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Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
105
load it was designed to carry. Most actuator devices
carry about 180 ma (12.6 volts at 70 ohms) to 500 ma
(12.6 volts at 25 ohms), but there are always exceptions
to the rule. Fuel injectors may carry much more amper-
age through their circuit (as much as 8 amps depending
on the type of injector). To check amperage draw, the
circuit must be powered up and have current flowing.
Set your meter for amperage draw and connect it in se-
ries between the solenoid negative terminal and ground,
or the actual driver circuit if you can energize it, figure 5-
27. Start by checking amps first, then move down to
"milli" amp scale. Leave the circuit energized for 1 to 2
minutes to check draw.
Remember, this test is for solenoids such as Canister
Purge, EGR, Air Management, only. Do not check fuel
injectors in this manner. Holding an injector on for any
length of time destroys it.
AC Ripple
On-board automotive computers do not like to see AC
ripples pass through the internal components. This e
fect can cause logic problems as well as many other
types of driveability problems. For example, a bad al-
ternator with a dropped diode can severely affect an au-
tomotive computer system. To check for AC ripple volt-
age, switch your DVOM to AC and connect the black
lead to a good ground and the red lead to the "BAT", or
power, terminal on the back of the alternator (not the
battery), figure 5-28. A good alternator should measure
less than 0.5 volts AC with the engine running and the
head lights on. A higher reading indicates damaged al-
ternator diodes.
Frequency
Frequency is the number of times a signal repeats itself
in one second. Frequency is measured in HERTZ. A sig-
nal that repeats itself 10 times a second, is operating at
a frequency of 10 hertz.
Many automotive computer systems read the frequen-
cy of a signal instead of the voltage. Ford MAP sensors
and AC Delco Mass Airflow Sensors are examples of
sensors that produce this type of signal. For example, a
Ford EEC-IV MAP sensor has a 5 volt reference voltage
applied to it. At a duty cycle of 50 percent (half of the
time on and half of the time off), the DVOM will average
the reading so you would see 2.5 volts. However, the
number of times the signal switches on and off in one
second will change depending on manifold vacuum. To
accurately diagnose these signals, you must have a
meter that can read frequency, figure 5-29.
Duty Cycle
Duty cycle is the percentage of time a digital signal is
high verses low. When measuring duty cycle, one com-
plete cycle is considered 100 percent. For a 5 volt sig-
Fig. 5-25. When checking resistance, the part must not be under
power or you will probably destroy your meter.
PCM
BATTERY
+ –
VHz~
~
10A
RPM
+
COM
A C F
RPM
V
%
OFF
VHz
GROUND
Fig. 5-26. Checking voltage drop at the PCM ground connection.