Chapter 25 Computer System Service 421
ming is completed, turn the ignition switch to the
position called for and disconnect the programming
computer. Then, use a scan tool to check the com-
puter and control system operation. While doing
this, make sure you have installed the proper pro-
gram into the computer by checking the program
calibration number.
Early Model PROM Service
Many older computers use a PROM to store data
for the specifi c make and model vehicle. You may
have to remove the PROM from the old computer
and install it in the new computer.
Remove the access cover over the PROM. Then,
use a PROM tool to grasp and pull the PROM out
of its socket. Most PROMs use a carrier, which is
a plastic housing that surrounds the outside of the
integrated circuit chip. Avoid touching the PROM
terminals with your fi ngers because the body oils
on your hand could adversely affect the PROM’s
operation.
Before installing the PROM in the new computer,
check that the pins (terminals) are straight. Check
for the presence of reference marks (indentations or
other markings to show how to reinstall the unit) on
the PROM or the carrier. If you install a PROM back-
wards, it will usually require replacement because
of physical damage.
To install the PROM, install the carrier and PROM
in the computer with the reference mark correctly
positioned. First press down on the carrier only.
Then, carefully press the PROM down into the com-
puter. Press on each corner until the PROM is fully
seated in its socket. If the PROM socket has locking
tabs, make sure they are connected to the carrier
housing.
After installing the PROM, install the access
cover, connect the wiring harness to the computer,
and install the computer into its mounts. Reconnect
the battery, turn on the ignition, and activate self-
diagnostics. As a fi nal check of the computer and
PROM, make sure no trouble codes are set. A code
might be set if the PROM is not fully seated or a pin
is bent over.
Computer Relearn Procedures
After being serviced, the computer system may
have to go through a relearn procedure. A relearn
procedure is a period of vehicle operation during
which the computer system adapts to new compo-
nents and programming information. The computer
must receive inputs to formulate adaptive strategies
to set some of its output parameters. The relearn
procedure can often be done by simply driving the
vehicle for several minutes. Some vehicles require a
specifi c relearn procedure. Always check the manu-
facturer’s service literature for exact relearn meth-
ods. In a few cases, sensors or actuators may have to
be manually adjusted using a scan tool.
DIAGNOSTIC CASE
STUDY
Concern: Mr. Hayes brought his vin-
tage Firebird into the shop, complain-
ing of a hesitation, or stumble, when he
accelerates the vehicle from a standing start.
Cause: Duff test drives the vehicle to double-check
the symptoms and to observe engine performance.
He immediately notices a hesitation during accelera-
tion. The vehicle seems to operate normally at cruising
speed.
Once back at the shop, Duff opens the hood and
inspects the engine compartment for obvious prob-
lems, such as loose connections, split vacuum hoses,
etc. Although he does not notice any obvious problems,
he does hear a slight hissing sound that seems to be
coming from somewhere near the top of the engine. He
turns the job over to one of his technicians, who checks
the vehicle for trouble codes but fi nds none.
Based on Duff’s initial inspection, the technician
uses a piece of vacuum hose to help locate the source
of the hissing sound. He holds one end of the hose
to his ear while moving the other end around the
engine compartment in an attempt to pinpoint the hiss-
ing sound. The sound seems to be loudest near the
engine’s MAP sensor.
Upon further inspection, the technician fi nds a hair-
line crack in the end of the vacuum hose going to the
sensor. The crack opens up as the engine vibrates
during acceleration. This causes a vacuum leak dur-
ing acceleration. The crack closes up once the vehicle
reaches cruising speed, sealing the leak. Even dur-
ing acceleration, the vacuum leak is too small to set a
trouble code.
Correction: The technician replaces the vacuum hose
leading to the MAP sensor. He then inspects the other
hoses for cracks and hardened ends. In all, he replaces
three vacuum hoses. He then test drives the vehicle to
make sure the repair fi xed the problem before releas-
ing the vehicle to the customer.
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