This is the first of three case studies presenting a complete A/C system using OEM (Original
Equipment Manufacturers) information. The case study is not written in textbook fashion.
This is because the purpose of each case study is for the reader to appreciate the breadth and
depth of information available and what information is required to enable the technician to
successfully work on such systems.The author has attempted to try to present the information
in a logical order. Photos of system components have been added to help visualise their pos-
ition and explanations have been written to assist in the understanding of system operation. If
the reader fails to understand a particular aspect of the case study then refer to a section which
covers that topic.
Information from Ford is usually sourced electronically via an intranet. The Ford informa-
tion system includes training and technical information, diagnostic routines, bulletins and wiring
diagrams.
Ford manual control system (Fiesta)
Described using OEM information.
Manual temperature control
Single evaporator
Single zone
System information
The manual control system used by this manufacturer has a manual blower control selection,
manual air distribution, manual temperature control, and switch operated A/C and recircula-
tion.The only information the module must process is based on A/C switch input, recirculation
input, and temperature variation for the solenoid operated water control.The blend door and
air distribution are all Bowden cable operated except for the recirculation door which is DC
motor operated. The electronic circuitry required for the operation of such a system is very
simple.The module has no memory functions (EPROM) cannot be programmed and is not a
part of a multiplex network.This means that the circuitry is designed as an ASIC (Application
Specific Integrated Circuit).The output from the unit can be monitored by more electronically
advanced modules like the Powertrain Control Module (PCM).
Filtering
A pollen filter is fitted as standard. This cleans the incoming fresh air of pollen and dirt par-
ticles. It must be renewed after every 40 000 km under normal operation conditions, or after
20 000 km under difficult conditions.
Air distribution
The blower motor has four speeds, controlled by the blower motor operating switch through
a resistor pack.The position of the air distribution flap is adjusted by a splined shaft.The recir-
culation air flap is operated by a servo motor. If the air-conditioning system is switched on
but the blower motor switched off, the air-conditioning system will not operate. The air-
conditioning system will only operate if the blower motor is on.The temperature control and
air distribution flaps are controlled by two operating cables.Air is supplied to the windscreen
and side windows regardless of the air distribution setting. If the air distribution rotary control
is turned to the ‘Defrost’ or ‘Foot well/Defrost’ position, the air-conditioning is switched on
regardless of the position of the air-conditioning switch or blower switch.As a result, the air in
the vehicle used to defrost the windows is dehumidified before being distributed.
Face level
The main flow of air enters the passenger compartment at face level.
Face level /floor level
The main flow of air enters the passenger compartment in equal parts at face and floor level,
and a small amount flows to the demister nozzles of the side windows.
Floor level
The main flow of air enters the passenger compartment at floor level, and a smaller amount
flows to the demister nozzles of the side windows.
Demist /floor level
The main flow of air enters the passenger compartment in equal parts at the windscreen and
at floor level, and a smaller amount flows to the demister nozzles of the side windows.
Demist
The main flow of air enters the passenger compartment at the windscreen and a smaller amount
flows to the demister nozzles of the side windows.
A/C system
The system uses an FOV valve which is explained in detail in Chapter 1, section 1.8. Later sys-
tems can have a variable orifice valve fitted.
Figure 3.131
A/C system operation
The temperature is set manually through the heater control module.The heater control mod-
ule operates the heater control valve cyclically (up to 18 cycles per minute) according to the
occupants’ temperature selection which is sensed by a potentiometer inside the heater control
module.This controls a heater control solenoid which regulates the flow of coolant to the heat
exchanger.The air-conditioning switch activates the air-conditioning relay through the heater
control module.This relay applies voltage to the low pressure switch.The low pressure switch
controls the operating cycles of the compressor according to the pressure on the low pressure
side of the refrigerant circuit. If the low pressure switch is closed then the PCM will receive a
voltage signal.When the air-conditioning is activated, the compressor clutch and cooling fan
(stage 1) are energised.
