TECHNICAL TIPS

Modern Diesel Testing

With Diesel fuelled vehicles accounting for more than 30 % of new vehicle sales, the modern light vehicle auto technician has to have a greater diagnostic skill set than ever before.The Sun Diagnostic help line Eurotech we are seeing an increasing number of calls related to diesel powered passenger vehicles. Like modern petrol vehicles the diesel vehicle uses an Electronic Control Unit that is capable of giving both live data and fault codes.Diesel powered light vehicle has advanced rapidly over the last 20 years from All mechanical and no electronics Mechanical with limited electronics, timing, start of injection feedback. Mechanical with electronic timing, mass, turbo, fuel quantity, shut off and injectors. Common rail type of systems with high-pressure rails and all electronic control.The ECU receives data from various sensors and then commands actuators to perform command functions. The ever increasing demands on the manufacturers to meet stringent emission figures has led to the use of more and more electronic systems. As the systems become more complex, the technician has to be well equipped and informed in diagnosis and faultfinding. The ability to interpret fault codes and live data is paramount to the efficient diagnosis and repair process.

As with petrol a vehicle, the fault code is only the start of the diagnostic road.

One of the most common codes we see is with the definition for the EGR system. Sometimes it is useful to understand what sets an ECU diagnostic trouble code. For example data does not correspond with the presets for an operating condition. The code can be set because the input signal from the map is not registering the data (a change in inlet pressure) to indicate the EGR has opened. The customer will have given a symptom of intermittent power loss or reduced performance.You can see the live data collected on road test. To see relevant data it is important to drive the vehicle when the fault occurs and save a movie. The data taken while under load on the road shows that the mass reading and boost pressures are low for this vehicle. The mass does have a high failure rate but this is not enough evidence to condemn it yet. The low boost could be the result of a number of reasons such as carbon around the EGR valve or a vacuum pipe is split or off.

Live data collected on road test low mass and boost data

Performance was not too bad but it was definitely underpowered. We have the luxury of ECU desired figures displayed for this vehicle. Comparing the actual readings with desired readings you can see that the mass and boost data is low. Not all manufacturers give the desired data so it is advisable to collect data from known good vehicles whenever the opportunity arises. It is always important to take into account such things as; The service history and what the vehicle is used for

Taxi. School run, Just purchased, Pool car.

The history is important, as one of the most common faults we record is low oil level. This causes the total or partial loss of vacuum due to the lack lubrication to the pump. The system needs good vacuum to operate the actuators.The test vehicle was in fact a pool car and a check on the oil level quickly indicates where the problem may be.The recorded reading after the oil level had been corrected. Improved by almost 100%, a good result for 3lt of oil.A correctly operating vacuum pump would give a reading of greater than -75.0 k/pas or 22in/hg. The recorded live data shows the difference in the actual readings after the oil level was corrected. We would expect to see a minimum mass and boost reading of but if in doubt always check with the manufacturers data 750gm/s with this 2lt engine.Boost reading of 1.5-2 bar.EGR should open under light load and close under load conditions.

On this vehicle we can compare the desired with the actual data. An interesting data stream is the EGR command. Remember that this is the command from the ECU and because of the low vacuum there may not be any movement on the actual valve. We believe the greater activity while the mass readings are low is the ECU trying to compensate for the perceived lack of air and limiting the combustion temperature so reducing the production of NOx. All codes were cleared after checking the oil level. The vehicle was road tested and checked again for DTC. Live recordings were taken using the Modis, the Handheld Diagnostic toolbox from Sun Diagnostics

