Transcript Auto- - mPortfolios.net

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Heating system
service diagnosis
Common heating system problems
• Insufficient heat.
• Blower motor not working or not working on all
speeds.
• Mode doors not working – system stuck in one mode.
• Too much heat – heat can’t be turned off.
• Antifreeze or oil on inside surface of windshield.
• Blower motor noise.
• Odor from HVAC vents.
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Insufficient heat – normal air volume
• With the fan on high speed the normal volume of air is
found at all outlets.
• Possible causes of insufficient heat:
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Low coolant level
Stuck open thermostat
Air in heater core
Restricted or plugged up heater core
Restricted heater hoses
Faulty blend door actuator
Faulty heat control/heater core shutoff valve
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Check the coolant level
• Serious personal injury can result if the radiator cap is
removed when the engine is hot.
• The coolant level must be checked before the engine is
started.
• The radiator should be completely full on cooling
systems that use overflow tanks.
• On systems that use an expansion tank the coolant level
should be between the min and max marks.
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Low radiator – normal overflow tank
level
Empty
tubes
Possible leaking
secondary seal
Possible leaking
overflow tube
• The condition here is caused by a faulty radiator cap, neck
or a leak in the overflow tube.
• The radiator has several rows of empty tubes.
• Air bubbles in that form in the radiator will work their way
into the heater core.
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Coolant leaks
• If the recovery tank is empty and the radiator is low the
system has a leak.
• Fill the radiator with coolant and connect a pressure
tester to the radiator neck.
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Pressure testing
• The pressure tester is a hand
operated air pump and gauge.
• The pressure tester is usually
connected to the radiator neck
with an adapter.
• The pump is operated until the
pressure is near the cap opening
pressure – typically 15 psi.
• The system can then be examined
for leaks
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Areas to look for coolant leaks
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Water pump weep hole
Radiator
Hoses, hose connections
Heat control/shutoff valves
Freeze out plugs
T-stat housing
Heater core
Head gasket
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Heater core leaks
• Since the heater core is enclosed inside the heater box
you wont be able to visually inspect it.
– Coolant leaks in the heater core normally work their way
down to the bottom of the evaporator case where they will
drip out of the evaporator drain tube.
– The evaporator drain tube is close to the bottom of the
firewall
– A leaking heater core will usually discharge a mist of coolant
out of the defroster vents where it will leave a greasy film on
the base of the windshield
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Head gasket leaks
• External head gasket leaks are rare but easy to spot.
• Head gaskets when they fail normally leak internally.
• The coolant leaks into the combustion chamber where
it is converted to steam in the combustion process.
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Head gasket leaks
• White smoke from the exhaust is evidence of a
relatively large leak.
• Small coolant leaks will produce a distinct exhaust
odor.
• Remove the spark plugs and examine them.
• If one spark plug is unusually clean and free of carbon
deposits it is evidence of a coolant leak.
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Finding leaks with ultraviolet light
• Small coolant leaks can
sometimes be difficult to
pinpoint.
• Dye can be added to the
coolant that glows when
illuminated with an ultraviolet
light source.
• Special UV absorbing glasses
are required when using a UV
light.
• Never look directly at the UV light – even when
wearing UV glasses.
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Bleeding the heater core
• When air gets into the cooling system during repair or
servicing the cooling system may need to be burped to
purge the air trapped in the heater core.
• Some cooling systems have a bleeder screw on the
heater outlet hose or one the high point on the engine
where air from the core will collect.
• The system is lightly pressurized. Open the bleeder and
allow coolant to leak out until a steady stream with no
bubbles is observed.
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Burping the cooling system
• If there is no bleeder screw there is sometimes a
temperature sensor that can be loosened.
• When all else fails you can loosen the heater hose
where it connects to the core and pry the hose away
from the tube with a hose removal tool.
• In some vehicles you may have to jack the front of the
car up a foot or more with a floor jack to allow the air
in the system to return to the expansion tank or radiator
core.
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Determining if there is air in the
system
• You can generally tell if there is air still in the system
by counting the number of strokes of the pressure tester
needed to pressurize the system a few psi.
• If it takes more than three strokes of the pump to get the
needle to move there is still some air in the system.
Low engine temperature
• Overcooling or low engine temperature is caused by a
stuck open, incorrect or missing thermostat.
• The OBDII system monitors the operation of the
cooling system and will set a code if the engine coolant
temperature does not rise to within 20 degrees of
thermostat opening temperature after 12 minutes of
driving above 15 mph.
• The check engine light will illuminate and a P0125
code will be set.
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Digital pyrometer
• A digital pyrometer displays the surface
temperature of objects by the measuring
the intensity of infrared light produced
by the object.
• The object to be measured cannot have a
shiny or highly reflective surface.
