AIC Refrigerant Pressure
Switch/Sensor Operation »

The purpose of this module is to compare

two methods used to sense system pressure:

+ F129 AIC Pressure Switch and

+ G65 High Pressure Sensor

F129 AIC Pressure Switch

G65 High Pressure Sensor

A/C Switch/Sensor Operation @

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Before the switch operation is covered,
let's take a look at basic A/C refrigerant
system operation.

A/C Switch/Sensor Operation @

The following components are used in
Volkswagen A/C refrigerant systems:

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The following components are used in
Volkswagen A/C refrigerant systems:

+ Compressor
- Abelt-driven accessory which
pumps refrigerant through the air
conditioning circuit.

The following components are used in
Volkswagen A/C refrigerant systems:

+ Compressor
+ Condenser
- The component where refrigerant
changes from a gas to a liquid.
This is also where the heat in the
system is given off to ambient
(outside) air.

The following components are used in
Volkswagen A/C refrigerant systems:

+ Compressor

+ Condenser

+ Expansion Valve / Restrictor Tube

-Arestrictor which the compressor

pumps against to create high
pressure. This component
controls the flow of refrigerant to
the evaporator.

The following components are used in
Volkswagen A/C refrigerant systems:

+ Compressor
+ Condenser
+ Expansion Valve / Restrictor Tube
+ Evaporator
- Refrigerant changes from a liquid
to a gaseous state in this
component, absorbing heat from
the passenger compartment.

The following components are used in
Volkswagen A/C refrigerant systems:

+ Compressor
+ Condenser
+ Expansion Valve / Restrictor Tube
+ Evaporator

+ Accumulator (Receiver/Drier)

- This component stores excess
refrigerant and contains a
dessicant to absorb moisture.
Older vehicles may be equipped
with a Receiver/Drier.

The following components are used in
Volkswagen A/C refrigerant systems:

+ Compressor

+ Condenser

+ Expansion Valve / Restrictor Tube

+ Evaporator

* Accumulator (Receiver/Drier)

+ AIC Pressure Sensor/Switch

- Senses the pressure in the
system and controls external
components (such as the cooling
fan) accordingly.

Two alternative methods used to sense
refrigerant pressure in the A/C system are:

+ F129 AIC Pressure Switch and
+ G65 High Pressure Sensor

Vehicles are fitted with either F129 or G65,
depending on the design of the A/C system.

F129 AIC Pressure Switch

G65 High Pressure Sensor

F129 and G65, depending on application, are both
located in the same area of the Air Conditioning
system; between the compressor and the condenser.
Both are pressure sensors, but each operates
differently.

F129 AIC Pressure Switch

G65 High Pressure Sensor

A/C Switch/Sensor Operation @
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F129 Operation

F129 uses two pressure-operated switches
to control:

+ Two high-pressure functions, and
+ One low-pressure function

F129 Operation

In the lower graphic, pins 1 and 2 receive
signals from a three-position pressure switch.

This switch is normally closed, but will open
whenever pressure either drops too low
(under 1.6 bar) or rises too high (over 28 bar).

Pins 1 and 2 send signals to either AJC
Cut-off Control Module J314 in manual
control systems, or A/C Control Head E87
in Climatronic systems.

To second speed
To AIC control coolant fan control
head -E87- (FC) relay
2

"y

Load reduction Ground (Gnd)

positive voltage

F129 Operation

In the lower graphic, pins 3 and 4 are
normally open, and complete a Ground
circuit for the Second Speed Coolant
Control Relay J101.

When pressure rises above 16 bar, the
switch closes, causing the relay to run the
second coolant fan.

A/C Switch/Sensor Operation @

To A/C control
head -E87-

To second speed
coolant fan control
(FC) relay

Load reduction
positive voltage

Ground (Gnd)

G65 Operation

G65 is being phased in on Volkswagen
vehicles to replace F129.

Itis mounted in the same location and
performs the same function as F129
but does so in a different way.

A/C Switch/Sensor Operation @
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G65 Operation

Instead of using a pressure-operated switch,
G65 uses a silicon crystal to sense pressure.

Low pressure

Silicon crystal
(resistance)
Voltage

Microprocessor

gue Pulse-width
321 modulated signal

A/C Switch/Sensor Ope!
G65 Operation
Low pressure
Voltage is applied to this silicon crystal. As
the crystal deforms under pressure, the Silicon crystal
appled voltage changes. (resistance)
An internal microprocessor converts this Vol
reference voltage into a pulse-width oltage
modulation signal. Microprocessor
gee Pulse-width
321 modulated signal
Period duration
f° ms = or
Pulse width
2.6ms
x x x
8 N XN
Low pressure
13% = 1.4 bar

G65 Operation

The pulse-width modulated signal
varies in proportion to system pressure.

+ Low duty cycle is a result of low pressure.

+ High duty cycle is a result of high pressure.

Silicon crystal

(resistance)
Voltage
Microprocessor
Pulse-width
modulated signal

Period duration

20 ms = 100°

Pulse width
18 ms

High pressure
90% = 37 bar

A/C Switch/Sensor Operation @
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System Testing

Manual A/C Systems:

Use Guided Fault Finding tests, where available, to diagnose the function of either
F129 or G65.

Climatronic A/C Systems:

Use Guided Fault Finding tests, where available, to diagnose the function of either
F129 or G65. View the function of F129 or G65 using Read Measuring Value Blocks
in Vehicle Self-Diagnosis. In addition, vehicles with Climatronic are equipped with
‘On-Board Diagnostic (OBD) capabilities. If Short or Open circuits occur in these
components, Diagnostic Trouble Codes (DTCs) will be stored in memory.