27 February 1998




Stay cool this coming

summer by doing the right

thing now. Andrew Pearce

chills out with advice from

Richard Swindells, partner

in specialists Agricultural

and Mobile Airconditioning

WE SEEM to have a national blind spot about air-con. On the one hand theres hardly a modern tractor, combine or self-propelled doo-dah without it on the options list or in the cab. On the other, its one of those black boxes thats fine until it packs up.

But, says Richard Swindells, keeping that cooled air coming (and the farm out of the hands of dodgy "specialists") is neither complicated nor brain-bendingly hard. Modern systems, if put in well, are pretty reliable. Its just that air-conditioning, like an engine or transmission, needs a little understanding and a good dose of service – and theres plenty that an operator can do.

At least half the battle is knowing how the outfit works; not in detail, but understanding the big picture. For this try Fig 1, which shows how heat is moved from the cab to the outside.

There are six main bits in the system, which these days can be arranged in two ways. As the only big visual difference is where the desiccant container sits, its shown as a dotted box in the diagram.

Moving heat around

Heres what goes on when you click air-con into action. The compressor clutches in, and in the cab a fan starts blowing air over a finned heat exchanger. Inside this is flowing cold refrigerant, so heat – being mobile stuff which never misses a chance to move to somewhere colder – leaves the air and jumps into the refrigerant, which turns into a vapour. Cooled air whooshes into the cab.

Warmed vapour trundles off to the compressor, where its pressure (and temperature) are boosted and its pushed round the circuit. Next port of call is the condenser, a second finned heat exchanger sitting somewhere in a stream of outside (ambient) air. In a tractor or car this one is usually up in the nose; in a self-propelled forager or combine its somewhere close to the engine. Either way the condenser usually sits in front of any other radiators so it gets first shot at the air stream, which is moved continuously past either by a fan, by vehicle forward movement or by both.

Now the hot/cold situation is reversed. Hot vapour is on the inside of the condenser, cooler ambient air is rushing by outside. So heat does its thing again, leaving the refrigerant for the cooler pastures of the outside world. The net result is that heat has been shifted from inside the cab to outside it, leaving the driver nicely cucumbered.

There are a couple of other details. Losing heat turns the vapour to a liquid, which has to be turned back to a vapour again before it gets back to the cab exchanger – a job for the expansion valve or the alarmingly-named orifice tube. And there has to be something in the circuit to take out moisture, which is a real air-con killer.

How come? Both the refrigerant and the compressors lubricating oil (which circulates with refrigerant) just love to hoover up water, even pulling it in through the walls of flexible hoses. The snag is that any free water in the system very soon starts corrosion, which if left, sludges up valves and chews through the metal parts. So somewhere in the circuit is a desiccant container which takes out water vapour on silica gel, and filters out any sludge.

What can you do?

Several areas can be tackled readily by the operator and one cant, but that can wait. The following jobs take little more than time but, stresses Richard Swindells, are far from trivial – each is crucial to good system performance and maximum life.

The best thing you can do for air-conditioning is to use it! At minimum, run the system at least once a month for at least one hour, whether on a tractor or stored machine. Thats the only way to circulate oil, keep the compressor limbered up, keep seals flexible and make sure valves and switches get a thorough work-out. And it wont do a laid-up combines engine and other bits any harm, either. In winter, put a fan heater or similar in the combine cab to give the aircon something to work against, or itll soon switch off.

&#8226 Low air flow over either heat exchanger limits the heat that can be moved. Dirty units mean poor performance and maybe worse. Richard Swindells reckons one of the surer ways to kill a system is to never clean the exchangers, particularly the out-of-sight, out-of-mind one in the cab.

Why? When all is working normally, water condenses out on it and collects all manner of crud from the air – particularly when the cab filter is past its best. Over time the exchangers fins slowly block, choking airflow.

But refrigerant is still pumping round inside as normal, so external water chills into ice. The systems de-icing thermostat shuts off the compressor each time, until finally its contacts weld shut and the compressor runs non-stop. Low heat exchange means not enough heat passes to the refrigerant to turn it fully into a vapour, so liquid gets to the compressor… and bang! Another big bill.

&#8226 Two things follow from this. Change or clean the cab air filter(s) regularly to keep plenty of relatively clean air floating round the cab, and look after both heat exchangers. The cab one should be done annually (a nice little winter job, particularly if the roof has to come off) and the engine-end one as soon as dirt builds up. Stick to a hosepipe and 100psi airline here – not a pressure washer – so the units fins arent flattened into flow-blockers.

&#8226 Keep the compressor drive belt tensioned and change it at the first sign of wear. An obvious one maybe, but the belt has to transmit significant power. If the compressor isnt kept up to speed, performance drops off.

&#8226 Keep the compressor clutch area clean, as oil on the clutch plates means potential slip. Slip produces wear, and eventually the clutchs magnetic grip wont be enough to bridge the widening gap between plates. Repeated oil film build-up may be telling you that the compressor shaft seal is leaking, letting out refrigerant and oil and letting in system-wrecking water vapour. To head off bigger trouble (like a seized compressor), dont hang about – have the seals changed and the system recharged. Special seal-extraction tools really are needed so its not a job to be bodged through.

&#8226 Farming gives pipework and the two heat exchangers a hard time, so keep everything tied down as the makers intended. Vibration is hard on pipes and finned units. Watch for local dirt build-up on hoses, connectors, the desiccant container and anywhere else. If it comes back after a clean, theres a leak underneath which needs to be fixed; see above concerning oil loss, and add the immediate environmental and money costs of losing refrigerant.

