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Today 4 degrees on the gauge, so I wanted some warm air, but only cold. [it all has worked perfectly for years]
Under the bonnet, the plumbing is warm all the way to the heater valve, then cold from the valve to the bulkhead.
I have assumed it's the heater valve, so I have been to the scrapyard and removed the valve and the heater pump, and all the associated pipe work [the AA man would never be able to supply that particular matrix] but now wonder about the reasons for the failure.
If the heater valve has failed, could it be the electrical supply on my car, the valve is hard wired.
The fuse list for a 96 X300 does list Air Con, in the the left and right rear seat positions, and I have checked those, and they are good, but nothing is listed for heater valve or pump.
How will I know that the 'new' valve that I have taken from the scrap car is serviceable, or even if my present valve is serviceable, but not getting an electrical supply?
I just don't want to take all the plumbing out of my car and then having fitted it all back, with new anti-freeze etc. that I'm still not getting any warm air.
With thanks for any info. Take care.
Ron. York City.

Have sent you a pm.

Thanks PopsJag, got all that information, and did the checks, which came up with a code number 24, which said: Face Vent Demand Potentiometer. Open/Short circuits.
Well that means absolutely nothing to me, or my mate Colin at the local garage, so we devised a plan.
He got his 'wiggly-amp' machine out, and did some unplugging of the heater valve and heater pump, and checked the circuits, which seemed to be in order.
We then plugged in both the heater valve and heater pump, which I had taken from the scrap X300, into my cars system, and with the toggling of the temperature switching, to full hot, then full cold, both the valve and the pump worked perfectly.
So, now we removed same valves and put all the electrics back to normal, knowing that the cars circuits were all good.
Did the same checks, with the toggling of temperature switching, and the valve worked, solenoid could be heard, but the pump was not running.
So, no problem, we had a pump which we knew worked, we would change it, but my man Colin suggested that before we do the job just give the pump some 'big hammer' treatment, see what the effect of that is.
I took the 10 inch socket bar, and carefully placed it on the pump, whilst Colin did all the switching in the cockpit, I gave it 3 heavy knocks and next thing is a shout of 'hot air', and it was like Bahrain at midday, the pump needed a belt up the bum, and everything is now perfect.
The lesson is to use it even in the summer months, I have never scrolled up to the higher temps all year, as we are often told to use the air-con in the winter, the same applies.
Thanks again PopsJag, it was an easy fix, but took some diagnosis, and I am not doing anything about the Potensiometer, code 24, everything seems to be working as designed, now spot on.
Take care. Ron. York City.

It must be working well if it is like Bahrain on a hot day !!!!!

Never the less - well done

Glad you got it sorted Ron - the good old-fashioned way!!

Good result.
It's very true that all machinery needs to be used to keep the wheels in motion.

Ron - congrats on the fix. It sounds to me like the motor brushes have worn and they've got a bit stuck in their holder so the springs aren't pushing them into the commutator like they should so contact is lost. A precision application of percussion and they move again. Hopefully they won't get stuck again.
It's always good to have a cheap fix and particularly satisfying when that fix is hitting something!