Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
88
When the PCM recognizes a signal or condition that is
absent or out of range, a DTC is stored in memory. A
DTC can indicate a problem in a particular circuit or sub-
system, but does not pinpoint the exact cause of the
problem. Checking codes can direct you to where to
start your troubleshooting. In general, trouble codes fall
into one of two categories: those that indicate a fault
that exists at the time of testing, and those that indicate
a fault that occurred in the past but is no longer present.
Although manufacturers use different terminology to de-
scribe the types of codes, common usage refers to
codes for faults present during testing as "hard" codes,
and codes for faults not present during testing as inter-
mittent or "soft" codes. This book will use the term "in-
termittent" for all codes of that type. As a rule, OBD II
vehicle store all codes relating to emission system com-
ponents as hard codes or faults. The PCM stores infor-
mation as to the number of starts or warm-up cycles
since the code was set.
Code Formats
Pre-OBD II vehicles generated DTCs based on two or
three digits. Each code indicated a particular compo-
nent or circuit that was at fault. These codes are not
necessarily specific to the exact nature of the problem
and can lead to extensive diagnosis to resolve the prob-
lem. OBD II DTCs are composed of a five digit alphanu-
meric sequence, figure 5-3. The OBD II DTC is much
more definitive as to the area of the problem and the cir-
cumstance under which it was set into memory.
Hard Codes
A hard code indicates a failure that is present at the time
of testing and remains permanently in the system until
the cause has been repaired. If the PCM memory is
erased, a hard code will reappear immediately or within
a few minutes after start up because the problem that
set the code still exists in the system. Hard codes indi-
cate a full-time problem that requires immediate atten-
tion. These are top priority problems, and must be re-
paired first, before any other code repairs are made.
Hard codes are generally easy to diagnose because the
conditions that caused the code to set are present at the
time of testing.
A hard code enables you to go right to a certain area, or
areas, and begin pinpoint testing to isolate the defective
component. Most diagnostic charts are designed to
• Run engine for 5 minutes in closed loop,
•
or until “check engine” light comes on.
1
2
3
Light on
Light off
Trouble is intermittent.
Make physical inspection
of circuit for intermittent
connections. Clear
memory.
Under 4 volts
4 volts or over
• Check resistance of coolant
•
sensor. It should be under
•
1000 ohms on a warm
•
engine.
Under 4 volts
4 volts or over
It is a faulty PCM
connection at
terminal “3” or “7”
or PCM. See Chart
C-1.
Replace sensor.
Not OK
OK
It is a poor sensor
connection or
low coolant level.
Check for open in
wires to PCM termi-
nals “3” and “7.”
• Check voltage from PCM
•
terminals. “3” to “7” (sensor
disconnected).
• Disconnect engine coolant
sensor.
• Ignition on, engine
•
stopped.
• Check voltage between
•
sensor connector terminals.
It should be about 5 volts.
4
Fig. 5-4.
Diagnostic charts, such as this one for an open engine coolant temperature (ECT) circuit, are designed for troubleshooting a hard code
failure.
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