• A laser pointer is used with most
infrared pyrometers. The laser beam is
used to aim the sensor and has no
bearing on the measurement.
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Determining if the engine is at normal
temperature
• The temperature of the coolant outlet – where the upper
radiator hose connects to the engine – should be within
20 deg of the thermostat rating.
• The temperature of the heater supply hose should also
be within 20 degrees of t-stat temperature.
• With the fan on high speed and the temperature at
maximum there should be a 40 degree or larger drop in
temperature between the heater inlet hose and outlet.
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Incorrect type thermostat
T-stat with bypass valve
Conventional T-stat
• If a bypass valve type thermostat is replaced with a
conventional type thermostat the system will overcool.
• The bypass valve closes the bypass passage when the tstat opens.
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Insufficient heat – engine temp normal
• Possible causes:
– Restricted heater core
– Faulty heater shutoff/control valve
– Faulty blend door actuator
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Restricted heater core test
• The classic test for heater core and heater valve testing
is to disconnect the heater hoses an run water from a
garden hose through the core.
• Use a section of universal heater hose and a funnel.
• Do not connect the heater core directly to the garden
hose or use shop air pressure to test for flow. It’s
possible to blow the side tanks off with that kind of
pressure.
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Damaged heater core tubes
Pinched tube
• The inlet tube shown here has been partially crushed
during heater hose service.
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Damaged heater core tubes
• The aluminum heater core
tubes is extremely soft and
can be easily damaged
when the heater hoses are
replaced by twisting the
hose ends off with pliers.
• If the hose is being replaced
slice the hose lengthwise
with a razor blade then peal
the hose away.
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Blend door actuator testing
• Cable type blend door operation can normally by
visually confirmed. Some trim panels may need to be
removed.
• Electric blend door actuators may be visible but you
usually can’t confirm there operation unless you can
observe the movement of the door itself.
• It may be possible to insert a bore scope through one of
the floor vents to observe the operation of the door.
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Too much heat
• Occasionally the complaint is that the heat won’t turn
off – the windows need to be rolled down even in the
winter to be comfortable.
• In nearly all cases the cause is a blend door stuck in the
hot position.
• Often there is something physically obstructing the
door from moving.
• Rodents often find a way into the HVAC system and
build nests. It’s not uncommon to find acorns and other
objects inside the heater box.
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Mode control failures
• In vehicles with vacuum operated mode doors a system
that has only the defrost mode operating no mater what
position the control is set to is likely to have a vacuum
leak between the intake manifold and the mode control
valve.
• If no air is coming out of the face level vents but the
other two modes work well the vacuum leak may be
between the control valve and the actuator or the
actuator may be faulty.
• Vacuum operated mode doors can be easily tested with
a hand operated vacuum pump.
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Recirculation mode testing
• The recirculating mode door operation is difficult to
test because it operates on the inlet side of the system.
• A smoke machine is a good method of determining if
the recirculation door is functioning
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Blower motor – not operating at any speed
• Check the blower motor fuse[s]
• Check for voltage between the two terminals on the
blower motor harness connector
• On older cars with a single wire connector check that
the motor housing is grounded to the body.
– Connect a test light to Battery + terminal and touch the fan
housing with the test light probe – it should illuminate if the
ground connection is good.
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Blower motor – not operating on low or
med
• Remove the blower resistor and check resistance
between terminals.
• There should be less than 10 ohms resistance between
any two terminals.
• If the resistor checks out OK the fault may be in the
blower motor switch.
– AllData should have a ‘truth chart’ that identifies which
terminals on the switch have continuity at each switch
position
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Electronically controlled HVAC
diagnosis
• A scan tool is normally used at a dealership to diagnose
electronically controlled heating and A/C components.
• The scan tool can display the position of the door or
speed of the fan commanded by the control head.
• The scan tool can also send a signal to the BCM to
operated a door or turn on the fan.
• Most electronic HVAC systems monitor their operation
and set failure codes if a fault in the system is detected.
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Fan noise
• Possible causes
– Leaves and twigs trapped between the fan and fan housing
• Rattling type noise
– Fan motor bearing failure
• Howling or screeching type noise
• Often a failed bearing only makes noise when cold or for a few
seconds after the fan is turned on
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Odor
• Common sources of HVAC odor:
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Rodent nests in HVAC plenum or heater box
Mold on A/C evaporator core
Evaporator leaks
Heater core leaks
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Rodent nests and mold removal
• Cleanup of rodent nests normally requires removal and
disassembly of the heater box so that it can be cleaned
and disinfected.
• Mold on the evaporator or heater core can be treated
with chemicals that can be sprayed into the heater box
with out removing it from the vehicle. This procedure
will only partially clean the evaporator.
• The best method of removing mold from the evaporator
and heater box is a complete disassembly of the box so
that the evaporator can be immersed in the disinfecting
chemicals.