Chapter
B
: Ignition System Diagnosis and Repair
51
tic trouble codes, are stored in volatile memory. Infor-
mation in the non-volatile memory is permanent and is
not dependant on battery power. Basic operating pro-
gram information and look-up tables, which are not
needed unless the engine is running, are stored in the
non-volatile memory or PROM.
Switching the ignition key to the start or run position di-
rects an additional source of system voltage to pin 12 of
the PCM. This provides the necessary power to operate
the processing circuits. Power applied here is subse-
quently directed to the input sensors through a voltage
regulator as a low-voltage reference signal, and as an
output voltage signal to the actuators. With the excep-
tion of ignition and fuel pump circuits, most input sen-
sors operate on low voltages that seldom exceed 5
volts. However, the internal processor of the PCM re-
quires system voltage. Most control systems will sus-
pend operations and revert to a fail-safe, or limp-home,
strategy should supply voltage fall below about 10 volts.
Input signal processing
To process information from an input sensor, the PCM
applies a low-voltage reference signal, usually about 5
volts, to the sensor. Most two-wire sensors have the
power source and a fixed resistor inside the PCM. The
PCM reads the voltage after the fixed resistor. Current
flows through the fixed resistor, and then through the
variable resistor in the sensor, and back to ground. As
the resistor in the sensor changes, the voltage in the
PCM changes. As the sensor resistance decreases, the
voltage drop decreases, and the read voltage decreas-
es.
A three-wire sensor is usually a potentiometer. A 5-volt
source is applied to one end of a resistor, and the other
end is grounded. The voltage drop is proportional to the
position of a sweep arm that sends the signal back to
the PCM.
Because the PCM operates on low voltage signals, even
small changes in resistance anywhere along a circuit
can effect system performance. Many driveability com-
plaints and control system malfunctions are a direct re-
sult of excessive resistance on the monitored circuits.
Poor (loose, dirty, or corroded) connections and dam-
aged (frayed, pinched, cracked, or broken) wiring and
insulation can all be a source of high circuit resistance.
The most effective way of locating a source of high re-
sistance on a computer-controlled circuit is voltage drop
testing.
Output signal processing
The PCM switches output circuits open and closed to
command system actuators to turn off and on. Power to
most actuators is provided through the ignition switch
circuit, and the PCM regulates voltage on the ground
side. Actuators respond to PCM commands and, in
turn, control other circuits or convert electrical energy
into mechanical work. PCM circuits that command ac-
tuators are often referred to as driver circuits. Typical
output devices include:
• Solenoids
• Relays
• Stepper motors
Almost all actuators in engine control systems contain
some kind of induction coil.
Every output device controlled by the PCM has a mini-
mum resistance specification, and this is an important
consideration when servicing the output side of the sys-
tem. Internal resistance in an actuator limits the amount
of current that is allowed to flow through the output con-
trol circuit. If an actuator is shorted, current in excess of
the safe maximum limit can flow to the PCM and cause
severe damage. Typically, a controlled output will allow
no more than 0.75 ampere (750 milliampere) to flow
through the circuit. Shorted actuators are a common
cause of PCM failure. If not replaced, a defective actu-
ator can quickly destroy a new PCM. Always check out-
put circuits for shorts and low resistance when replac-
ing a failed PCM.
Fig. 2-29.
All computers perform 4 basic functions.
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