I've never been entirely sure what the two heater control levers actually do. Following a coolant leak into the passenger footwell I've taken out the heater box to investigate so while it was out I've worked out what the two levers actually do so thought I'd post my findings in case it is of interest/help to anyone else.

These are the two levers operated by the cables from the two sliders, one for heat, one for distribution


Here I've marked the two main paths that air can take through the box. It enters from the fan into a grp duct that I'm guessing was a GTM part. It can then either go through the heater matrix or not depending on the heat flaps and then go to either the screen demisters or the footwell and demisters depending on the position of the other flap.


When the heat flaps are open the air coming through the box comes through the heater matrix so is warmed up


When that pair of flaps is closed the heater matrix is isolated from the air flow. Coolant still flows through the matrix but without airflow it shouldn't escape much. These flaps can only move as a pair.


When the distribution flap is closed air can't easily get to the demisters or footwell so must favour the eyeball vents.


When the flap is in the central position the air flow favours the screen demisters. I hadn't realised this from my previous trial and error so when I'm demisting in the future I'll now know to put the distribution flap in the centre


When the flap is fully open air flow goes to the cabin and screen however as the path of least resistance is the cabin not much will go to the screen vents. Good to know!


Anyway, having now tested the matrix by attaching hoses, pumping it up with air and submerging it in a bucket of water I found the matrix itself is fine but the was a decent build up of gunge on the inlet/outlet pipes and although the hose clamps were tight air was escaping there through the crud. Having now given them a good clean they seem air tight so will hopefully be coolant tight too. There was no way of getting to the hose clamps without removing the whole heater box so not wasted. In preparation for the job I sourced a whole R100 heater box with matrix which I now won't need. I've tested that matrix too and its good. I'll hang onto it in case anyone needs one.

Hi Dave,

How much of a job was it getting it out?

Mine needs to come out as although the Air Diverter flap sort of works, the Hot/Cold flap has no effect.

At the moment I've got round this by fitting an electric diverter valve in the heater pipes, as close to the heater as I could get, so that the flow to and from the engine is maintained even when the valve is closed cutting out the heater. This works well - so much cooler in he Summer without all that hot air coming into the cabin.

However, taht's only a bodge until I take the heater out because although the levers/cables move the bits they should, I can't get any cold air into the cabin, so either somethings broken inside or, as I suspect, the foam seals are shot (something that happened on daughters R100 years ago).

Depending what I find when I take it out, I may well be a customer for your spare heater unit!

Hi Martin,
It not actually too bad removing it. Especially if there is a bit of slack on the hoses as there was on mine.
The whole box is held in place by just two bolts as shown in this pic

Dash top has to come off to access the bolts. Once you've removed those two the whole box can drop down. It takes a bit of a wiggle as the foam seals will hold it in place but it will come. Having released the p clips holding the heater hoses under the front clam you should have enough slack to drop it far enough to get to the jubilee clips on the matrix. Also, removing the control cables will be required to get it all the way out.

The foam on the spare box is quite old too but should be possible to glue new on but I don't think it makes a huge difference to the seal on the flaps that isolate the heater matrix as they close quite tightly.