Chapter 24 Sensor, Actuator, Computer Service 477
Saving Memory Contents
Saving memory contents can be done by connecting
a small battery (such as a 9-volt battery) across the two
battery cables before the vehicle’s battery is disconnected.
The small battery cannot produce enough current to cause
an electrical fi re or operate the starting system, but it will
provide enough power to keep the clock, stereo, and com-
puter from losing the information stored in their memories.
When using a memory saver, turn off all accessories (radio,
blower, etc.). The current drain from these devices, com-
bined with even the smallest voltage drop, could cause
electronic devices (computer, clock, radio, etc.) to lose
their preprogrammed data.
Computer Replacement
When removing a computer, the ignition key should
be off and the car battery disconnected. This will prevent
any voltage spike from possibly damaging components
when the ECU harness connectors are pulled apart. Unplug
the computer connectors and unbolt the brackets holding
the computer in place.
Identifi cation information is usually stamped or printed
on the computer. Use this data and the year, make, and
model of car to order the correct replacement computer.
The vehicle VIN (vehicle identifi cation number) may some-
times be helpful.
Computer Adjustments
A few computers or electronic control units have provi-
sions for various adjustments. Most often, a screw might be
located on the side of the computer. Turning the screw will
change an operating parameter. This might be base ignition
timing, cruise control sensitivity, and other functions. Keep
this in mind and refer to a service manual when in doubt.
Updated PROMs
An updated PROM is a modifi ed integrated circuit
produced by the auto manufacturers to correct a driveabil-
ity problem or improve a vehicle’s performance. The old
PROM is simply removed from the computer and replaced
with the updated PROM.
Updated PROMs are produced to correct problems
like surging, extended cranking periods, excessive emis-
sions, cold and hot start problem, and unusual driveability
problems that cannot be isolated to one system. If you are
faced with a problem and cannot fi nd the cause, you should
check with the local dealership to fi nd out if there are any
updated PROMs for the vehicle that address the problem.
Due to the popularity of aftermarket chips, or so-
called “hot PROM” performance chips (PROM chips
that enhance engine performance, and in many cases,
increase exhaust emissions), federal regulations require
the computers in all new vehicles to be equipped with
fi xed PROMS. With the frequent number of updated
PROMs released in the past, electrically erasable PROMs,
or EEPROMs, are now used. EEPROM programming is
covered in the next section.
EEPROMs can be reprogrammed to correct drive-
ability problems or improve performance. EEPROMs are
permanently fi xed to the circuit board, which makes the
installation of a hot PROM almost impossible.
EEPROM Programming Using
Computerized Equipment
Most computers use Electrically Erasable Programmable
Read Only Memory (EEPROM) or Flash Erasable
Programmable Read Only Memory (FEPROM) chips that
are permanently soldered to the circuit board. These
chips must be programmed using electronic equipment.
They can also be reprogrammed to correct driveability
and performance problems. See Figure 24-32.
EEPROMs are programmed using a method referred
to as fl ash programming. Flash programming may be per-
formed by downloading the vehicle’s information through
a computer, a computerized diagnostic analyzer, or a scan
tool. Actual programming details vary between manufactur-
ers, but the basic procedure begins by placing the computer
Figure 24-32. Many late-model computers can be repro-
grammed to change internal mapping to correct performance
problems. The dealership usually has to do this with specialized
equipment and software. (Toyota Motor Co.)
Dash
Panel
Side
Brace
Computer
Bracket
Computer
or ECU
Computer
Harness
Computer
Connectors