Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
101
the cycle. A zero percent or a 100 percent duty cycle in-
dicates that the solenoid is fully off or fully on, respec-
tively.
Attempting to read voltage on a PWM solenoid with a
DMM can be misleading because of the way the meter
samples the signal and the rapid speed at which the
pulses occur. For example, a DMM set to the voltage
scale and connected to a 5 volt PWM solenoid operat-
ing at a 50 percent duty cycle would read 2.5 volts.
However, a DMM designed for automotive use general-
ly has a duty cycle setting. In this mode, the meter dis-
plays the duty cycle of the signal as a percentage.
Pulse width and duty cycle can also be measured on a
lab scope trace. When viewing a trace, look for a good,
clean transition as the solenoid energizes. The slope
should be a vertical line between system voltage and
ground. The collapsing coil creates a spike, or inductive
kick, which can go either up or down, depending upon
how the unit switches. Resistance on the ground circuit
may cause a trace to not pull to the baseline.
Relay
A relay is a digital device that uses a low current
circuit to open and close a switch and control current on
another circuit, figure 5-20. The PCM switches the relay
control circuit to energize and de-energize the relay coil.
An electromagnet in the coil closes the power circuit
contacts to allow a high current signal through the
power circuit. Typically, relays used in electronic engine
control systems include the fuel pump relay, lamp driver
relays, and system power relays. Relays can be
switched on either the power or ground side of the cir-
cuit, and both types can be used on the same vehicle.
Two common relay failures are defective coil windings
and insufficient contact between the points.
Stepper Motor
A stepper motor operates by moving its armature in
small increments in one direction or the other. A pintel,
which does not rotate, is threaded onto the end of the
armature shaft. As the armature rotates, the pintel
moves in or out on the threads depending upon the di-
rection of the armature. These motors contain either two
or four coils. When two coils are used, the PCM controls
the polarity of each coil, as well as the sequence of en-
ergizing them. The PCM can apply a positive charge at
one end of the coil and negative at the other, then switch
to reverse the polarity and magnetic pull on a part of the
armature. By pulsing these two coils in sequence, the
PCM commands the armature to move in small incre-
ments or steps. Stepper motors commonly operate idle
air control valves and fuel or air bleed controls in some
feedback carburetors.
Diagnostic Tools
Accurate diagnosis of computer controlled systems re-
lies on the use of the proper tools. Several different tools
are required for the various test that must be performed.
Among those are:
• Digital multimeter (DMM)
• Diagnostic scan tool
• Oscilloscope
• Breakout box (BOB)
Voltmeter
A voltmeter measures the electrical pressure differential,
in volts, between the two points connected by the leads,
figure 5-21. A voltmeter is connected in parallel, or
across, a component or circuit. A very small amount of
current passes through the meter, while the rest of the
current travels through the normal circuit path. There-
fore, the circuit functions normally with the meter con-
nected.
Ohmmeter
An ohmmeter, which measures the resistance to electri-
cal flow in ohms, can be used to determine continuity
and the amount of circuit resistance, figure 5-22. Unlike
a voltmeter or ammeter, an ohmmeter contains its own
voltage source. Since an ohmmeter does not use circuit
voltage, either lead can be connected to any test point
unless checking diodes or other solid-state devices.
Ammeter
An ammeter, which measures current in amperes, is
connected to the circuit in series, figure 5-23. This per-
mits the current in the circuit to pass through the meter
for measurement. Unlike a voltmeter, an ammeter has
SIGNAL
VOLTAGE
GROUND
RETURN
COMPUTER
5VREF
PULL-UP RESISTOR
THERMISTOR
(LOAD)
Fig. 5-18.
The resistance of a thermistor, which varies in proportion
to temperature, determines how much of the signal voltage is
dropped to ground.
Summary of Contents for ASE-A8
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