Chapter 18 Diagnosis and Repair of the Emission Control and Exhaust Systems 375
Copyright by Goodheart-Willcox Co., Inc.
Placing the Vehicle on the Dynamometer
If a static test is being performed, the dynamometer
will not be used. Nevertheless, many technicians prefer to
place the vehicle on the dynamometer during a static test.
To place the vehicle on the dynamometer, drive the
vehicle onto the apparatus and place it in park or neutral.
Leave the vehicle running to maintain engine temperature
and prevent the buildup of vapors in the engine.
Once the vehicle is on the dynamometer, install the
securing devices. These may be snubbing chains, wheel
chocks, or another means of securing the vehicle. These
devices keep the vehicle from coming off of the dynamom-
eter rollers.
After securing the vehicle on the dynamometer, posi-
tion the cooling fans in front of the vehicle’s radiator. The
cooling fans should be operated during the test to prevent
the engine from overheating. A vehicle properly positioned
on a dynamometer is shown in Figure 18-1.
Warning: Never attempt to operate the
vehicle on the dynamometer without
installing the securing devices.
Attaching the Test Equipment
Once the vehicle is placed on the dynamometer and
properly secured, install the analyzer probe 10″–16″ (25 –
40 cm) into the tailpipe. A collection cone is sometimes
used in place of the tailpipe probe. Next, install the engine
tachometer. Some tachometers can simply be placed on the
vehicle’s hood. Others must be attached to the engine. If
necessary, attach the evaporative emissions pressure tester
to the vehicle’s test fi tting. Also, attach a scan tool to the
vehicle’s data link connector, if required.
Performing the Emissions Test
Once all preliminary steps have been taken, you can
perform the emissions test. Both static and enhanced test
procedures are outlined in the following sections. Many
test sequences are automated and the inspector simply
responds to prompts from the computer screen. Exact
procedures vary greatly from state to state, so contact your
local vehicle emission inspection authority to determine
the exact sequence.
Enhanced Emissions Test. If an enhanced emissions
test is being performed, enter the vehicle and make sure the
hand controls on the dynamometer are within easy reach.
Begin by lowering the dynamometer drive-on plates so the
drive wheels are resting on the dynamometer rollers. Place
the vehicle in gear and accelerate to the specifi ed speed.
The test may require that several sequences of acceleration
and braking be performed. Figure 18-2 shows the sequence
of acceleration and deceleration required for one specifi c
enhanced test.
Many test systems will automatically begin the test
when the vehicle reaches the desired speed. Some test sys-
tems require operator input to begin and end the test. Most
test sequences last no longer than four minutes.
The entire test sequence must be performed to obtain
fi nal emissions readings. Often, the test system will stop the
test early when the vehicle emissions are very low or very
high. If you do not maintain the required speeds during
the test, the computer will prompt you to perform the test
again. After the test is stopped, the vehicle will be passed
or failed as applicable.
Note: The simulated speeds vary between
states. Some test speeds are as low as
15 mph (24 kph), while others can reach
58 mph (93 kph). The simulated driving distance can
vary between one-half mile (0.8 km) and two miles
(3.2 km).
Static Emissions Test. If a static emissions test is being
performed, allow the vehicle to idle as the tailpipe probe or
cone picks up exhaust gases. The gases will be drawn into
the analyzer and read. If necessary, accelerate the engine
to the desired speed for the second phase of the static test.
As with the enhanced test, the procedure may be stopped
when the vehicle passes or fails the test.
Figure 18-1. Typical setup for performing an enhanced emissions test. (Photo: Honda)
Computer
Exhaust
analyzer
Fan
Dynamometer
Inertia
flywheels
Computer
Tie-down
straps
Tie-down
straps