Diagnosis Equipment; Glow Plug Test; Scan Tool Use - Hummer H1 Service Manual

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2-6 Engine
Diesel Fuel Knock or Rattle
The rattling sound unique to diesel engine is normal. It is a
function of high compression ratio, injector nozzle spray pat-
tern and pressure, and compression ignition. However, incor-
rect injection pump timing, low quality fuel, or injector nozzle
faults can make the normal sound much more pronounced. If
the normal rattling is accompanied by, or becomes a knocking
sound, the fuel injectors, injection pump, pump timing, and
fuel quality must all be tested.

DIAGNOSIS EQUIPMENT

Diagnosing engine mechanical problems can be performed
with normal shop equipment.
Compression testing with a good quality gauge will reveal con-
dition of the engine rings, pistons, cylinder bores, valves, and
guides.
Vacuum diaphragm operation is quickly tested with a hand op-
erated vacuum pump. These pumps are available from compa-
nies such as Kent-Moore, Snap-On, RobinAire and Lisle.
A digital or analog multimeter is used for many tasks such as
checking wire continuity, shorted-open circuits and compo-
nents, ground connections, battery condition, glow plug cur-
rent levels, and so on.
A tachometer is required for checking/setting idle speeds. On
diesel engines, an especially accurate tachometer is needed to
check cranking speeds of 100-180 rpm.
An oil pressure gauge is necessary in order to accurately test
engine oil pressure. The best gauges are mineral oil filled to
avoid indicator needle bounce. The gauge should have a dial
range up to 100 psi in 2 psi increments. Necessary adapters to
connect the gauge are available from either the gauge manufac-
turer, or Kent-Moore Tool Corp.
An all purpose dial indicator set such as J
needed. Accurate measurement of clearances, component
wear, chain deflection, and overhaul dimensional settings is
only possible with a dial indicator. A good quality vernier cali-
per and micrometer set are also needed to check part toler-
ances.

SCAN TOOL USE

The Tech 2 scan tool is primarily designed to test/interrogate
PCM/TCM circuits. The tool is extremely useful in locating
sensor, switch, or computer faults that affect engine perfor-
mance. However, the tool is not really designed to replace vac-
uum gauges, compression testers, multimeters, or 12 volt test
lamps.
GLOW PLUG OPERATION AND TESTING
The engine glow plugs are used to heat air entering the com-
bustion chamber. Heating the air results in improved cold tem-
perature starting and engine warm-up.
A total of eight glow plugs are used; one for each cylinder.
Each glow plug is threaded into a cylinder head port located
just below the injector nozzle (Figure 2-5). The tip of each plug
extends into a pre-chamber which concentrates the fuel/air
mixture prior to combustion.
The glow plugs are operated by a relay mounted on a bracket
attached to the right side intake manifold. The relay is in circuit
with the ignition switch and is energized only when the switch
is in the ÒstartÓ and ÒrunÓ positions.
Turning the ignition switch to the ÒstartÓ position allows cur-
rent to flow through the switch to the relay and the PCM. Once
energized, the PCM uses information from the coolant temper-
ature sensor and the intake air temperature sensor to determine
glow plug Òon timeÓ. The PCM commands the relay to allow
current to flow through it and to the individual glow plugs and
monitors each glow plug circuit voltage individually. If a glow
plug failure is detected, a DTC P0380 will be set.

GLOW PLUG TEST

1. Check all glow plug connections. They should be tight and
corrosion free.
2. Verify that engine temperature is at or below 95¡F (35¡C).
3. Turn multimeter to amps or current flow setting.
4. Check current at each glow plug (Figure 2-6). Place
ammeter clamp around glow plug wire. Current should be
13-14 amps.
5. If current at glow plug is less than 13-14 amps, further
diagnosis is required. If current is zero check fuses and
circuit for shorts, grounds and opens. Check the fusible
link between the starter and the glow plug relay.
VALVE
Ð
8001, is also
BLOCK
Figure 2-5: Glow Plug Location In Cylinder
CYLINDER
HEAD
COMBUSTION
PISTON
Head and Pre-Chamber
FUEL
INJECTOR
GLOW
PLUG
PRE-CHAMBER
CHAMBER
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