Fitting a Defender Heater

This entry is largely in response to frequently asked questions on Land Rover forums about fitting later heaters to SIIIs in an effort to improve airflow and heating capacity. Defender heater units are vastly superior to the SIII units, and are an upgrade that should be high on most peoples’ list.

heaterheater sideThe Defender unit differs significantly from the original SIII heater, though they are very similar in operation. Both have a two-speed electrically powered blower motor that forces external air through the heater matrix, where it is warmed to a selected temperature by an engine coolant heat exchanger, and then exits the heater matrix housing through a hole in the bulkhead. Once through the bulkhead, it enters the lower facia assmbly, which acts as a low pressure duct. This duct has two upper vent holes that lead to the windscreen demisters and two lower vents for the footwells. A cable operated torque rod with vent flaps inside the facia blocks airflow through the unwanted vents to control the proportion of air used for demisting and cab heating.

The differences in the equipment are:

Location of the intake – side of the passenger wing on SIII, wing top on Defender;
Separate blower and matrix housings on SIII, combined unit on Defender;
Two positive feeds from switch on SIII, one positive motor feed and two switched earths on Defender;
Temperature controled by water control valve on SIII (front of cylinder head), air bypass flap on Defender.

heater - steering bracketFitting the Defender unit to a SIII is pretty simple. First, remove the SIII heater matrix housing, blower motor and ducting. Next, cut away the top rear corner of the redundant steering box support bracket above where it meets the footwell front edge (the photos show more removed to allow the snorkel ducting to pass through the same location), and remove the rear-most 4″ of the wing top’s return lip to alow easier installation of the bulky Defender unit. The Defender unit’s top mountings use the same captive nuts as the SIII, but two new bolt holes will need to be drilled for the bottom bracket near the bottom edge of the top of the foortwell. This is best done by trial fitting the unit with the top bolts and marking the footwell before removing for drilling. It is also a good idea to blank any unused bolt holes, including the holes that were occupied the old matrix housing side bolts and the blower motor mountings.

heater controlOnce the Defender unit is fixed in position, connect the two rubber coolant hoses. The SIII temperature control valve (if retained) should be opened to the fully hot position and locked. The Bowden cable for this valve should be disconnected and used to operate the arm on the top of the Defender housing, which moves the matrix bypass flap inside. You may be unlucky and find that the SIII cable does not reach, in which case the standard Defender cable should be used. The control levers next to the instrument panel are identical between models, so the Defender cable will fit without modification.

Now that the coolant hoses and control cable are fitted, you can wire up the electrics. As mentioned, the current direction is reversed, so you can’t just connect the green wires from the switch to the green wires from the motor. The way around this problem is to use the existing dash switch to control two relays (one for each speed circuit) to act as remote switches to earth, replicating the Defender’s dash switch which runs to earth.

The motor’s purple wire is now the positive feed. This can be run from any positive pick up (such as the battery or starter motor terminals or the fuse box). A 15A fuse must be fitted in this feed. The green wires from the switches are used as relay control wires, with the motor’s corresponding green wires connecting to the relays’ positive feed terminals. The relay outputs are to earth. Remember to earth the relay control circuit as normal too. No fuses are needed in the relay circuits as they are only earthing the current that has passed through the motor and its fused purple feed.

The final element of installation is the intake. There are three ways of doing this:

1) Leave the intake duct off and use under-bonnet air. This would require 100% gas tight manifolds and exhaust joints to prevent exhaust gasses entering the cabin. It;s the easiest installation, but also the riskiest method, requiring frequent check for gas tight joints. I don’t know whether it would pass road worthiness inspections;
2) Fabricate a duct that would take air from the existing SIII side intake grille. There isn’t much space between the grille and motor casing, but it may be possible;
heater duct3) Use a Defender wing top intake and duct, as I did, and cut the wheelarch mud shield to fit around the plastic duct. It’s fiddly, but once the mudshield is removed, a piece of cardboard can be used to make a template to transfer the required shape to the steel mudshield before cutting. Rubber strips can be used to seal up any gaps.


  1. Fitting a defender heater.
    Quote: The final element of installation is the intake. There are three ways of doing this: i see another.
    4. Put a LHD volute on the RHD box and then the intake is on the right side to either feed in from the existing series intake or vai a reciculating vent vai the bulkhead..

  2. That would work, Martin, but you’d need to source two Defender heater units, one LHD and one RHD, so it’s be quite expensive. Modifying the existing blower with a few pieces of sheet steel, silicone sealant and a pop-riveter would be considerably cheaper, though not as neat.

