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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 don’t 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 compressor’s 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 it’s 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 isn’t 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 don’t 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. I’ve 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 doesn’t 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 didn’t know when to stop! The safety valve is preset to release the line pressure if it exceeds 150psi. As I said, absolutely vital for safety’s sake.

The feed for a pressure gauge is exactly that. I’ve just mounted a small pressure gauge in the engine bay so I can see what’s going on. This is temporary for me, I intend to mount a nice VDO type gauge in the dash eventually and I’ll 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 didn’t 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.

Here’s 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 – Installing the Cooling Loop

I originally intended to use a line of ½ 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 – The Manifold

There are many variations possible with this. Indeed, the use of any type of manifold is optional anyway, it’s 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 isn’t essential to do it this way.

Here’s 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 compressor’s magnetic clutch. It’s not essential to have this in place, but since most pressure switches and non-return valves have the facility anyway, it certainly can’t hurt.

Here’s the manifold installed under the right wing, under that blank plate:


Step 4 – 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 – 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 you’re 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, they’re 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.

Here’s 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 33” (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 that’s fast!

Edited by white90
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