Pinhole leaks may stem from internal corrosion (likely if the desiccant container has not been changed regularly, see below) or from electrolytic corrosion between an aluminium unit core and steel sideplates. Many cab evaporators are integral with the heater matrix, and can pinhole if the engine coolant has the wrong corrosion inhibitor or none at all.

Now heres the one routine job that needs outside help – changing the desiccant container. In most systems this is plumbed in with screw fittings, so to swap it the circuit has to be opened. See Box "Know where to stop".

But you cant forget container renewal if the system is to live a long and chilling life. Its desiccant can only hold so much water, and system corrosion starts once it runs out of capacity. Service manuals may be vague about container-change timing, but Richard Swindells isnt. "As a rule, have it changed after the second season and then every year," he stresses.

When cool is not

cool enough

How do you know if your air-con is working reasonably, and whether or not to call a specialist?

Heres a rule of thumb. Shove a thermometer in one of the outlet vents, switch the system on and take a reading as the compressor shuts off – engine note changes slightly. On a 15C day, conditioned air should be 6-10C. If its under 6C, the system is a belter. But if its over the limit or at 0C or less, theres a problem.

A couple of quick checks can take you further. Somewhere in the system – often on or by the compressor – will be a switch or switches which protect the compressor against low pressure (no gas = no oil supply) or overpressure. Bridge the terminals, run the compressor for no more than 30secs and feel its supply/exit pipes. If one is hot, one cold, the system has refrigerant gas. No obvious temperature difference means no gas, or the compressor has stopped compressing.

If this test shows the system is working but only warm air is coming out of the vents, have a look at the heater. On most modern kit the heater matrix sits downstream of the air-con heat exchanger. So, if the heater valve is even slightly open, cold conditioned air will be re-warmed before it gets to the vents. To see if the heater valve is shutting right off, feel the units supply/return pipes. One cold pipe suggests water is passing through the heater and being chilled by aircon air.

If the system works fine until the weather really hots up, the cause may be something quite separate. Plenty of modern tractors have a temperature-sensitive viscous fan, which should shift more air through the radiators as temperature rises. If this fan slips when it should be pulling, air flow drops off and the air-cons heat exchanger cant work too well. Check by running up the tractor in the yard and spraying cold water on the air-con heat exchanger. If system effectiveness perks up, you may have found the problem.

A final word from Richard Swindells. "Once air-conditioning has lost its gas, dont just have it recharged. Find and fix the cause, or youre throwing away money and risking expensive damage."


Agricultural & Mobile Airconditioning (01453-832884) designs, makes, installs and services systems, supplies parts and generally looks after farming equipment. Theres also a technical helpline.

The sun is beaming, the dust is rising. Arent you glad you looked after the air-conditioning over the winter?

Fig 1: How air-conditioning works. The compressor (C) pumps refrigerant round the circuit. The cab fan (F, top) blows warm cab air over a heat exchanger. Heat moves to the refrigerant; cooled, dehumidified air comes out to cheer up the driver. Warmed refrigerant vapour goes through the compressor to the engine-side heat exchanger, loses cab heat to the outside air, changes to a liquid and flows back to the cab to start again. A desiccant container (D) somewhere in the circuit filters the refrigerant and takes out any moisture from it, the expansion valve or orifice tube (EV/OT) changes liquid refrigerant back to a vapour.

The best way to keep a system healthy is to run it, summer and winter.

Clean filters pass plenty of air into the cab for heat exchange – good for the operator, good for the system.

Dirt build-up around a union suggests a leak. See to it promptly. Nip it up to stop refrigerant and oil going out, moisture going in – both are very bad news.

Left: Chafed hoses lead ultimately to trouble. Follow pipe runs and get busy with the cable ties.

Below: Keep the compressor clutch clean. Recurring oil film points to seal trouble or a bearing on the way out; quick repair will probably save a bigger bill.


Just sitting there, an air-con system holds high-pressure refrigerant. If you open a connection or damage a pipe, it literally boils out – and sucked through a burning fag or in contact with hot metal, turns into particularly unpleasant gases. Skin, eyes and health are all at risk unless the right equipment is used; there are legal/environmental considerations, and lost refrigerant simply cant be put back without the right gear. So Richard Swindells message is simple – look after the outside, but dont undo anything. Its just not DIY territory.


If youre keen to persuade management that air-con is a good idea, try this. Its obvious trick is pulling heat from cabin air and passing this to the outside, keeping the operator cool. But it also conditions that air by condensing out water vapour on the cold evaporator, bringing down humidity and pulling much of the dust, pollen and general airborne grot out of circulation. So air-cons net effect is less irritability, better concentration and potentially more productivity all year round. And it demists the windows pretty sharply.

A job often forgotten but crucial to long system life is cleaning the cab heat exchanger – do it once a year. Like most, this one sits upstream of the heater matrix in a combined unit. Unlike many, its easy to get at.

Looking good. With a temperature probe scenting the breeze from a cab vent, this healthy system shows it can pull cab air down to 5C.

A wash with mains water then a blow out with the airline keeps the engine-side heat exchanger clear. Dont use a pressure washer though.

A quick check. If running the system produces no substantial temperature difference between the compressors inlet and outlet pipes, either the system is out of gas or the compressor isnt compressing. If the former is the case, find and fix the leak before recharging with refrigerant!

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