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.
The 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.
Fitting 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.
Once 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;
3) 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.