The A/C system will be deactivated in the event of the following:
1. Engine overheating sensed by the coolant temperature sensor 120°C.
2. Engine under load, TP sensor output 3.4 volts. So the full engine power is available for
vehicle acceleration.
3. Compressor cycling switch is below lower threshold approximately 1.6 bar.
4. High pressure switch is open, indicating excessive pressure in the system 30 bar.
5. Internal blower fan not operating.
6. Compressor thermal protection device becomes open circuit.
When the air-conditioning is switched off, the compressor clutch is immediately de-energised.
The cooling fan continues working for a further 40 seconds.The maximum cut-off time for the
compressor clutch with a wide open throttle is 12 seconds.When the air-conditioning is switched
on, the cooling fan (stage 1) is switched on at once.Then, after a delay of about 2 seconds, the
compressor clutch is switched on. If the cooling fan is already switched on, this time is reduced
to 1 second. Switching the cooling fan on in advance ensures that the condenser is cooled
before hot refrigerant gas flows through it. For the first stage the voltage of the cooling fan is
restricted by a ballast resistor. This circuit is closed by means of a relay. For the second stage
the full voltage is provided for the cooling fan through a second relay. Both relays are con-
trolled by the PCM.
The cut-in conditions for the relay for the first stage are as follows:
● Coolant temperature exceeding 95°C once and then remaining over 92°C.
● Air-conditioning switched on.
● Relay for the second stage pulled up.
The cut-in conditions for the relay for the second stage are as follows:
● High pressure switch (P1) closed for at least 5 seconds.
● Coolant temperature exceeding 100°C and then remaining over 97°C.
● Coolant temperature over 95°C for more than 15 seconds and engine speed over 4000 rpm at
full load (WOT).
The vehicle battery has two terminals. A positive terminal and a negative (earth/ground)
terminal.All the electrical power required to run electrical systems will come from this source.
The power supply the battery provides would eventually diminish unless it was topped up. The
alternator carries out this task and maintains the battery’s power supply while the engine is run-
ning.There is a network of cables that runs from the battery power supply to various electrical
systems.Most electrical systems are protected by a fuse.These are located in fuse boxes.
Most manufacturers use a Battery Junction Box (BJB fitted as close to the battery as pos-
sible) and Central Junction Box (CJB generally inside the vehicle) to house fuses, relays,
diodes and sometimes diagnostic connectors. The BJB will contain fuses and relays mainly
related to systems under the bonnet (hood) of the vehicle.The CJB will support the rest of the
power supply to all the other systems.
Battery (01) supply has a DIN designation 30 BA610RD (Fig. 3.132).There is a permanent
feed (not switched), charging system designation (BA), large diameter (10mm2
), high current carrying capacity wire unprotected by a fuse going to the generator (alternator). This is the main power supply of the starter motor and the main feed to the battery from the alternator
to keep the power supply above 12V.
Figure 3.132
Figure 3.133
This power supply from the battery also provides power to the BJB bus bar, a bar inside the
BJB which is fitted to all the fuses and acts as a supply line. In Figure 3.131 the FSC codes are
all 30 meaning permanent feed.This means that the fuses will always remain live at this point.
It does not mean that the circuit will be fully operational all the time. Remember the circuit
will only be live when current can flow through it. Often the BJB supplies a feed to the CJB
(Fig. 3.133).This is why there are often high current rated fuses in the BJB because it feeds a
range of other circuits elsewhere in the system.
This also means that if one of these fuses blows (becomes open circuit) a large number of
systems will not operate. Power distribution is like a pyramid system, at the very top is the bat-
tery positive terminal and at the bottom all the circuits that receive their power from a fuse,
control unit/module or the ignition switch.
