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Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
93
Computer Controlled Timing Service
As discussed in Chapter Two, many late-model vehicles
utilize the PCM to monitor and control the ignition tim-
ing. Timing adjustments are made based on several fac-
tors:
• Engine coolant temperature
• Intake air temperature
• Engine load
• Throttle position
• PCM operating strategies
• Knock sensor input
When one or more of these inputs is out of range the
PCM may not be able to accurately control the timing.
When the PCm determines an input is invalid it will sub-
stitute a value in an attempt to get the best possible per-
formance and emission output from the engine. Some
of the driveability symptoms include:
• Poor fuel economy
• Emissions failure
• Detonation or pinging on acceleration
• Hard starting
As with any computer controlled system use a scan tool
to check for DTC’s before attempting any repair.
Operating Range Tests
The analog signal from a sensor can drift out of range
as the sensor ages or wears. Some sensors can devel-
op an erratic signal, or dropout, at one point in the sig-
nal range. A loose or corroded ground connection for a
sensor can also force the signal out of range, but not far
enough to set a code.
These and similar problems can cause driveability prob-
lems without setting a code. The operation of many sen-
sors can be checked using the operating range charts
provided by the manufacturer. These charts list signal
range specifications for voltage, resistance, frequency,
or temperature that the sensor provides under varying
conditions, figure 5-10.
Use a DMM or scope to test the sensor signal. Take ini-
tial readings at the sensor connector; then, if necessary,
at the main connector to the PCM. If possible, operate
the sensor through its full range and check the signal at
several points.
Road Test and Record Data
If the vehicle transmits computer data in a road test or
normal operating condition, drive it and try to duplicate
the problem. A quality scan tool can electronically
record the data stream when the problem occurs while
driving. Analyzing the recorded data can help locate the
Fig. 5-7. When the catalyst is working efficiently most oxygen is used
to oxidation and reduction, so post converter voltage fluctuations are
minimal.
Fig. 5-8 . As catalyst performance becomes less efficient, less
oxygen is used and voltage fluctuations from the post converter
begin to increase.