Monkeys guide to AC in your Mk2.

July 16, 2025, 02:03:28 PM

When installing AC into a mk2, there are many ways you can do it.
I’m going to write down how I did it, no idea if it’s the best way, but it’s working for me and as always, I take no responsibility for anything, ever.

Unless you have a 3.2/24v, you’re almost certainly not going to be copying this exactly.
But as number of people have asked me to write down how I installed AC into my mk2, here goes…

Basics:
Air conditioning relies on the expansion of refrigerant gas.
When it does this, it loses energy rapidly and immediately tries to recover it from the atmosphere around it, making everything cold.
These gasses are very toxic, hence having to be controlled by specialist garages and I’m just going to call it gas from here on in.

The system comprises of 4 main stages:

Compressor
This is the alternator looking thing that bolts to your engine.
It compresses the gas for expansion later. This process heats up the gas. It's controlled by an electromagnetic clutch that engages when you need ac and free spins when you don't to reduce load on the engine.

Condenser
This is the radiator that is mounted in front of your coolant radiator.
This chills the compressed gas down as much as possible before it hits the expansion valve.
The more it can chill here, the colder your air will be in the car.

Expansion Valve
This allows the compressed gas to expand, lose it’s energy and cool down.
It’s usually mounted on the bulkhead, directly to the Evaporator.

Evaporator
The matrix in the car that the air passes through to and is chilled through the now expanded gas.
It’s located inside the fan housing, inside the car.

Ancillaries
Other bits on the system:
Drier – captures any moisture created through condensation.
Pressure switch – this protects the compressor from running empty and possibly failing in the event of a loss of gas.
Schrader valves – to fill/empty the system
Pipework – this has to put up with pressures between 32-155 psi
Wiring – while basic, you need to be able to control the system.

So now that’s out the way, I’ll move on to what I used:

Compressor:
As I have an Audi TT 3.2 engine, I fitted the standard compressor to it.
It’s worth pointing out that everyone who fit’s a 3.2 in a mk2 removes the compressor as it fouls the front crossmember.
I had to cut mine down and substantially weld it back up to restore the strength.
Had the welding done by a professional to ensure it was done right!

Untitled by Dave Ackerman, on Flickr

Condenser:
I was running out of room at the front of the car and had already cut into my slam panel to allow the Corrado VR coolant radiator.
That being said, I found that a Lupo condenser is not only half the width of a normal one but also bolts directly up to a VR6 radiator.
Had to cut the top bracket a little to allow for the bonnet latch to sit home properly.

Untitled by Dave Ackerman, on Flickr

Expansion valve:
Dead easy, use a mk3 item to match the evaporator below…

Evaporator/fan housing:
So, this is the biggest and by far the trickiest part of the build…
I used a mk3 item and since buying it, I’ve heard that a mk4 item fits better.

Mk2 vs mk3 unit

Untitled by Dave Ackerman, on Flickr
No idea if this is a fact and how it looks when it’s in, but I had what I had and pressed on.
I didn’t even consider trying to source a mk2 setup, saw one a few years ago hit nearly a grand and it was all worn out!
Seems obvious, but don’t get a LHD fan housing for this, it will not fit (unless you have a LHD car).
I wanted to retain the standard flap controls and keep everything looking as OE as possible.
What I found was by chopping off a small bracket, I was able to mount the mk2 matrix housing to the mk3 fan housing using all the stock clips.

Untitled by Dave Ackerman, on Flickr
Whatever matrix housing you’re using, it’ll be very old by now, so take advantage of it being apart and swap the matrix and refurbish the heater flaps.

Untitled by Dave Ackerman, on Flickr

Now here’s the hard part, fitting the new assembly.
You need to take a brave pill and cut a not tiny hole in your bulkhead to allow the mount for the expansion valve to sit properly and for the fan inlet to line up with the stock hole in your scuttle panel. There’s no super easy way to do this, you have to mount up the two heater matrix screws on the bulkhead for reference and then mark up and go for it.

Untitled by Dave Ackerman, on Flickr

I started small and gradually got bigger and bigger with the hole until everything lined up.
Shot of the mk3 fan inlet matching the hole in the mk2 scuttle.

Untitled by Dave Ackerman, on Flickr
I then had to make a small enclosure to tie everything back together.

Untitled by Dave Ackerman, on Flickr

Untitled by Dave Ackerman, on Flickr

Is it pretty? No.
Is it watertight? Yes
Is it buried down the back of the engine bay and no one will really see unless they’re looking for it? Also yes.

Now the part that really adds time to this project is what you have to do to achieve the above.
I completely removed my engine (not strictly necessary but made the job much easier) and stripped the dash right back to the metal bulkhead.
I also pulled the carpet back as far as I could and covered the interior in a damp sheet when I was cutting/welding.
Still managed two small fires.
Between the strip down, the constant removing the installing of the box, the slowly cutting the hole out and fabricating the enclosure, this probably took me 5 or 6 weekends.
Didn’t help I also got distracted, fitted a timing chain, overhauled the cooling system and painted the block, put new sound deadening in… be warned – project creep happens!

