent conditions will appear as an irregular change in the pattern of the voltage trace.
Any change in voltage when a circuit is wiggled indicates that the intermittent connection is
close to the area where the harness was disturbed.
Scan Tool Interface
The increasing complexity of vehicle technology led manufacturers to develop ways to
effectively diagnose vehicle problems as a result of new electronic hardware. Thus, the
earliest form of vehicle on-board diagnostics was developed by auto manufacturers to
decrease the downtime spent diagnosing vehicles. Other then flash code diagnostics that
were used on some vehicles, the scan tool became the popular diagnostic tool on many
vehicles.
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Servicing the Engine Management System (continued)
ECM/PCM Electrical Testing (continued)
Scan Tool Interface (continued)
This device that service technicians use worldwide today is used to diagnose various vehicle
management systems. This tool originally began as an in-plant quality control tool. The
"assembly line communication link" was used in the late 1970s in assembly lines that built
California versions of the Cutlass Supreme. A simple single-row connector provided the data
to the plant employees.
A scan tool is an interface the technician can use to see what’s occurring in the on-board
engine management and other systems such as ABS, Body, and HVAC etc. The scan tool
may be a handheld unit or a PC based system using special software programming. The
connection between the scan tool and the onboard computer system may be utilizing a direct
cable or in some instances, a wireless interface. Any configuration available is a scan tool if it
communicates with the onboard computer system.
As the complexity of the EMS increased, the functionality of the scan tool had to increase
also. Many of today’s scan tools are able to:
•
Display DTCs
•
Display data parameters
•
Display Readiness Monitor Status (OBD-II)
•
Display Freeze Frame data (OBD-II)
•
Clear OBD data
•
Perform bi-directional testing
Diagnostic Trouble Codes
Diagnostic Trouble Codes (DTCs) can be accessed when a failure occurs that can be
identified by the engine management system. The code will identify the circuit where the
failure occurred. A DTC does not necessarily mean the component is defective. It could be a
short or open in the wiring harness or the component itself. The technician must use a
diagnostic trouble tree to isolate the problem.
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Servicing the Engine Management System (continued)
ECM/PCM Electrical Testing (continued)
Scan Tool Interface (continued)
When a DTC is stored in the
ECM/PCM memory, Freeze Frame
Data is also stored. Freeze frame
data is a ‘snap shot’ of the most
important data parameters as seen
by the ECM/PCM at the exact point
in time the DTC was stored. This
information is often helpful to the
Technician when troubleshooting
the vehicle.
Reading Scan Tool Data Stream
Serial data is electronically coded information transmitted by the ECM/PCM. Using an
analog/digital circuit, this information from sensors, actuators and other calculated data is
digitized and displayed on the scan tool.
In order to display the data in familiar terms, the scan tool interprets each binary word as it is
received and displays it as an analog voltage, temperature, speed, time or another form of
measurement.
When reading scan tool data stream or Parameter Identification (PID) we are looking at three
different types of readings. We have sensor circuit inputs in voltage or hertz, output
commands from the ECM/PCM to actuators in yes/no, on/off; or percentage and calculated
readings, such as throttle opening in percentage and coolant temperature in degrees. And
that does not take into account other PIDs such as open/closed loop.
Clearing OBD Data
A scan tool is also capable of erasing the OBD data stored in the ECM/PCM. This data
includes:
•
DTCs
•
Freeze Frame Data
•
Readiness Monitor Status (OBD-II)
•
Adaptive Information (Fuel Trim, shift timing, etc.)
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Servicing the Engine Management System (continued)
ECM/PCM Electrical Testing (continued)
Scan Tool Interface (continued)
Bi-directional Testing
Bi-Directional tests can be
invaluable when diagnosing
certain malfunctions. Bidirectional is a generic term
used to describe sending and
receiving information between
one device and another. We
could refer to bi-directional
controls as functional tests,
actuator tests, inspection tests,
or system tests. Resetting
basic settings and
reprogramming also could also
be included as bi-directional
controls.
Most enhanced scan tools have the ability to activate relays, injectors, ignition coils and
perform system tests. We can also turn the fuel pump on and off, cycle the a/c clutch on and
off and perform an evaporative emissions leak test. The options programmed into both the
vehicle and the scan tool will determine the range of options available.
When available, bi-directional tests will open up a new world of diagnostics even without
DTCs. As an example, we could activate a solenoid and test its amperage draw. We could
activate the IAC and look for a change in rpm where no change may indicate a clogged
passage. We could operate the transmission shift and pressure solenoids while using a
transmission pressure gauge. This will help us isolate some transmission problems. The list is
endless as manufacturers add more bi-directional features.