This article is brought to you by the Sun EuroTech Diagnostic Helpline

Facts about refrigeration

“By evaporation, heat is used” this physical truth is the background for all refrigeration. We use refrigerants which have a lower evaporating temperature by 0 Bar than 0° C. The reason is that we sometimes want lower temperatures and we do not want to have a suction pressure (low pressure) below 0 Bar because the moisture and air would in that case heavily enter the A/C systems in case of leakages. R12 and R134a have an evaporating temperature by 0 Bar at approximately –30° C which means that, by evaporating temperatures at –10° C the pressure will be 1,5 Bar . If moisture enters the A/C system, a first effect would be the throttle device to freeze (expansion valve or orifice). The moisture would then mix with the refrigerant and generate Sulphur-Acid (HCL) and this immediately causes the corrosion of bearings, flexible hoses, the copper tubes thus generating Cu Phosphate which will attack all moving parts reducing tolerances and causing, by the end, the stop of the compressor. Another point is that, when the suction pressure is sinking below 0,6 Bar the oil in the compressor will start to leave the compressor and enter the A/C system as a vapour. This means that there will be less oil left in the compressor and this, again, will mean that leakages, clogged driers, clogged expansion valves, stopped fan motor on evaporator could spoil the compressor. To avoid these problems, all A/C manufacturers have a low pressure switch placed somewhere in the system, mostly in connection with the drier. It is an electrical switch which shuts off the clutch of the compressor. Normally, it switches off when the refrigerant quantity is below 200 Gr. On cars and lorries. On busses it reacts by half the refrigerant quantity as normal. Remind that a bus is usually charged with 12-18 Kg. of refrigerant.

Normal charges on A/C systems:
Cars ………………………………. 350-750 gr. Approx.
Lorries ……………………………. 700-900 gr. Approx.
Busses……………………………. 5 – 18 Kg. Approx
Agriculture vehicles …………….. 450 gr. Aprox.
The charge is usually checked by the discharge side (high pressure) showing 15° C + surrounding temperature (refer to a temperature- pressure related table).

Compressor

What the compressor has to do is to move a refrigerant through a pipe and compress it. The compressor forces the liquid refrigerant from the evaporator into the condenser and expansion valve and then back to the evaporator. There are three common types of compressor:

a. Two cylinder reciprocating piston type
b. Four cylinder radial type
c. Six cylinder Axial type

The engine drives the compressor by means of a transmission belt. When it is working the compressor takes low pressure refrigerant from the evaporator and compresses it. The inlet side is known as suction side or low pressure side and the outlet side as high pressure or discharge side Compressor clutchThe compressor has an electromagnetic clutch which can engage or disengage the compressor pulley. The compressor pulley always turns when the engine is running, but the compressor only runs when the pulley is engaged to the compressor driving shaft. Compressor relay A capillary tube from a cycling switch lets the switch know what the temperature is in the evaporator. This switch turns the compressor on and off to keep the evaporator temperature constant and avoids the moisture to freeze on the evaporator core.

The condenser is a long tube going back and forth through a multitude of cooling fins, quite similar to the evaporator in structure. The condenser is mounted in front of the radiator to take advantage of the air provided by the fan the motion of the car. The refrigerant hands its heat off to the air and flows as a cooler liquid in the expansion valve Throttle Devices
There are 3 different forms of throttle device:

1. Expansion valve (Block Valve)(cars and lorries).
2. Thermostatic Expansion Valve, busses.
3. Orifice (fixed opening), cars.

When using the block valve there is always a drier placed in the liquid line, normally black painted or aluminium. The thermostatic expansion valve is normally used in lorries and busses. They are usually equipped with industrial refrigeration plants. When a orifice is used (which is hidden, cannot be seen) there is NO drier in the liquid line but at suction accumulator/filter/receiver on the suction line and it is normally in aluminium The throttle device determines the correct amount of refrigerant going into the evaporator and it lowers the pressure of the refrigerant. It maintains the balance between the heat
load and the cooling efficiency of the evaporator. Evaporator The evaporator is a long tube going back and forth through a multitude of cooling fins, quite similar to the condenser in structure. The refrigerant enters th evaporator as a liquid. A fan blows warm air over the evaporator causing the refrigerant to boil (evaporate). The refrigerant absorbs heat from the warm air which becomes “cool” and is blown into the car interior. The evaporator also removes moisture from the air coming through its fins and turns it into water. The water just drains off the car, this is the reason why your car could drop water while using the
A/C system. Dryers In Cars and Lorries it is not needed to change them every year. In Busses it is needed to change every year. If you do not respect this, the drier could dissolve and the desiccant flow freely in the system and fasten in the expansion valves, and make them not functioning.
To make them work again, the complete system has to be flushed or the expansion valve has to be changed. You do not have to use a special drier but a drier for R134a. Every brand can be used but the connections are different. Therefore, you normally buy from the importer.