Manual A/C schematics
Block diagram
Figure 3.134
Components
A128 heater control module
The heater control module has the heating, ventilation and air-conditioning switches built into
the unit (see Figure 3.88). It is this unit which communicates the information on heating tem-
perature, air recirculation and A/C demand to other components. The heater temperature is
set using the temperature control switch built into the heater control module called a poten-
tiometer. The potentiometer operates a heater control solenoid valve (Y100), which controls
the flow of hot coolant through the heat exchanger. Depending on the setting of this switch,
the heater control module monitors the position of Y100 and varies the voltage applied to it to
control the coolant flow through the heat exchanger.
Figure 3.135
A147 powertrain control module
The powertrain control module controls all of the electrical operations of the engine, for
example fuelling, ignition and diagnostics. It is also used to control the A/C.The block diagram
in Figure 3.134 shows that a wire goes from K32 to the engine control module A147. K47 is
used to supply power to the compressor clutch and can only be operated by A147 when pro-
viding an earth path to the relay.
N75 A/C compressor cycling switch (low pressure and cycling switch)
As described earlier in this chapter this switch acts as a safety device to prevent icing of the
evaporator and operation if no refrigerant is present in the system by becoming open circuit
(open so no current can flow).
Air-conditioning system – low pressure switch/cycling switch
The low pressure/cycling switch is fitted on the accumulator housing which is situated between
the evaporator and the compressor.When the air-conditioning is switched on, the clutch only
receives 12V if the low pressure switch is closed.When the pressure falls below 1.6 bar, the low
pressure switch is opened and the supply of current to the compressor clutch is interrupted.
The low pressure switch functions as a de-icing switch and controls the compressor clutch
on/off cycle.The pressures in the low pressure line and evaporator are practically equal.As the
pressure in the low pressure line drops, the temperature in the evaporator approaches freezing
point. There is a risk that the evaporator will ice up and lose its cooling power. The low pres-
sure switch opens the compressor clutch when the pressure drops to a certain level and will not
close it again until the pressure has risen to a suitable level.
N76 dual pressure switch
Switch one is normally open (high speed fan switch). The high pressure switch is fitted in the
high pressure line between the condenser and orifice valve. The function of the high pressure
switch is to protect the high pressure section of the refrigerant circuit.When pressure exceeds
the maximum value of 31.4 bar because, for instance, the air flow through the condenser is
impeded or the high pressure line is blocked, the high pressure switch turns the compressor off.
The compressor will not be switched on again until the pressure has dropped back to 17.2 bar.
Switch two is normally closed (high pressure switch). The fan speed must increase when
the pressure is more than 20.7 bar to enable additional cooling of the condensor. A147, the
powertrain control module (main engine control module), will not switch the fan to a lower
speed until the pressure has dropped back to 17.2 bar.
Figure 3.136
K158 A/C switch relay
Located in the BJB this relay is operated by the heater control module once A/C has been acti-
vated by the occupants.Upon activation a voltage will go to the low pressure switch and then
to the A147 powertrain control module where under the right conditions the module will acti-
vate K32 to send power to the A/C compressor clutch.
K45 engine cooling fan relay
Operated by A147 (providing an earth path to the relay) this allows the fan to operate at a low
speed due to reduced current flow via a resistor R27.
K46 high speed engine cooling fan relay
Operated by A147 (providing an earth path to the relay) this allows the fan to operate at max-
imum speed to a direct power supply.
K32 A/C wide open throttle relay
Operated by A147 and provides power to the compressor clutch Y16.
N125 blower motor switch
A four speed, five position switch which directs current through a resistor pack R21.
R21 heater blower series resistor
A series resistor pack, providing a range of resistances to reduce the current flowing through
the motor M3 enabling speed control.
R27 engine cooling fan resistor
A resistor which enables reduced current flow to the engine cooling fan motor M37.
P91 Central Junction Box (CJB)
Situated inside of the vehicle and contains relays and fuses.The wiring schematics provide all
the information on the fuse and relay requirements.
Figure 3.137
Figure 3.138
Figure 3.139
P93 Battery Junction Box (BJB)
Situated near the battery and contains fuses and relays.
