418 Section 4 Computer Control Technology
find the source of complex problems. This will be
discussed in a later chapter.
Again, always refer to the service manual for exact
procedures when testing a computer system. One
wrong electrical connection can destroy the computer.
CAUTION
Never use an ohmmeter to check
a computer, as it will damage the
computer’s internal circuitry. If it
is necessary to use an ohmmeter to check the
continuity of a wire or circuit in the computer
harness, disconnect all wiring harnesses from the
computer before testing.
Saving Memory
Saving memory can be done by connecting a
memory saver to the accessory port (lighter port)
or the data link connector before disconnecting the
vehicle’s battery. The memory saver supplies enough
power to keep the clock, stereo, and computer from
losing the information stored in their memories.
Some memory savers use a 9-volt battery and plug
into the vehicle’s accessory port. Other memory sav-
ers contain a rechargeable 12-volt battery and plug
into the vehicle’s accessory port or the data link con-
nector. See Figure 25-11. In some cases, a battery
jump start box can be used as a memory saver. A
specially designed cable connects the jump start box
to the vehicle’s accessory port or data link connector.
Before using a memory saver, turn off all accesso-
ries (radio, blower, etc.). The current drain from these
devices, combined with even the smallest voltage
drop, could cause electronic devices (computer, clock,
radio, etc.) to lose their preprogrammed data. You
are still disconnecting the vehicle battery for safety.
The memory saver cannot supply enough current to
cause an electrical fi re or operate the starting system.
Computer Replacement
Before disconnecting the battery and removing
the computer from the vehicle, you should scan the
computer and obtain the PROM identifi cation num-
ber or the EEPROM calibration number. This infor-
mation is needed to check for updated PROMs that
should be installed and calibration programs that
should be downloaded to the new computer. Exter-
nal identifi cation numbers are not always placed on
the PROM.
When removing a computer, the ignition key
should be off and the vehicle’s negative battery
cable disconnected. This will prevent voltage spikes
from damaging the computer when the harness con-
nectors are removed. Remove any shields or compo-
nents necessary to access the computer. Unbolt the
brackets holding the computer in place and unplug
the computer connectors.
Identifi cation information is usually stamped
or printed on the computer. Use this data and the
year, make, and model of vehicle to order the correct
replacement computer. The VIN (vehicle identifi ca-
tion number) may be helpful, as well.
CAUTION
When handling computers, keep one
hand on chassis ground and use the
other to remove the component. This
will prevent a static electrical charge from entering
and damaging the electronic circuitry. If available,
wear an anti-static wrist strap when working on
computer circuits. Static electricity may not instantly
ruin an electronic part, but it can reduce the part’s
useful service life from years to days.
Memory saver
Memory saver-to-data link
connector cable
Memory saver-to-accessory
port cable
Figure 25-11. This memory saver has cables for your vehicle’s
accessory port and for the data link connector. (OTC/SPX Corp.)