I found this The tutorial uses a Sanden compressor.... From old Rangie A/c
Onboard Air system for a 200 Tdi Defender, using a Sanden compressor.

Onboard air systems using converted air conditioning compressors have considerable benefits over the usual 12v electric compressors. The best of the 12v models will produce a max of about 3cfm, whereas most air con compressors can comfortably produce at least 8 cfm (some as high as 12.5cfm), and do it on a continuous basis, unlike the limited duty cycle of the 12v models.

There are two basic types of compressors used in automotive air conditioning.

Less common nowadays is the York type compressor. These are 2 piston reciprocating compressors and are generally of a higher output than the rotaries, and also have the benefit of a separate oil sump which can be converted to be isolated from the airflow of the compressor. The downside of the York style is the sheer size and weight of the unit. This makes fitting difficult in most Landrovers, which donít usually have a lot of spare space in the engine bay.

More common is the rotary compressor, using either 5 or 7 cylinders mounted on a rotating wobble plate. Of these, the Sanden brand is the most common. Although generally of less capacity than the Yorks, the Sanden compressors still produce either 8 or 10cfm (at 2000rpm), depending on the model used. The Sandens have both an upside and a downside for use as an Onboard Air system:

The downside is that they rely on lubrication from an oil charge that travels around the system with the refrigerant gas, and cannot be converted to run a separated oil sump. However, this can be addressed by using an air tool oiler to feed oil into the compressor intake to provide the necessary lubrication. To remove this oil from the compressed air output, a coalescing filter is used. These filters will remove both oil and moisture from the compressorís output, ensuring only clean, dry air reaches the storage tank (and your tyres).

The upside is the compact size and shape of the Sanden compressor. In vehicles without Air Conditioning already fitted, it often is simply a matter of getting hold of the factory A/C mounting brackets and using them to mount the compressor as you would for Air conditioning. If A/C is already fitted, then itís usually possible to mount the extra compressor off the side of the A/C compressor. This is what I did in my Tdi Defender.

So, first off, the schematic:

Here you can see both the air flow and electrical for my particular system.

The compressor draws air in through the oiler (to which I also attached a small cheap K & N type filter), which will add a small amount of oil to keep the internals of the pump happy.

Compressed air is then sent via a coil of pipe (in my case aluminium, but copper would be good too), before arriving at the check (or non-return) valve. This will ensure that when the compressor isnít pumping, the air in the tank will not flow back past the pistons and out through the intake line. The purpose of the coil of pipe is to function as a cooling line. The compressed air coming out of the compressor will get extremely hot, 150 deg C or more. At this temperature many of the downstream devices (filter, check valve etc) are a bit at risk because they contain some plastic parts. More importantly though, the filters function more effectively on cooler air and we donít want oil getting past into the tyres.

Once past the check valve, the air passes through the coalescing filter which will remove the oil and any moisture present, before reaching the manifold. Iíve used a manifold arrangement simply as a convenient way of mounting the pressure switch, safety valve and feed for a pressure gauge in the engine bay.

What do these things do?

The pressure switch does pretty much what you would imagine from the name. A simple electrical switch that will cut the power feed to the compressors magnetic clutch (and hence stop it pumping) when a preset pressure is reached. Likewise, when the pressure drops to a lower preset level, it will then trigger the power back to the magnetic clutch and start the compressor pumping again. In my case the pressure switch is set to cut in at about 80psi and cut out at about 120psi.

The safety valve is vital in these systems and is simply there as a back up in case the pressure switch for some reason doesnít cut the power to the compressor once the 120psi limit is reached. These compressors are fully capable of pumping well beyond 200psi, and can exceed the safe level for things like your tank and hoses. Obviously it would get a bit exciting if this were to happen and you had you air tank blow apart because the compressor didnít know when to stop! The safety valve is preset to release the line pressure if it exceeds 150psi. As I said, absolutely vital for safetyís sake.

The feed for a pressure gauge is exactly that. Iíve just mounted a small pressure gauge in the engine bay so I can see whatís going on. This is temporary for me, I intend to mount a nice VDO type gauge in the dash eventually and Iíll take the existing one out altogether.

Step 1- Mounting the Compressor

I thought this would be the easiest part of the whole job, but the tight confines
of the engine bay made it a bit of a challenge.

The first part of this exercise was to alter the head of the pump to make hookup easier. Air conditioning compressors usually have an odd type of thread and flare fitting for connection of the inlet and outlet hoses. Also, these fitting are quite large and I knew I didnít have a lot of space to operate in. My solution was to take the head off the compressor and cut off the threaded section for the flare fittings. I then drilled and tapped the holes so that I could fit normal threaded brass elbows and barb fittings.

The typical compressor with the large flare fittings:

My modified compressor with the threaded bits cut off, and drilled and tapped for normal fittings. The fittings installed are just there temporarily to keep muck out of the holes:

Since the Sanden unit had exactly the same mounting dimensions as the existing Diesel Niki A/C compressor, my idea was to mount it off the side, using the lug mounts on each compressor to both mount and pivot from. That would enable me to fit a slotted flat bar for belt tensioning, much like many alternators do.

Reality in the form of lack of space soon put paid the Mark 1, so Mark 2 was invented that utilised a tensioning pulley purchase for an automotive A/C parts supplier.