Y100 heater control solenoid valve
A solenoid operated valve controlling the flow of heated coolant (hot water mixed with
antifreeze) to the heat exchanger inside the vehicle.
Figure 3.140
Figure 3.141
Y16 A/C compressor clutch with built-in thermo-time switch
The compressor clutch is an electromagnetic unit with an internal resistance of 3 .A thermo
switch is integrated in the compressor; at a temperature of 115°C in the compressor outlet
(high pressure side), this switch cuts off the power supply to the magnetic clutch in the event
of the unit overheating.
V7 A/C compressor clutch diode
A diode is used to protect the electrical system from any back-emf (electromotive force) or
voltage spikes caused by the clutch being operated.
M69 recirculation air actuator
The recirculation air flap is operated by an electronic stepper motor (see section 3.2). The elec-
tric stepper motor has two cables attached to a lever which when operated opens and closes
the recirculation doors (see Figure 1.29).
M3 blower motor
The blower motor has four speeds, controlled by the blower motor switch (N125) through a series
resistor pack (R21). If the air-conditioning system is switched on but the blower motor switch
is off, the air-conditioning system will not operate unless the rotary control switch is turned to the
‘Defrost’ or ‘Footwell/Defrost’ position, the air-conditioning is switched on regardless of the posi-
tion of the air-conditioning switch or blower switch.As a result, the air in the vehicle used to
defrost the windows is dehumidified before being distributed. In demist/defrost mode the heater
control module provides an earth path to the blower motor switch enabling the blower motor
to operate.
Electrical system
Explanation of manual control system wiring schematic
When vehicle electricians analyse a vehicle wiring schematic they generally follow a set pattern:
1. Power supply signals.
2. Earth/ground signals.
3. Input – sensors.
4. Output – actuators.
When working on a system which incorporates an ECU (Electronic Control Unit,A128,A147)
electricians will often take all electrical readings from the ECU connector. Some systems have
built-in diagnostics and will display a fault via a code; other systems require a diagnostic tester
to be plugged into a standard diagnostic connector on the vehicle to retrieve information
based on the system’s performance. Even when this has been achieved wiring schematics are
still required to be examined.What happens if the diagnostic plug is faulty? Often technicians
are not taught to read wiring schematics and rely too heavily on diagnostic test equipment.
Research and development engineers still set up rigs in test cells and have to wire systems up
manually. Understanding wiring diagrams promotes a greater depth of system operation and
develops excellent diagnostic skills.
Heater control module circuit
Voltage supply
Voltage is always present at the heater control module (A128) pin 2 C41 (connector 41).With
the ignition switched on, the heater control module (A128), the heater control solenoid valve
(Y100), the recirculation air actuator (M69) and the heater blower motor (M3) receive volt-
age. The heater control motor is controlled by switching the earth path hence 31S at pin 5.
When the air-conditioning is switched on using the switch at the heater control module (A128),
the heater control module (A128) provides a ground signal via pin 11 of A128 (earth, 31S) for
the A/C switch relay pin 1 (K158).The A/C will not be switched on unless the blower motor is
on or the air distribution is switched to defrost. Pin 4 of A128 is live when the sidelights are
switched on to illuminate the display. The heating temperature is controlled by the tempera-
ture selector switch (13) at the heater control module (A128).The heater control solenoid valve
(Y100) is timed by the heater control module (A128 – see Figure 3.129 on page 201).
In this way the supply of heating water to the heat exchanger is controlled. Changes from
outside air to recirculated air operation are made using the switch (1) at the heater control
module (A128).The heater control module (A128) then sends a switched ground signal to the
recirculation air actuator (M69) via pin 9 of the module (A128).
Blower motor circuit
The blower motor and recirculation air actuator receive power from F16 once the ignition
switch has been operated to start or run (DIN code 15).The speed of the heater blower moto
(M3) is controlled via the heater blower switch (N14). If the blower motor is off and the air
conditioning is operating in demist conditions then the heater blower motor (M3) receives
ground signal from the heater control module (A128) pin 7 – see Figure 3.127 on page 196.
