Blue Print instruction on Air Con System Diagnostics | Part 2

This is the second in a series of articles written to provide technicians with an improved understanding of manual air conditioning systems.

The A/C electrical system consists of an ECU, (usually referred to as an amplifier), a selection of inputs – earths, lives and sensor values – with the primary output control function being the relay for the magnetic clutch. Providing all the sensor values are acceptable the amplifier will power up the magnetic clutch relay, the magnetic clutch will engage and refrigerant will be pumped around the circuit by the now operational compressor.Blueprint ac_pt2_001

The circuit in detail

Blower switch

The blower switch performs 2 functions in 1 unit. Its first function is to provide a route to earth for the heater relay. When the switch is turned to any of its variable speed “on” settings, current can flow though the switch and down to earth to allow the relay to switch on. Once the relay is switched on, a feed is then provided to the A/C switch. Once the A/C switch is turned on, a feed is provided to the A/C amplifier to enable it to operate.

The second function of the blower switch is to perform the function of a variable resistor. Depending upon which position the switch is rotated to, its internal resistance will change accordingly. Position 1 – high resistance, 2 slightly less 3 slightly less still. This varying resistance controls how much current is able to pass through the blower motor; the higher the resistance, the lower the amount of current able to pass through the blower motor circuit. The less current that can pass through the motor, the slower it will rotate. This will in turn control the rate of air being passed though the evaporator and/or heater matrix depending upon the position of the vents in the heater box.

A/C compressor clutch

A fixed capacity compressor is usually fitted with an electromagnetic clutch giving the A/C system an ability to engage and disengage drive to the compressor. The clutch typically consists of 3 parts, a field coil or stator, a rotor assembly or pulley and a pressure plate. The pressure plate is connected to the central compressor shaft. If the pressure plate rotates, so will the compressor internals.Blueprint ac_pt2_002

The pulley is free to rotate at all times and is permanently driven by the auxiliary belt when the engine is running. When the stator is energised a magnetic field is generated. Current flows through the A/C clutch relay closed contact, through the stator winding and down to earth usually through the compressor casing and into the chassis of the vehicle. This magnetic field pulls the pressure plate along the splines of the compressor shaft where it comes into contact with the pulley. Once there is frictional contact between the pulley and the pressure plate the compressor shaft will start to rotate at pulley speed. When this frictional contact occurs the compressor is commonly described as being engaged. When the supply of current to the stator is removed by the A/C clutch relay opening, a spring, seated against the pressure plate, pushes the pressure plate out of contact with the pulley and the pulley is again disengaged from driving the compressor and rotates freely.Blueprint ac_pt2_003

Compressor speed sensor

Some compressors are additionally fitted with a revolution speed sensor. This speed sensor value is monitored in relation to engine speed by the A/C amplifier (amplifier programming takes into account the pulley drive ratio). This comparison gives the A/C amplifier the ability to identify belt and / or magnetic clutch slippage, or even a locked up compressor. In this instance, to protect the system the magnetic clutch is disengaged and A/C operation will cease. The driver is notified about this fault through the light on the A/C on/off button flashing or by some other means dependent upon manufacturer.

High pressure switch

A pressure switch is fitted in the high-pressure side of the system. It can be placed in a selection of locations but it will always be found somewhere in the high pressure part of the circuit in between the compressor output and the thermal expansion valve. Two pressure sensitive switches (diaphragm operated) are located into the one unit.blueprint ac_pt2_005

The two switches combined are able to indicate three conditions. The first switch (switch 1), under normal operating pressures (typically 15bar) will be in its closed or in its “on” state. If the pressure applied to it by the refrigerant is too high (26-30 bar), the switch contacts will open. Once the circuit is broken the amplifier detects the voltage change in this circuit and de-energises the magnetic clutch relay, turning the compressor off. This is done to prevent the system over-pressurising which could cause permanent damage to numerous components in the circuit. Conversely, if the pressure in the high pressure part of the circuit drops significantly (to 1 – 2 bar), the same switch opens in the other direction, again opening the contacts. This again leads to the same outcome of turning off the compressor. The most likely cause of such low pressure in the high pressure side of the system would be insufficient refrigerant. As the refrigerant is used as a ‘vehicle’ for the systems lubricating oil, this could lead to seizure, hence the action of switching off the compressor.

The second mechanical switch located in the switch unit is designed to close when the refrigerant pressure rises to just above normal operating pressure. (16 – 18 bar). When the switch closes, the A/C amplifier switches the condenser cooling fan to high speed. This increases the quantity of air flowing over the condenser causing the refrigerant to cool further and contract. This reduces the pressure back to a normal range which is ideal for optimum cooling of the evaporator.

Evaporator temperature sensor (anti-frosting device).blueprint ac_pt2_006

This is used to sense an excessively over-cooled evaporator condition to prevent frosting on the evaporator fins (ice is an excellent heat insulator and the efficiency of the air conditioning will reduce drastically if the evaporator becomes frosted).

The sensor is an NTC type thermistor and will therefore experience an increase in electrical resistance as its temperature reduces (and vice versa). This change in resistance provides a detectable temperature condition (measured by the amplifier). When the sensor indicates that the evaporator is at risk of frosting, the ECU will disengage the magnetic clutch to prevent this.blueprint ac_pt2_007


Much like the coolant temperature circuit on a typical engine management system, this uses 2 resistances in series, the first resistance inside the amplifier has a fixed value and the second resistance (the thermistor) has a varying value according to temperature. As current flows through the circuit a volt drop occurs across each resistance.

As the temperature drops, the thermistor resistance increases. Conversely, as the temperature increases the resistance of the thermistor drops. If the resistance inside the amplifier is equal to the resistance in the thermistor the volt drop across the first resistance would be 2.5 volts (the 5 volts applied is halved). The remaining 2.5 volts will drop to zero as current flows through the second resistance (the thermistor). As the voltage is monitored inside the amplifier after the first resistance (where the arrow is located on the diagram) the voltage value can be interpreted as a temperature and the amplifier can act accordingly. As the temperature of the evaporator drops, the resistance of the thermistor will increase. This means that there will be a higher volt drop across the evaporator temp sensor than across the resistance inside the amplifier. Therefore the measured voltage value (at the arrow inside the amplifier) will be higher. This higher voltage reading will be interpreted by the amplifier that the temp sensor is detecting a colder temperature and again, the amplifier can act accordingly. The action will typically involve the opening of the A/C compressor relay. This will lead to a reduction of refrigerant flow around the system which will reduce the cooling effect of refrigerant evaporating inside the evaporator. Finally, as the evaporator warms up the resistance of the evaporator temp sensor will drop. This will mean comparatively there will be a larger volt drop across the resistance inside the amplifier and a smaller volt drop across the evaporator temp sensor. As there is a larger volt drop across the resistance inside the amplifier the voltage measured after it will be lower. This reduction in measured voltage at the arrow inside the amplifier will be interpreted by the amplifier that the temperature of the evaporator has increased to a level where frosting is no longer a risk and the magnetic clutch will be re-engaged through control of the relay.

In summary, study the diagram in detail and check the inputs and outputs of the A/C system. A good clear understanding of voltage in a circuit is essential to the accurate electrical diagnosis of the A/C system. It is only when the correct voltages are applied to the A/C amplifier from the key inputs, will the primary output (the A/C compressor relay) be energised.

In next month’s article we will be studying diagnostic techniques which can take you quickly to the problem area, be it electrical, mechanical or gas!

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Published on: June 30, 2015

Filled Under: News, Technical Articles

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