Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
104
problems. One example of this may be a car that idles
too high because the ECM monitors the battery voltage.
If the supply voltage is low the ECM may raise the idle
speed so the charging system could charge what the
ECM thinks is a low battery.
To check resistance, make sure that the circuit to be
tested is not under power. Place the leads across the
circuit or component to be tested, figure 5-25. To read
ohms, place the meter on auto-ranging or start at the
higher scales and work down.
Voltage Drop
Checking voltage drop is one of the most important
tests that a technician can perform on a circuit. A volt-
age drop test measures the difference in electrical pres-
sure between two points in a live circuit. Voltage drops
can cause major driveability symptoms in on-board
computer systems. A voltage drop on a ECM power
ground can cause sensor voltage references to be high-
er than normal, throwing off the overall sensor calibra-
tion of the entire engine control system, figure 5-26.
Another example of a driveability symptom might be a
car with an idle speed that continuously hunts. To start
diagnosis, you connect the scan tool to check trouble
codes and the idle smoothes out. This is usually caused
by a poor ground.
To check voltage drop, the circuit must be powered up
and have current flowing. The circuit also must have the
maximum amount of current flowing under normal con-
ditions by which the circuit was designed.
Although there is no exact amount voltage drop that is
considered acceptable, you should remember that low
current circuits that draw milli amps will be affected by
very small voltage drops. A good rule of thumb would
be a drop of 0.2 volt or less. However, even this is too
much for some circuits. A power ground circuit should
have a voltage drop of no more than 0.1 volt. A com-
puter ground circuit should have a voltage drop of no
more than 0.05 volt.
Amperage
Too much amperage flow through an ECM actuator dri-
ver circuit can partially damage that circuit and cause
severe driveability problems. Most ECM actuator com-
ponents carry "milli" amps through their circuits. Using
a ohmmeter and calculating amperage draw from resis-
tance and voltage readings is not always accurate be-
cause the device under test does not carry the actual
VHz~
~
10A
RPM
+
COM
A
C F
RPM
V
%
OFF
VHz
Fig. 5-22.
Ohmmeter connection testing the resistance of a pickup
coil.
VHz~
~
10A
RPM
+
COM
A
C F
RPM
V
%
OFF
VHz
DISTRIBUTOR
BATTERY
IGNITION
SWITCH
COIL
Fig. 5-23.
Using an ammeter to check coil primary circuit current
draw.
Fig. 5-24.
Data Link Connector for all OBD II vehicles.
Summary of Contents for ASE-A8
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