Turbocharger Operation; Boost Pressure Test - Hummer H1 Service Manual

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2-16 Engine

TURBOCHARGER OPERATION

The wastegate valve controls boost. It is operated by the vac-
uum actuator. Opening the valve stops boost and closing the
valve generates boost. Boost levels are continuously monitored
by the PCM.
In the operation, an increase in accelerator pedal sensor angle
to boost detent, causes the pedal sensor to signal the PCM that
engine load is increasing. At this point, the PCM closes the
wastegate valve with the actuator and increases fuel flow to the
injectors.
Closing the wastegate valve allows full exhaust gas flow into
the turbine housing. As the gas enters the housing, it acts
against the blades on the turbine impeller causing it to rotate.
Since the turbine and compressor impellers are fixed to the
same shaft, the turbine impeller causes the compressor impel-
ler to rotate as well.
Once the compressor impeller begins rotating, it draws fresh air
into the compressor housing. The air is then compressed and
pumped into the intake manifold. Fresh air source for the com-
pressor section is through the air cleaner and related ducting.
A decrease in pedal sensor angle signals the PCM that boost
can be reduced or stopped entirely. The PCM then opens the
wastegate valve by reducing vacuum to the actuator. Once
open, the wastegate causes exhaust gas to bypass the turbine
housing and impeller.
Electronic Accelerator Pedal
The electronic pedal is unique to models equipped with a turbo-
charged engine. It consists of a pedal, arm, and position sensor
(potentiometer), attached to a mounting bracket (Figure 2-9).
POSITION SENSOR
(POTENTIOMETER)
MOUNTING
BRACKET
Figure 2-9: Electronic Accelerator Pedal
The position sensor is a potentiometer that operates like a TPS.
It signals pedal position to the PCM. At boost detent position,
the potentiometer signal causes the PCM to close the wastegate
valve and generate boost.
Turbocharger Service
The turbocharger is not a repairable component. It must be re-
placed as an assembly when diagnosis indicates this is necessary.
Wastegate Actuator Test
The actuator is tested with a hand operated vacuum pump.
Testing is performed with the engine off (not running) as fol-
lows:
1. Operate wastegate lever by hand. Lever should move back
and forth freely without bind or drag.
2. Disconnect vacuum line at actuator and connect vacuum
pump to actuator.
3. Apply 5 inches vacuum to actuator and note action.
Actuator rod should close wastegate smoothly. If rod fails
to move, release vacuum and proceed to next step.
4. Disconnect actuator rod at wastegate lever. Apply 5 inches
vacuum to actuator, and note rod operation. If rod fails to
move replace actuator. But if rod does move wastegate
valve or lever is binding. Free up lever with heat valve
lubricant. Replace turbocharger if wastegate valve is
seized or binding.

Boost Pressure Test

Boost pressure is checked with a compound vacuum/pressure
gauge. A road test under boost conditions is required. Proce-
dure is as follows:
1. Attach compound gauge hose to Þtting on air inlet tube, air
horn, or turbocharger outlet.
2. Route hose to passenger side-window and into vehicle
interior.
3. Road test vehicle. Have helper either drive or observe test
gauge. Press accelerator pedal to wide open position and
note gauge reading. A gauge reading of 2 psi (14 kPa) or
more indicates normal operation.
PEDAL AND
ARM
NOTE: During deceleration, boost level may decrease, this is
normal.
4. If gauge reading is zero, or well below 2 psi (14 kPa),
check complete air intake and exhaust system for leaks
and repair as needed. Also check vacuum actuator, and
accelerator pedal potentiometer. Run electrical check with
Tech 2 scan tool if electrical system problems are
suspected.
Vacuum Pump Test
The pump can be checked with a standard vacuum gauge. Out-
put should be minimum of 20-21 inches vacuum.
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