Testing blower motors
All tests should include power and earth connections/fuses etc. Testing the circuit operation is
best suited to current measurement.This is also the most dangerous.This could be achieved by
removing fuse F16 and placing an ammeter capable of drawing 25–30 amps or using an amp clamp
around the wires.This will give the health of the electric motor and resistors.Alternatively dis-
connect switch N125 and provide an earth path with a fusible link. Be very careful the blower
motor draws a high current which can create heat or damage testers.Often shunts are used to
reduce the possibility of damage and provide the ability to measure high current flow.Testing
using the wrong cable thickness could act as a hot wire! Always look at the cable thickness and
fuse rating if in doubt:M3 pin 2,wire code 31S – CA18 4BK/RD which is a 4mm2
cross-sectional
area of high current carrying capacity.Any switched earth/grounds 31S can often be tested using
an LED or power probe. If the circuit is constantly being switched on and off the LED will
flash green for earth and red for live.
A/C signal to the PCM (Fig. 3.142)
The relay (K158) then closes, thereby sending a signal to pin 10 of the Powertrain Control Module
(PCM) (A147) via the A/C compressor cycling switch (N75). If there is no refrigerant in the sys-
tem (below 1.6 bar) then the switch will be open circuit and the signal will not reach A147, thus no
A/C system operation. The PCM (A147) controls the A/C Wide Open Throttle (WOT) relay
(K32) via pin 47.By providing a ground (earth) to the relay, power can flow from fuse 32 (F32) to
dual pressure switch (N76) and providing system pressure is not excessive the power will go to the
WOT relay (K32). The A/C compressor clutch (Y16) and the A/C compressor are switched on.
The A/C compressor cycling switch (A75) interrupts the voltage supply to pin 41 of the PCM
(A147) when the system pressure decreases. Consequently, the compressor is turned off by the
PCM (A147) via the A/C WOT relay (K32) in order to avoid ice formation in the evaporator.
When the pressure increases, the A/C compressor is turned on again.
High speed condenser fan (Fig. 3.143)
At the same time the second contact of the dual pressure switch (N76) closes and signals the
Powertrain Control Module (PCM) (A147) to run the engine cooling fan motor (M37) at high
speed.The PCM (A147) then sends a signal to the high speed engine cooling fan relay (K46)
which bridges the engine cooling fan resistor (R27).
System diagnostics
Assuming the clutch is not engaging, refrigerant pressure exists and the technician has selected
to carry out some electrical tests.
Initial diagnostics
Initially, treat the A/C system as operating between two control systems, one being the heater
control module and the other the Powertrain Control Module (PCM). The heater control
module sends an A/C demand signal via the low pressure switch to the PCM. This is a good
starting point if the A/C cycling switch is easy to access. The voltage level should be battery
voltage.A connector view is available from the manufacturer if unsure which wire to check. If
the A/C cycling switch is difficult to access then remove the A/C relay K150 and place either a
power probe,LED tester, or multimeter (continuity or current test) on the relay coil pin which
is provided with a ground signal by the heater control module. If using an LED/power probe
the green LED will light up. Switch the A/C on and off to check the operation of the heater
control module.Remember the A/C switch and blower fan must be running for the ground sig-
nal to be sent. If the signal is present then system diagnostics will focus on the PCM receiving
the signal and the A/C coil being operated. If the signal is not present then diagnostics will
work backward to verify why the signal is not being sent.
Figure 3.142
If the signal is being sent, carry out a relay test on K32.Test to see if the PCM is providing a
ground signal to the relay to activate the A/C system. If the ground is not provided then the fault
exists where the PCM does not want the A/C system to run.This is due to the following reasons:
● It has not received the signal from the A/C relay.