When the thermostat of the car exceeds the temperature which has been set, the clutch of the compressor (1) activates and refrigerant vapour is pumped round in the system. The air cooled condenser (2) reduces the vapour superheat. When the condensing temperature has been reached in the condenser, the vapour begins to condense by constant pressure and temperature. In the last end the condensed vapour (which has now become a liquid) is under cooled at least 6 to 8 degrees C from the condensing temperature in order to withstand the pressure loss from the end of the condenser to the throttle device (3) From the condenser the liquid is pressed through a drier which removes moisture and particles from the refrigerant. The drier also functions as a receiver and on this receiver there is normally also a low pressure cut out, sometimes both high pressure and low pressure cut out and a sight glass. From the drier the liquid is pressed to the throttle device (3) which can be an orifice, a thermostatic expansion valve or a block valve (constant suction pressure). In front of the inlet of the throttle valve there usually is a needle filter to retain all bigger particles, on top of the throttle valve (constant expansion valve) the suction line from the evaporator is led through, to control the quantity of liquid which is led to the valve from the liquid line. From the throttle valve, the mixture (approx. 85% liquid and 15% vapour) is led through the evaporator (4) and the liquid part of he mixture evaporates absorbing heat from the air the fan forces through its fins.. The evaporated liquid (superheated vapour) is suctioned through the suction line by the compressor and the cycle is ready to start again. On the suction line (low pressure line) a service connection ( a quick coupling in case of R134a, or a ¼” flare for R12) is mounted, and also on the discharge line (high pressure line) a connection can be found, usually near the drier. The working pressures on A/C plants are usually, both by R12 and R134a, 1,0 –1,5 Bar on the suction line and a pressure equal to 15° C + surrounding temperature on the discharge (high pressure) side. (refer to a temperature-pressure comparison table). To work in a professional way, do not simply follow the high pressure statement on the car A/C manual. The discharge side must have a pressure equal to 15° C + surrounding temperature. This takes into consideration the external conditions during the pressure test.

Common failures on A/C plants

Compressor

a) Leakage of the shaft seal (shaft seal ring)
b) Defective valve plates (suction/discharge pressure equalises)
c) Leakage on service nipples (Schrader needle)
d) No oil left in the compressor
e) Wrong oil has been used (Ester, Mineral PAG)

Discharge line

a) Leakage on the connection to the compressor
b) Leakage on the connection to the condenser
c) Leakage on the security valve on the condenser

Condenser

a) Leakage of the tubes in th condenser, Voltage corrosion.
b) Leakage on the liquid line connection/condenser
c) Clogged condenser on the air side, paper test. Check the painting on the Intercooler , water cooler. Check the fan. Some lorries have a hydraulic pin which lifts by reduced motor rounds. Air in the A/C system.

Liquid line

a) Leakage on liquid line/condenser connection

Drier

a) Clogged drier (feels cold)
b) Moisture in drier/clogged drier (feels cold)
c) Bubbles in the sight glass, lack of refrigerant

Throttle device (expansion valve)

a) Moisture in the throttle device/suction pressure below 0,3 Bar. Warm up with warm cloth
b) Icy ottle valve / dirt in filter before throttle
c) Icy suction line/sensor from expansion lin loose on suction line. Fasten and isolate it.
d) Throttle device closes./Spring inside could be defective
e) Throttle device opens/sensor leaking/spring defective
f) Throttle valve opens/noise between liquid side/ suction side leaking Note: if something is wrong with the throttle valve, replace it, do not try to repair.

Evaporator

a) Evaporator blocked on air side/no cooling/change air filter.
b) Leakage in the evaporator
c) Leakage in the connection suction line/evaporator
d) Low suction pressure/no cooling/fan stopped
e) No refrigeration/no cooling/grid does not close
f) No cooling/high discharge pressure/recover refrigerant until discharge pressure is
equal to 15° C + surrounding temperature

Suction Line

a) Leakage in the connection evaporator/suction line
b) Leakage nin quick connection/tightened Schrader needle
c) Leakage in the connection suction line/compressor