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Written By: Gibson Hull
April 06, 2025
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The Defender is not known for its leg room. Many air conditioning systems installed in these old Defenders were retrofit jobs that were tacked onto the orginal interior design and took up valuable knee space. The Brits did not have to worry much about cooling off in the UK where the Defender was designed. Alot of Defenders delivered to the rest of the world had air conditioning systems installed by the dealer who purchased them. Although, this was true for a lot of cars back in the day.
The system I chose to go with was not the easiest option. I could have chosen the factory/dealer installed design, but I did not want to give up valuable knee real estate. It would have also taken passenger footwell space where the new fan motor would have lived. I also did not want a seperate heating and air conditioning system adding more weight and reducing interior space. I chose to go with the 319 HVAC System from Ministry of Defender (MOD). The MOD system is great beacuse it replaces the heater box in the engine bay with a dual heater/AC system. It does require cutting a hole for the interior air, but it was the best option to save space and still retain both heat and AC.
One of the drawbacks of the method I chose is that there are not air vents in the dash of the classic defender. The air vents in the retrofit system from the factory and dealers is what takes up the valuable knee and leg space. MOD provides some 3D printed swiveling vents for the stock lower heater ducts on the Defender dash. The thing is, the entire lower dash of the defender is one big air duct for the heater. The driver’s vent controls either send heat to the windshield defroster vents or down to the footwells with one mechaniical flap in the heater box. I plan to modify this further in the future so stay tuned!
This took me about a month to complete. Not bad considering it was the first time I have ever installed or worked on an automotive AC system. I wish I had taken more pictures at the time, but it was before I launched Corranta and really focused on recording each step.
Step 02: Gather your tools. This is a big job and not for the faint hearted. You’ll stretch your Stage 1 Tools, and start to dig into Stage 2 Tools as the work becomes more complex. Doing the job in an apartment garage means a lot of time spent making a mess and cleaning it up every night before turning in.
Step 03: Make a plan. The majority of time spent on this job was really the wiring. I did weeks of research to ensure I had all my ducks in a row, and I still ended up making mistakes and needing to redo things. Having a super clear understanding of how the wiring would work, the colors of wire I would use, and finally how the AC system is routed, made the job so much easier than it could have been.
Step 04: Make your own wiring diagram. While I based my diagram on the one that came with the kit, it was not very clear, and I wanted to clean it up and add my own wire colors, etc. This gave a good opportunity to understand clearly how my truck is wired and how the AC system would fit into it.
Step 04a: This is the completed diagram. Drawing things out makes it easy to quickly make changes as you go and keep track of any adjustments.
Step 02: Gather your tools.
Step 03: Make a plan.
Step 04: Make your own wiring diagram.
Step 04a: The completed wiring diagram.
Step 06: Replace crank pulley, Install AC Compressor. (Not pictured). A relatively easy step, remove the radiator fan shroud, the fan (with a special long thin wrench), loosen alternator, and remove all belts. Install the new AC Compressor Mount and Compressor, which mounts to the passenger side of this 200tdi engine. A note, the kit did not come with a bolt to put through the back of the mount, and I noticed when the AC belt was tightened the mount did not feel 100% solidly mounted. I went out and bought the right bolt to thread through the back of the mount into the engine.
Step 07: Mount the condenser and fan. The condenser spreads the AC fluid through all the tiny fins, which allows heat to escape the AC fluid and cools it down. Mounting it is another issue. This kit does not come with mounts, so I fabricated my own with some steel flat stock that I bent and drilled to mount the condenser in front of the engine’s radiator. I painted the condenser flat black so it did not stick out from behind the grill of the Defender. I mounted the electric fan to the condenser.
Step 08: Mount the AC dryer bottle. (Not pictured). The Dryer Bottle helps remove moisture from the AC system. Moisture kills the AC systems ability to work properly, and you want as much moisture as possible out of the system. I mounted mine underneath the passenger front fender in the engine bay, so that my AC lines would be as short as possible, and to improve efficiency.
Step 05: Remove old heater box, cut inlet hole, install new AC box.
Step 07: Mount the condenser and fan.
Step 10: Insert nutserts. These handy nuts are compressed in place with a tool so that the nut is affixed to the bulkhead, and you can easily install and remove fasteners.
Step 11: Install aluminum mounting bracket.
Step 12: install heater valve in bracket.
Step 13: Cut heater hose to length and install. The kit comes with extra lengths of heater hose, and you can pick some up at the local parts store with ease. I strategically located the heater valve in a place that would fall directly in line with the existing heater lines, maintain clean flow, and be up and out of the way. All that's left is cutting the line to the proper length to meet up to the valve. On the return side, a coupler allows some extra length of hose to be added to get it just right.
Step 09: Find location to mount the heater valve and drill holes for nutserts.
Step 10: Insert nutserts.
Step 11: Install aluminum mounting bracket.
Step 12: install heater valve in bracket.
Step 13: Cut Heater Hose to Length and Install.
Step 15: Wiring. This is the really hard part. There are a ton of wires needed for this job, for the fan, the HVAC box, the compressor, the heater valve, new switches in the cab to turn everything on and off, fuses, and a couple of relays to make it all work. I also moved and cleaned up the wiring for the two front fog lights while I was at it. I wanted to ensure everything stayed as factory in appearance as possible, so I ran all wires through factory holes and looms, and also located the fuses and relays alongside all the existing relays and fuses in the cab. Take your time, and enjoy this part, it can be very methodical.
Step 16: Refurbish lower dash/air duct. The lower dash on these old Defenders that act as the air duct for the heater are known to rust and mine was no exception. Lucky for me the rust only affected the top plate which screws down onto the box to cap it off and form the bottom of the dash-long tray. I picked up a replacement top plate, and added some closed cell foam to one side to match the factory seal/insulation. Then it is just a matter of screwing it all back in place with new stainless screws.
Step 15: Wiring.
Step 15a: Wiring.
Step 18: Troubleshooting. One issue I ran into was the AC fuse blowing every time I turned it on. Turns out the fan in the kit ran at a higher output than the recommended fan relay and wire size could support. I ended up stripping out all the wiring for the fan , upgrading the wire and the fuse to the appropriate amount, and it all worked like a charm.
Step 19: New dash buttons installed. The new Carling switches that came in the kit work well. I really want to try and integrate all the functionality of the orignal switches by the instrument cluster, instead of these new switches. Will be fun to modify and create new switches. Stay tuned for that.
Step 17: Charge the AC System.
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