● It has received an input from a sensor providing a signal which is out of operational parame-
ters, i.e. engine coolant over 120°C or below 5°C, throttle position sensor over 3.8V.
Figure 3.143
In this event the PCM will not provide a ground signal to the WOT relay.
A/C relays are always a good starting point for initial electrical diagnostics.
WOT relay is not receiving a ground signal
Check that the A/C signal is being received by the PCM.This can be done at the module con-
nector pin 10 C1159.A connector view can be obtained.A break-out box is sometimes useful
and reduces the possibility of any damage to the connector block.A multimeter or oscilloscope
can be used to verify the correct voltage level. First measure the voltage at pin 10 by placing the
meter probe (red) on pin 10 and the ground probe (black) on ground (for zero reference). Then
carry out a power-to-power check.This is done by placing the red probe on the battery positive
terminal and the ground probe on pin 10.This will check the potential difference between the
two signals.A difference no greater than 0.2V should be present. If greater than this amount
then a resistance exists in the system. If no voltage is present is the first instance then an open
circuit exists possibly due to a faulty A/C switch.
If the signal is being received then a full ‘PCM pin’ test is required starting with all PCM
power supplies and ground signal. ‘Power-to-power’ and ‘ground-to-ground’ tests must also be
carried out to check pds across supplies. On successful completion the coolant, throttle and
vehicle speed sensors should be checked.
During these tests you can provide your own feeds and earths using power probes to test the
system’s response. Never trick the A/C system to switch on if you have no evidence that the
system has sufficient refrigerant and oil.Always seek advice when unsure.All tests should be
carried out at the connector pins using the correct connector terminal fittings (connector pin
kits or break-out box kits).
Serial testing
The heater control module is not accessible via serial testing.The PCM will provide DTCs and
data lists but the A/C information will be limited to the cycling switch, throttle position sensor,
vehicle speed sensor and engine coolant temperature sensor. The data list will provide infor-
mation on whether the PCM is functioning correctly and if it is receiving power.
The serial test is certainly a quick and easy test to carry out and is worth conducting before
any detailed electrical diagnostic testing is done.
Erasing trouble codes
The manual control system uses the PCM to operate the A/C clutch.The PCM receives a sig-
nal from the heater control module via the A/C relay and cycling switch. The only relevant
diagnostic trouble codes available are A/C cycling switch signal, engine related vehicle speed
sensor, engine coolant temperature sensor, and throttle position sensor. Diagnostic trouble
codes are removed using a diagnostic scan tool. Carry out a dealer drive cycle upon retrieval
and removal of codes.
Self-diagnostics
No self-diagnostics are available on this model and year.
System control
Three pressure switches are used to control A/C operation. A compressor cycling switch is
used due to an FOV being fitted. A dual pressure switch is used to protect the system from
overpressure and two operate the secondary condenser fan when the system is under load.
figure .......................
Refrigerant pressures
Preparatory conditions:
1. Pollen filter in good condition.
2. Engine idling and above 85°C.
3. A/C running (5 minutes).
4. Blower speed maximum.
5. Recirculation on.
6. A/C temperature to maximum cold selection.
7. All vents open.
figure..................
Delivery temperature
Preparatory conditions:
1. All windows and doors closed.
2. All ventilation outlets fully open.
3. Engine idling.
4. Select cold on the A/C temperature control knob.
5. A/C/heater blower motor set to maximum speed.
6. Recirculation mode selected.
7. Air distribution knob set to face (centre vent).
Checking:
1. Run engine at 1500–2000 rpm.
2. Position temperature probe 100mm into fascia ventilation centre outlet.
3. Measure temperature after 5 minutes.
figure............
Description Fill capacity/adjustment
figure.................
Since 07/99 vehicles have a fluorescent tracer dye tablet inserted into the A/C system. If tracer
dye is present, there is a green cross on the suction accumulator.
System repairs
The dash panel must be removed to gain access to the evaporator on this model.
0 comments :
Post a Comment