Replacing the Electronic or Powertrain Control Module (ECM/PCM)
Occasionally the diagnostic process indicates the ECM/PCM is the cause of the malfunction.
It is important to remember that a defective ECM/PCM is usually a result of another
malfunction.
Many times on earlier engine management systems, a shorted component will damage a
driver (transistor) in the ECM/PCM requiring ECM/PCM replacement. If the shorted
component is not identified and replaced there will be a reoccurrence of ECM/PCM failure.
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Servicing the Engine Management System (continued)
Replacing the Electronic or Powertrain Control Module (ECM/PCM) (continued)
Prior to replacing a ECM/PCM, any suspected component(s) and circuit(s) should be tested
for specified resistance.
Harness connectors should also be checked for:
• Loose, bent, or corroded terminals.
•
Backed out pins.
•
Broken locking tabs.
•
Missing weather packs.
These conditions can cause unwanted resistances, which can cause the ECM/PCM to appear
defective.
ECM/PCM Software Issues
As mentioned earlier, the software that operates the electronic control module is stored in the
Read Only Memory (ROM), Random Access Memory (RAM), and Programmable Read Only
Memory (PROM).
Many ECM/PCMs manufactured since 1993 use an Electronic Erasable PROM (EEPROM).
This means that the PROM can be “reflashed” or reprogrammed for changes in operating
procedures or conditions. Even new ECM/PCMs must be programmed for the specific vehicle
before installation. All equipment is available for the aftermarket; however, most reflashing is
performed at the Parts Supplier.
The next capability that is still to come is
updating vehicle software by means of a passthrough reprogramming device. Because
reprogramming of a vehicle's software will be a
common job in the near future, the Society of
Automotive Engineers (SAE) created a
standard for just such a device -- the SAEJ2534 pass-through device.
The J-2534 unit will connect to the vehicle and to a personal computer (PC). This will simplify
diagnostics and reprogramming. This will require a late-model PC and a PC-friendly
connection. The connection, could be a serial connector, a USB connector as now seen on
PCs of the last few years, a Fire wire connection, an IEEE 802-111x connector or even a
Bluetooth short-range wireless transmission system. Tool and equipment manufacturers of
the J-2534 pass-through device will have to supply a device driver for the PC that will work
with the Application Programming Interface (API) provided by an OEM.
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Servicing the Engine Management System (continued)
Replacing the Electronic or Powertrain Control Module (ECM/PCM) (continued)
ECM/PCM Software Issues (continued)
Using this pass-through device, it will be possible to retrieve new calibration updates or OEM
software upgrades from any one of a few sources, including CDs or an Internet OEM Web site
connection.
Reprogramming of Vehicle ECM/PCM
The ECM/PCM uses an EEPROM (Electronically
Erasable Programmable Read Only Memory) chip
that can be re-programmed. This procedure can
be performed in the shop by downloading the
essential data from a scan tool, personal
computer, or modem that are equipped with the
necessary software. This is required whenever
the ECM/PCM is replaced or there is a factory
update for a drivability or emission problem.
There are different reflashing procedures used.
Remote
You attach a suitable scan tool to the DCL to retrieve the current calibration, then take it to a
PC, which looks up the calibration and, if available, uploads a new one from CD to the scan
tool. Go back to the car and let it download the fresh program from the tool's memory.
Pass-Through
Drive the vehicle up to your in-shop computer work
station, attach cables from the DCL through a scan
tool or dedicated reflashing device and on to the PC,
which will identify the calibration, offer any new ones
that are available, then upload your choice through the
scan tool or a reflash unit. (Either of which acts as a
conduit).
Off-board
Remove the ECM/PCM from the car, take it to your computer-equipped workbench, attach
power and ground, connect to your PC through the reflashing unit and requisite cables, and
fire up the software.
NOTE: Always follow the instructions with your equipment when reprogramming a
ECM/PCM.
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Servicing the Engine Management System (continued)
Replacing the Electronic or Powertrain Control Module (ECM/PCM) (continued)
ECM/PCM Software Issues (continued)
Identifying the Correct & Updated Calibration Software
When confronted with a problem that may be corrected by a factory software update, it is
advisable to first check Technical Service Bulletins (TSBs), for symptoms that are similar to
the problem at hand. The TSB will list the update (reflash) that is required to correct the
problem.
Some manufacturers presently have a website where a VIN can be entered to check for any
updates that are pertinent to that particular vehicle. Some scan tools will display the current
calibration identification that is present in the vehicle.
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