Hi People
The heater motor on my 1987 series III xj12 sovereign is on all the time. It stays on low speed even when the ignition is off.
The symptom is that when I leave my car parked for the day the battery is flat.

It works fine on medium and high speed.

I suspect it is a sticking relay but I do not know where to look for it. Please can anyone help?

Thanks

 

The most common cause of this problem is a faulty circuit board inside one of the blower assemblies. Each board (one for each blower) has a relay, a Darlington transistor, and....a diode or resistor or some such (memory a bit foggy at the momet)

You can remove the blower assemblies (big-ish job) and rebuild the circuit boards yourself if you're handy with a soldering gun .

I find pics and post them later

Cheers
DD

 

Um, I completely stripped the climate control out of a 1987 S-III in a wrecking yard a year ago specifically for the blowers as mine were a bit hinky. I found them to be Exactly like the ones in my S-II containing no electronic components at all.

I didn't know relays were involved (I suppose I should have) but I didn't need them anyway and I don't know where they might have been in that car.
(';')

 

Here's some info on the transistors, etc

Air conditioning blower control

Cheers
DD

 

In August 1986 the Ser III V12s got the new Delanair Mk III climate control system which had, among many changes, circuit boards inside the blower assemblies

Prior to this, on V12s, and all Ser II cars, and six cylinder Ser III cars, the Delanair Mk II sytem was used.

Cheers
DD

 

Nice assist Doug, thanks, should be FAQ I think.

 

Thank you, Doug, it's all clear to me now.
The car I worked on was a 6.
Sorry for the misunderstanding.
(';')

 

Hiya All
Thanks very much indeed, especially Doug. My car is February 1987 and therefore must have the Delanair mkIII. So it makes sense that it looks different to all the pictures I've seen in the manuals. I will study the information you provided and let you all know how I get on. I'm an electronic design engineer by day so I rekon I should be able to work out why my motor wont stop

Dave

 

Hiya Doug and everyone.
Just to update the situation with the blower on my series III. I eventually managed to remove the blower with some help from a local mechanic. He had to move the steering column to get it out in the end.
The good news is that all that had to be replaced was the darlington transistor and it was all systems go.
One thing to note is that I believe someone had already replaced the factory fitted 2N6284 with a MJ11016. I am an electronic engineer and I think it may be that the original design is flawed. The diode and 68R resistor are there to limit the voltage at the collector of the darlington when the motor is switched off by allowing the back emf current to flow. My concern is that the 68R resistor (which is there to protect the diode from dissipating too much power) may develop too much voltage across it and the voltage may then exceed the 120v the MJ11016 is specified to withstand.
That said however I simply replaced the MJ11016 with another MJ11016 and will hope for the best.
Thanks again for the help
Dave