Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
85
COMPUTER CONTROLLED ENGINE
FUNCTIONS
The output commands of an engine control system reg-
ulate most, or all, of the following operations:
• Fuel metering, fuel injection or a carburetor
• Ignition timing advance
• Idle speed
• Electric fuel pump control
• Exhaust gas recirculation (EGR)
• Secondary air injection (AIR)
• Evaporative emissions vapor (EVAP) canister purging
• Intake mixture preheating, or early fuel evaporation
(EFE)
• Automatic transmission torque converter clutch
(TCC) lockup, and transmission shifting
• Air conditioning (A/C) compressor clutch engagement
• Electric cooling fan operation
Self-Diagnostic Programs
Most engine control systems have a self-diagnostic pro-
gram that monitors operating signals and records a di-
agnostic trouble code (DTC) in the event of a communi-
cated failure. Although a DTC does not identify the exact
source of a failure, it does isolate the offending circuit
and can save a considerable amount of diagnostic time.
To retrieve codes from memory, the powertrain control
module (PCM) must be placed in diagnostic mode. Al-
though codes are often displayed as sequential flashes
of the MIL, the most accurate method of code retrieval
is with a scan tool. In addition to stored codes, a scan
tool can display data parameters or parameter identi-
fiers (PID’s) transmitted by the PCM on the serial data
stream. What information is available to the scan tool
varies by the system being tested and the tool itself. Fol-
low procedures from the vehicle and equipment manu-
facturers.
System Diagnosis
The electronic control system does not monitor or con-
trol all functions of the engine. Mechanical systems and
components, such as vacuum and coolant lines, wiring
and connections, fuel delivery, ignition operation, and
cooling system condition must be inspected before
troubleshooting the electronic control system. Also
check for body damage, mechanical damage, tamper-
ing, and newly installed accessories that might com-
promise system operation. Conducting a thorough pre-
liminary evaluation often reveals the cause of, or
conditions that contribute to, common driveability com-
plaints.
Prepare the Engine for Testing
To test an engine control system thoroughly, the engine
should be warmed to normal operating temperature.
However, some initial tests, such as troubleshooting a
no-start problem, are performed on a cold engine. Other
instances would include problems that only occur when
the engine is cold, or operating in open-loop. In many
cases, it is advantageous to check engine operation im-
mediately after a cold start, then watch the data read-
ings as the engine warms and goes into closed-loop.
Many driveability complaints and system trouble codes
relate to warm, closed-loop operation. One basic test of
any engine control system is to verify that it goes into
closed-loop operation. As a minimum, the engine
should be run for two to three minutes above 2000
RPM. This allows the oxygen sensor, catalytic convert-
er, and coolant temperature to come up to operating
range and permits the system to enter closed-loop.
Even when warm, many systems require several min-
utes of engine operation to let the open-loop timer in the
PCM expire and before they will enter closed-loop. To
warm the engine completely, you may have to run it for
10 to 15 minutes or drive the vehicle for several miles.
Check MIL Function
All OBD II vehicles are required to have a malfunction in-
dicator lamp (MIL) on the instrument panel that lights to
indicate major system problems. The lamp may be la-
beled:
• ENGINE
• CHECK ENGINE
• SERVICE ENGINE SOON
• Some other term that indicates a system problem
Any MIL should illuminate when the ignition key is
turned on without starting the engine, figure 5-2. This is
a basic bulb check similar to the bulb check for gener-
ator or brake system warning lamps.
If the MIL does not light with the key on, the PCM prob-
ably will not go into the diagnostic mode. The problem
may be as simple as a burned out bulb, it may be a
faulty circuit, or it could be an internal PCM fault. Refer
CHECK
ENGINE
Fig. 5-2.
The MIL, which lights or flashes when a DTC sets, should
come on as a bulb check when the ignition is switched on.
Summary of Contents for ASE-A8
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