Hereís the compressor mounted up:

A close up of the small brackets linking the two compressors together:

The underside:

The bracket holding the tensioning pulley:

The tensioning pulley from behind:

Having thought I'd cleverly solved the mounting and tensioning problem, I then closed the bonnet. Bugger, the bonnet stay folds neatly against the newly installed belt. Didn't think that one through very well. Fortunately, the stay has a lot of flex in it, so a small length of flat bar was bolted in place to guide the folding stay away from the belt. Some rubber strip stuck to the stay stops any rattles (which we don't want in a Defender):

Step 2 Ė Installing the Cooling Loop

I originally intended to use a line of Ĺ inch copper pipe tucked under the front wing somewhere as a cooling line. However, I just happened to be around for the decommissioning of a large industrial compressor which had cooling coils mounted on the output of each of its cylinders. It actually had four cylinders with a separate coil on each one. Obviously one of these (which is about 2 metres long if straightened out) was meant for Landrover service:

It mounted up well (eventually) behind the right front headlight. I made up a simple bracket the mounted from the bolts in the stiffening ribs on the coil to the bracket holding the PAS reservoir:

Step 3 Ė The Manifold

There are many variations possible with this. Indeed, the use of any type of manifold is optional anyway, itís just a convenient way of putting together some of the items that you need installed on the output line.

In my case, I chose to mount up the Check (Non-return) valve, pressure switch and safety valve in a manifold constructed from a combination of galv and brass fittings. They were just what I already had in the garage, so cut down on what I had to buy or make.

These three items normally live together in most air compressor environments, so fitting them into some sort of manifold makes sense, but it isnít essential to do it this way.

Hereís the manifold ready to be installed:

In the above photo you can see the thin black tube running from the Non-return valve to the pressure switch. This is the unloader line. Its function is to release the pressure from the line between the compressor and the Non-return valve, once the pressure switch has reached its maximum level. At this point a small valve in the pressure switch opens up releasing the air via this line. This ensures that when the compressor needs to re-start, it does so without any backpressure, making life easier in the compressor and compressorís magnetic clutch. Itís not essential to have this in place, but since most pressure switches and non-return valves have the facility anyway, it certainly canít hurt.

Hereís the manifold installed under the right wing, under that blank plate:

Step 4 Ė The Intake Oiler and Outlet Filter

I made a bracket to mount both of these on the top of the brake booster. It was about the only space left in the engine bay that was big enough to house them.

In this picture the filter is nearest the camera, the oiler and small K & N type filter for the inlet are on the other side of the brake booster:

From the other side showing the oiler and line to the compressor:

Step 5 Ė The Receiver (Tank)

This turned out to be the easy part. There are once again a number of options for a tank. Some people have used things like old fire extinguishers, LP Gas cylinders or even purpose made cylinders. I found the easiest (and probably cheapest) was to use the air brake reservoir tank from an old truck. These are usually rated to well in excess of the pressure youíre likely to use, are available in a variety of shapes and sizes, often can be had with the original mounting bracket, and best of all, theyíre cheap.

My tank came courtesy of an old Ford Trader truck. I did a quick measure up of the space I had to work with, then visited the local truck wrecking yard. The owner pointed to the trucks and said to go a find the one I wanted. The second truck I looked at was the Ford Trader. It had two tanks, one was far too large, but the smaller secondary tank was just the ticket and my later calculations tell me it holds about 16 litres. The yard owner sent the young apprentice out to take it off the truck for me complete with the mounting bracket. He even gave it a steam clean before handing it over. The price? Just under ₤10.

Hereís the (repainted) tank ready to fit:

My chosen location was behind the left rear wheel:

The outlet from the tank goes to a quick disconnect coupling on the rear crossmember:


So, how does it work?? Well, to say I'm impressed is an understatement! I know the specs say these compressors should be capable of about 8cfm, but it's not until you get the thing running that you see just how serious they are. It takes just under 20 seconds to fill the system, including the 16 litre tank, from empty to 120psi. That includes the almost 2 metres of 12mm bore cooling loop, plus the 12mm hose running to the tank behind the left rear wheel.

The unloader valve works just like it says on the packet, a very nifty pssssst when the 120psi mark arrives and the compressor starts freewheeling. At about 80psi, the pressure switch fires things up again, and before you know it, psssst, we've got 120psi.

With the engine doing about 2000rpm, the compressor can pump air faster than my air duster attachment can let it out of the tank. That'll do me I think!!

Real world use is no less impressive. I timed the compressor pumping up my 33Ē (255/85r16) BFG M/Ts, from 18 psi to 35 psi, which is what I generally do coming out of the sand. Time taken was just 25 seconds per tyre. Having used the 12v compressors in the past for that very exercise, I can tell you thatís fast!

And this

Air conditioning compressor converted for ĎOn Board Airí:

There are two excellent Ďon board airí 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 donít 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 donít 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 compressorís outlet fitting. This is connected to some Ĺ 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 Ĺ 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 Yorkís 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 Ďcrank and conrodí design of the York, the Sanden design works well for onboard air when the Ďsumpí 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 didnít 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 Ė I havenít 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 donít need an Ďunloaderí type pressure switch for onboard air. Compressor will easily cope with full system pressure in the pipe work etc on startup.

You donít 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 doesnít 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 ľ bsp and are the narrowest points in the system. Tool airline is 3/8 inch bore.


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 Ė probably not very efficient).

Useful links:

I discovered reasonable value pressure switches, safety blow off valves etc can be bought from Matt Savage at: http://www.mattsavag...ompressors.html

I have no connection etc.

Sanden specs and workshop manual:



Have fun with your onboard air!