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Air conditioning compressor converted for ‘On Board Air’:
There are two excellent ‘on board air’ articles in the Tech archive, and I thought I would share details of my slightly simplified system together with relevant info for those contemplating such a useful modification.
I have converted a Sanden SD709 Range Rover air con pump for onboard air.
The newer versions which take the more environmentally friendly refrigerants are numbered SD7H15 and are mechanically and dimensionally the same.
The Sanden air con compressors are commonly found on Range Rovers and Discoverys with both V8 and Tdi engines. They come with either V belt pulleys or serpentine belt pulleys depending on age.
All 2.5 diesel, turbo diesel, tdi and v8 engines have the necessary mounting points on the engine (though some diesels don’t have the four mounting bosses on top of the timing case drilled and tapped). With the right factory bracket, tensioner and bottom crank pulley, fitting is easy.
The Land Rover 4cyl petrol engines don’t have the timing case mountings as standard, so I had to make a bracket for my application. I chose to run mine from a second standard water pump pulley bolted on top of the original.
To keep things simple and cheap, I chose not to buy lots of threaded fittings. I welded a short piece of 13mm steel tube to the compressor’s outlet fitting. This is connected to some ½ inch bore rubber air line with some construction adhesive and a jubilee clip. This rubber air line is connected in the same way to a length of standard 15mm domestic copper tube which runs along the chassis. Towards the end of this copper pipe, I fitted a 15mm domestic water system non return valve. This is then connected to another piece of rubber ½ inch air line to the tank/receiver.
For simplicity, I used the complete tank, switch, pressure relief valve etc from a very small workshop compressor (4 litre tank). The pressure switch was adjustable (although these switches are from 240v compressors, they all work fine with 12volts), so I set it to come on at 90psi and switch off at 130psi. The tank fits neatly on top of the rear wheelbox.
Wiring is very simple. A fused ignition controlled supply goes to the on off/pressure switch on the tank, and from here to the clutch on the compressor in the engine bay.
The York compressor (not commonly fitted to Land Rover vehicles) is sometimes chosen over the Sanden because of the rumours that only the York has a sump for oil lubrication. The Sanden also has a 'sump' in that the wobble plate area under the pistons is also designed to be filled with oil in its original air conditioning application.
Just like the York’s sump, this area is connected to the head by a small passage which is designed to let some of the oil circulate with the refrigerant.
Because of the rotary design of the Sanden as opposed to the ‘crank and conrod’ design of the York, the Sanden design works well for onboard air when the ‘sump’ is filled with grease.
This means that there is no need to fit a coalescing filter to prevent oil getting into the tyres.
The valves in the head of the compressor are simple flaps of stainless steel and require virtually no lubrication. For this reason, I didn’t feel the need to fit an inline oiler to the air intake. A little squirt of wd40 or similar into the intake once in a blue moon is all I do.
The simplest way to get grease into the compressor, is to unscrew the top hex filler plug, and put the nozzle of the grease gun into the hole and pump until full-ish, then replace the plug. It may be that the thixotropic swivel housing grease would be good in this application – I haven’t tried it.
I put a filter on the end of my airline to check that no oil or moisture would get to the tyres. The filter never collects anything. I drain off any moisture from the tank before and after each offroad day, and that seems to be all it needs.
Points to note:
You don’t need an ‘unloader’ type pressure switch for onboard air. Compressor will easily cope with full system pressure in the pipe work etc on startup.
You don’t need a tank for airing up tyres, but it helps to collect any condensation/moisture/oil residue from the air, which is therefore prevented from reaching the tyres, and can be drained off regularly.
I used a 15mm domestic water system non return valve (from Toolstation - £1) because the one fitted to the original compressor tank looked small and restrictive. Conventional air ones are readily available but at higher cost.
Domestic copper tube and fittings are perfectly capable of handling the air pressures.
If you use copper pipe, it should be fitted in a way so that it doesn’t fatigue due to flexing/vibration (use flexible hose at either end).
Use rubber or silicone air hose close to compressor as some plastic types may get soft and weak with the heat. If you wanted to run ARB air difflocks, you can either set the pressure switch to keep the system pressure at the required pressure (will reduce tyre inflation performance a bit), or fit a cheap regulator to the system to feed the ARBs, whilst keeping system pressure high for tyre inflation.
I used a cheap K&N style crankcase filter on the compressor inlet (about £2 new from ebay) I have a large plastic aerosol cap which I fix over the filter which still allows plenty of air flow but stops the filter getting splashed with muddy water.
You can use all sorts of different pressure vessels for the tank, eg fire extinguishers, gas bottles, truck air brake tanks, or whatever. Being able to regularly drain them is important.
A pressure relief valve/safety blow off valve is vital in case the pressure switch fails or the clutch fails to disengage.
I enlarged the compressor head and fitting orifices with a drill (this may not have been necessary) and used as big a bore hoses and pipes as I could, in the belief that it would keep any restrictions to a minimum. Fittings where necessary are ¼ bsp and are the narrowest points in the system. Tool airline is 3/8 inch bore.
Performance:
I only have a tiny 4 litre air tank, but my system will inflate a 32inch tyre from Zero psi to 32psi in only 25 seconds (with the engine at around 2000rpm). It will pump air faster than it can go through the tyre valve, so the compressor will turn itself off and on during filling.
Even with such a small tank the impact gun will undo wheel nuts, as long as they aren't over tightened (cheap impact gun – probably not very efficient).
Useful links:
I discovered reasonable value pressure switches, safety blow off valves etc can be bought from Matt Savage at:
Matt Savage Land Rover Parts VIAIR Compressors
I have no connection etc.
Sanden specs and workshop manual:
http://www.sanden.com/support/pdf/sd7servicemanual.pdf
Photos: 2006_1212Onboardair060001.jpg ( 71.77K ) Number of downloads: 210
rear_of_compresssor.jpg ( 71.91K ) Number of downloads: 186
non_returnvalve.jpg ( 55.37K ) Number of downloads: 103
2006_1212Onboardair060006.jpg ( 59.66K ) Number of downloads: 209
2006_1212Onboardair060007.jpg ( 64.38K ) Number of downloads: 143
Another On board air write up - LR4x4 - The Land Rover Forum
Have fun with your onboard air!
