Actron GM Code Scanner CP9001 - Manual (Page 13)

13
2) Basic Mechanical Checks
Don’t overlook the basic checks listed
on the next page. Mechanical problems
by themselves can always create engine
troubles. Even worse, these problems
can make a good sensor send an
incorrect signal to the computer. Then
the computer runs the engine improp-
erly or sets a trouble code.
• Cylinder compression:
–Check for proper
compression in
each cylinder.
–Refer to vehicle
service manual for
specifications.
• Exhaust backpressure:
–Check for any
restrictions in the
exhaust system.
• Ignition timing (If adjustable):
–Verify timing is within
specifica-
tion.
–Refer to
vehicle
service manual,
or Vehicle
Emission Control
Information
(VECI) decal
located in the
engine compartment.
–Be sure to disable computer spark
advance timing circuit, if used,
when checking basic timing!
• Air induction system:
–Check for
intake
manifold
vacuum
leaks.
–Check for
carbon or
varnish
build-up on throttle valve or idle
speed control device.
3) Read Codes
Refer to Section 3, “Reading Codes”.
Remember there are two types of
codes:
• “Hard” codes – codes for problems
which are present now.
• “Intermittent” codes – codes for
problems which happened in the
past, but are not happening now.
Remember...
–“Check Engine” light ON: You have
at least one “hard” code stored in
memory. (You may have more
“hard” or “intermittent” codes
stored, also.)
– “Check Engine” light OFF: Stored
codes are for “intermittent”
problems. (Exception: sometimes
there are minor “hard” faults which
do not turn on the “Check Engine”
light.)
How to Tell “Hard” Codes from
“Intermittent” Codes
Do the following if you are not sure:
• Write down all codes (except code
12). For example: 15, 34.
• Erase codes from computer
memory. (Refer to Step 5.)
• Drive vehicle for at least 10 minutes
at normal temperature, cruise speed
and load. (The computer may want
to verify a fault for several minutes
before storing a code.)
• Read codes again. Codes which
return are “hard” faults. Codes which
do not return are “intermittent” faults.
For example, if you see code 15 (but
not 34) then you know code 15 is a
“hard” code and code 34 was
“intermittent”.
You troubleshoot the “hard” problems
differently from the “intermittent” ones.
4) Use Vehicle Service Manual
Dealing with “Hard” Codes
• Refer to
the vehicle
service
manual
diagnostic
code
charts.
These will be in Section 6E in the
GM manual. Other publications have
this information in books or sections
by
INDUCTIVE TIMING LIGHT CP7515
Made in U.S.A.
30
60
90
120
150
210
180
240
270
300
3
6
9 12
15
18
21
COMPRESSION TESTER
®