Untitled by Dave Ackerman, on Flickr

Untitled by Dave Ackerman, on Flickr

Ancillaries:
Ok, now all of that was done, I refitted the engine and started planning the pipework.

Untitled by Dave Ackerman, on Flickr

This is the part that will vary the most for you, depending on what engine you’re running.
I managed to buy a full Lupo pipework setup from eBay, this made life so much easier.
Most ac pipework (mk3/mk4 and on) comprises of solid pipework and a small flexible pipes near the connections.
Lupo stuff is mostly flexible, making it much better when sorting out the routing.
In fact, I was able to use all the Lupo pipe work (including air dryer) and a single mk4 high pressure to expansion valve pipe.
Doing it this way, I was able to only have one non OE join in the whole system.
I did this on a 10mm straight section, next to the brake servo.
As both pipes were 10mm and solid, I used a stainless compression fitting to join them.
This particular one is rated to 90 bar (around 1300 psi!) so it was more than enough to deal with the pressure.
I also used brand new green o rings for every connection, you can buy boxes of them for about a tenner.
Mk4 section

Untitled by Dave Ackerman, on Flickr

Below is the join at the end of the mk4 pipe, turned out to be a great fit, you can even see it dodge the brake servo.

Untitled by Dave Ackerman, on Flickr

Wiring:
Once that was all in, all it needed was to be wired up.

Untitled by Dave Ackerman, on Flickr
This was something I actually started before the dash and engine refit, it just makes life much easier.
I used a US spec AC switch as it’s OE and matches the other switches on the dash.
I also bought a small IP rated relay box off of Amazon that had 6 fused relays.
I may yet move it, but as I had some real estate on the drivers chassis leg, that’s where it went.
The first two I (again got distracted) used for the high/low beams for my headlight upgrade.
Essentially, my plan for wiring is I take a switched live feed, go via the dash switch, then the pressure switch and use it to trigger the compressor clutch and relay 1.
Relay 1 then runs the radiator fan.
I also have a permanent live that goes through the radiator fan switch and triggers relay 2.
Relay 2 also runs the fan switch.
That way I can have two independent sources to run the radiator fan and don’t have to worry about anything back feeding anything else.
If anyone wants, I can look to draw up a wiring diagram to give you an idea.

Testing:
So once it was all in, I tested it with the engine off and a danger wire (think short piece of wire with 12v at one end and whatever I want to test at the other), making sure everything reacted as expected.
When I was happy, I used an AC refill kit to put some charge in the system, while this was happening, I sprayed some soapy water around all joints to look for any leaks.
Note – not all recharge kits come with lubricant, running it without can damage the compressor.
I then ran the system up and while it wasn’t great, it worked and blew slightly colder air out the vents.
Reason for doing this was so I could make sure the system worked without leaking before spending money on a professional to vacuum test it and then put gas in.
When they test the system, it has to hold a vacuum for at least 15 minutes, happy to say it passed without a hitch.
Afterwards, (using the Lupo as the car to select the capacity) 575g was installed in the system and it finally worked properly!

Afterthoughts:
Now while this was specifically for my car using my engine, it should be quite possible to use this as a guide, but when you do the compressor, buy a second hand mk3 setup. Twice in the past, I've managed to bolt up a mk3 accessory bracket to a mk2 block and get the multi v belt, this contains the alternator/PAS pump/water pump and if you get the right one, an AC compressor. Just make sure to get the loom (or a cut of it) to capture the correct connectors and splice them into your wiring. It's also worth noting you should get the correct donor for your car - 8v for an 8v and 16v for a 16v. Also, with all VW compressors being in roughly the same place, the lupo/mk4 pipework trick should work too.

Re: Monkeys guide to AC in your Mk2.

Reply #1 – July 17, 2025, 07:42:18 AM

Now one other thing to consider is how much it's going to cost.
I've tried to do everything myself, but here's a breakdown of what it's cost me to get installed:
Full mk3 AC setup (only really need the evaporator box, but I didn't know that then) £120
Mk2 Switch - £65
Crossmember Welding - £60
Full Lupo AC pipework - £110
Compression fitting - £7
Lupo AC Drier - £20
Lupo Condenser - £65
TT Compressor - £180
Mk3 Expansion Valve - £22
AC Regas Can - £30
Processional Regas - £46
Fusebox - £35
Consumables (welding/grinding/wiring/o-rings etc) - £40-£50
Pollen Filter Housing (3D Printed) £10/£15
Pollen Filter £15

So at just under £700, it's not a cheap undertaking, but if you to it like me over the course of a number of years, it's not too bad.