Discovery heater controls

[mikeant]

I have a 1994 Disco, full Middle East spec, 3.9i petrol engine, AC installed. Initially supplied new by main dealer in Muscat, now with me in Dubai.

How to turn on the heater? There is a water valve that controls the flow of water to the heater matrix and this is vacuum operated by a vacuum switch. The vacuum switch is electrically operated by what? Logically by the heater controls inside the cabin but unless there is some microswitch asssociated with the two controls that operate bowden cables to move flaps about I cannot discover how. No manuals are available here as the main dealers have lost the old books and this car is too old for the present CD based part lookup system. If I connect a vacuum line directly to the water valve the heater gets hot, so the valve and all the pipework are OK, the vacuum switch is new so should work but where does it get it's signal to turn on?

I need to be able to turn on the heater as grinding about in 55degC in low box in soft sand is rather too much for the cooling system. Maybe Land Rover never envisaged such conditions in their design scenarios! My fellow nutters  driving Merc 280 powered 4x4s have fitted extra radiators.

[ChrisW]

Mike, firstly I haven't got a clue, I would have said maybe an airlock in the plumbing somewhere but as you have managed to get it running by bypassing the vacuum switch obviously not.

Would it be possible to rig up a switch on the dash to operate the valve manually, or via a relay that is energised by the ignition?

Good luck

Chris W

[mikeant]

Chris, thanks for the reply. Yes I could always hotwire the switch to a relay or an external switch but how did Land Rover do it in the first place? A look at the appropriate wiring diagram for the 1994 Disco would supply the answer.  Anyone can look this up for me?

[ChrisW]

Sorry Mike, my rave cd has got a few nice cracks and scratches on it  

Thinking further on it tho... could the valve be operated by the thermostat somehow. The coolant system when the engine is first started obviously is cold so perhaps the valve is kept closed to keep the coolant pumping around the block to help bring the engine up to temperature, once the 'stat operating temp is reached a one-off signal operates the valve for the heating.
I would imagine that the valve and whatever is signalling it is probably wired back to an ECU of some sort maybe.

Purely thinking out loud though I'm afraid and probably talking through my hat!

Chris W

[mikeant]

Chris, This is not such an off-the-wall idea.

I believe that on the more modern Discos the heater is on all the time and the heating effect into the cab is controlled by switching the airflow more, less or not at all through the heater matrix inside the cab.

So if that is how they do it now then perhaps this is how it was always done and you are on the right track.

The ECU will send a signal to the vacuum switch once it realises that the engine is up to temperature and the hot water will then pass to the heater matrix. Whether the cab gets any heat will then depend upon the deployment of the air flaps in the trunking system, which latter are controlled by the knobs on the dash.

This question will only be resolved by looking at an appropriate wiring diagram.

As a quick and dirty fix when I am in the desert I connect the vacuum line that feeds the vacuum switch directly to the water valve and turn the air controls to hot inside the cab. This works fine but I wanted to know how Land Rover originally made their system work.

On another matter. Leakage from the steering pump. All Rangies and Discos of this sort of vintage leak from the steering box to some extent sooner or later. Well my leak has now got to an unacceptable amount. The Haynes manual for the Disco does not allow that a Private Owner would ever get involved in sorting such a problem. The Range Rover one has a section upon stripping and rebuilding the box. Is it so difficult?  

Mike A