Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
102
extremely low internal resistance. If connected in paral-
lel, too much current will pass through the meter and
damage it.
When current draw specifications for a circuit or com-
ponent are known, an ammeter reading can determine
if a short or grounded circuit is present. A reading
greater than that specified indicates a short in the cir-
cuit. An ammeter also can detect excessive circuit re-
sistance, which causes a low reading.
Oscilloscope
Another tool used to measure voltage in many applica-
tions is the oscilloscope. Oscilloscopes or "scopes" are
available in either analog or digital styles. The analog
scope incorporates a true live display much like older ig-
nition analyzers. A beam of electrons writes on a phos-
phorus screen which is laid out in a grid. The analog
scope has the advantage of providing a true live display.
The disadvantage lies in the fact that it cannot store sig-
nals for future review.
Technically, the digital scope is known as the digital
storage oscilloscope or DSO. Inputs to the digital scope
are digitized, processed and displayed on the screen for
viewing. The digital scope has the advantage storing the
information so that in can be saved, parameters can be
calculated, and intermittent glitches can be captured. It
also allows for automatic setup and operation. The dig-
ital scope does not provide a true live readout and may
miss certain short-duration voltage variances.
Diagnostic Scan Tool
The diagnostic tools discussed to this point rely on mak-
ing an actual connection to the circuit or component in
question to make a reading. In today's vehicles it is
often necessary to access information stored within one
of the onboard electronic control modules. Scan tools
are usually hand-held computers containing program-
ming allowing communication with the control modules
They connect to the vehicle via the data link connector
or DLC, figure 5-24. The scan tool sends requests for in-
formation to the control module and displays the results
on its screen.
Most scan tools can be used to read operating para-
meters, read and clear DTCs, and command certain
components to operate.
The scan tool must always be used in conjunction with
the appropriate vehicle service or diagnostic manual.
The manuals will provide specific instructions for con-
necting the tool, reading the information, and interpret-
ing the results. Random use of the scan tool can result
in misdiagnosis and wasted diagnostic efforts. Once re-
sults are obtained they should be compared to pub-
lished guidelines and the appropriate repairs complet-
ed.
Circuit Testing Using Serial Data
Using serial data to test ECM circuits can be of great
value during driveability diagnosis, however, there are
some items to remember. The data that is being read on
DUTY CYCLE =
t/T = 3/4 = 75%
DUTY CYCLE =
t/T = 1/4 = 25%
OFF
OFF
ON
ON
1
2
3
4
5
6
7
8
UNITS OF TIME
PULSE
WIDTH
1
2
3
4
5
6
7
8
UNITS OF TIME
PULSE
WIDTH
T = COMPLETE CYCLE TIME
t = DUTY CYCLE TIME (PULSE WIDTH)
T
T
t
t
Fig. 5-19.
The duty cycle of a PWM signal is the amount of time the solenoid is on in relation to the total cycle time.
Summary of Contents for ASE-A8
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