    You wouldn’t be able to connect it to the SIII wing intake, but you could have an intake in the top of the passenger foot well as you suggested. This would be very effective at warming the cabin as it would be recirculating the interior air rather than continually heating cold external air, much like the Smiths heaters in the SIs, SIIs and SIII Lightweights. It does mean that the cabin air becomes quite stale, though, and you’d have to open a window or the vents to get any fresh air in.

    It’s a good alternative, though, if you don’t want to fit the wing top vent.

  3. Does anyone know if Any other blower motors can be adapted to fit a defender blower unit. There’s loads of different ones on eBay but the defender ones are at least twice to three times the price of any other make.

  4. Hi Nick
    What are size of the two trays that you used for the speed circuits
    And what is the best way to to cut wing for defender intake top and scoop
    Thanks Darren

  5. Hi David,

    I took measurements from a Defender, of the distance from the bulkhead to the rear of the intake and from the outboard edge of the intake to the joint between wing top and wing side panel and transferred these measurements to the SIII wing top (covered in masking tape to prevent scratching when cutting and also to make drawing easier). Remember to allow for the thickness of the edge of the intake grille – it and the duct below need an overlap of aluminium to screw into. The hole was cut using a drill to make pilot holes and a jigsaw.

    I used 17A wiring and fuse for the motor control, connecting the purple motor wire (permanent live) via the fuse to the battery terminal on the starter motor solenoid. The striped green wires connect to the SIII wires of the same colour, and you then disconnect the live from the SIII switch and connect that terminal to earth (the Defender unit is wired backwards compared to the SIII, where the switch runs to earth instead of the motor, increasing efficiency and also extending switch life).

  6. Hi Nick, I’ve read your site with great interest many times, cracking work!

    I’ve just installed a Defender heater assembly in my 109 SW, I’ve used a RHD assembly, but split the motor off the casing and turned it 90degrees like the Series blower. I’ve fabricated a bracket to hold it to the footwell, and a ducting to the orginal matrix housing. It seems to work ok, but I think I could have done better….I’ll be doing a mark 2 version for my other 109! Just got the wiring to do now, but it works straight off battery.


  7. if you consider to replicate the heater conversion, connected to the standard S III intake, then get in contact with someone in Germany or switzerland who wants to do the same, so they could exchange the intakes and keep the cost low.
    About taking the air from passenger footwell recycling problem :
    one could do so but connect a hose from original S III intake to the footwell intake with a flap to control fresh air supplye, so you get both recycling and fresh air supplye.

  8. I don’t think you can do it quite that way, but if you’re keen to use the original SIII wing intake, I suspect you could (perhaps with a litlle trimming and fabrication work) fit the Defender matrix to the SIII housing and the Defender blower motor and impeller to the SIII blower casing.

  9. Just a thought. Many modern cars have the option to recycle the cars internal air (assumably to keep nasty fumes out of the car) back through the heater. I can’t see any reason why this couldn’t be done for a series landrover, but do you think summer driving would be even hotter?

  10. It would help significantly in reducing the time to warm the cabin. My Range Rover has just such a facility at the touch of a button, and it also helps cooling time when using aircon. However, humidity levels increase rapidly with this system when the vehicle is occupied, so more than a few minutes use leads to greater window fogging than having external air selected. Aircon is not affected in this manner because the vapour condenses in the matrix housing and is drained away. Series I and II vehicles and Lightweights had heaters that recirculated cabin air, and earned the nickname “fog stirrers”.

  11. Iain Mackay says

    Hi Nick,
    After you fitted the defender heater, did you refit the wing mudshield in any form?
    I’ve got my defender heater and wing top air intake fitted, but obviously this gets in the way of refitting the mudshield, I had to remove the bracket from the underside of the wingtop.
    I’m keen to put something back that will both protect the heater and engine bay from debris kicked up by the wheel, and also re-enforce the wing for the many occasions when I stand on it. I think the original shield is a too sturdy piece of steel to be just a mud shield. It must add a fair bit of strength and rigidity to the wing.
    I’ve got a few ideas I’m trying out, but thought I’d check in case you’ve invented that wheel already.
    Cheers again,

  12. Hi Ian

    Yes, I refit the original SIII mudsheild, cut to fit around the trunking and fitted with rubber strips to create a splash seal, riveted onto the mud shield. I used a cardboard template to determine the area to be removed and had the complication of fitting it around the side intake duct (Defender type also) that connects the air filter to the snorkel, but that won’t affect most people.


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