Chapter 24 Sensor, Actuator, Computer Service 461
❑ Never disconnect the battery cables while the engine
is running. In the past, some technicians would do this
to see if the alternator was working. This can destroy
or weaken electronic circuits, causing failure in a short
period of time.
❑ Do not disconnect or connect wiring, especially
the computer wiring, with the ignition key on. This
can cause a current surge that can damage the
computer.
❑ Make sure you do not reverse the battery cable con-
nections. This can destroy electronic components.
❑ Wear an anti-static wrist strap whenever you handle
static-sensitive components (removable PROM chips
for example) to protect them from damage.
❑ Only use high-impedance test lights and meters when
checking electronic circuits or their wiring. A conven-
tional test light or meter will draw too much current
and destroy electrical components.
❑ Do not disconnect a scan tool from the data link con-
nector while the ignition key is on. This could create a
voltage spike that can damage the computer.
Computer System Circuit Problems
Almost all electrical-electronic problems are actually
basic circuit problems. A basic circuit problem is caused by
a problem in a circuit that increases or decreases current,
resistance, or voltage. For example, a broken wire could
stop current fl ow or a charging system problem could
decrease output voltage and current fl ow.
Unfortunately, when a minor problem occurs in a
complex circuit like a computer control system, it may not
seem like a minor problem. For example, a poor electrical
connection in a feed wire to a body ground may cause the
computer system or one of the systems it controls to lose
power. This can result in a shutdown of the fuel injection
system, emissions control system, ignition system, or the
entire vehicle.
You might think that any of these systems of the
computer itself is at fault. Several systems could appear
to have a problem. In reality, it is simply a poor electrical
connection in one wire that is causing all the problems. It
is important for an automotive technician to remain calm
Figure 24-1. Scan tool will usually direct you to the area of the vehicle causing the problem. A—“Read DTC Options” allows you to
select stored codes, pending codes (codes that are almost out of normal operating parameters), and manufacturer data. B—Scan
tool denotes the vehicle has tripped two trouble codes. C—Scan tool readout explains that the mass or volume air fl ow sensor circuit
is showing low voltage or current input to the computer. D—Scan tool readout shows there is no engine misfi re problem, the fuel
injection system is working normally, and the catalytic converter is working. E—No signal from the heated catalyst may be due to the
engine not warming up completely. Evaporative emission control system is working normally. Air conditioning has no signal because
it might not be turned on. F—Scan tool indicates both O2 sensors are working normally. The EGR system may not have a signal
because the engine has not warmed fully. (Snap-on Incorporated)
A B C
D E F