Heater fan stuck on

[RobC]

Before I start pulling the dash apart, just wanted to double check if anyone could advise if the fault lies elsewhere (eg in the scuttle ?)

Symptoms:
- as soon as you touch the dial the heater fan is stuck permanently at semi-full speed (ie max on the dial without the full speed relay).
- full speed relay still works as normal and toggles on/off and takes the fans ‘up to 11’
- turning the ignition off and on again cuts the fan, but if you turn the dial again it comes on nearly-full speed and the dial does nothing again (apart from the full speed relay still working properly, as above)

Cheers
Rob

[Kitch]

Could it be the resistor pack? I can't think how it would prevent you from switching it off again, but stranger things have happened. Can only be that or the control panel.

[RobC]

Cheer Kitch- is that the little box of tricks attached to the fan itself?

[RobC]

Aha, this post from the archive implies I have a failed transistor due to the symptoms being permanent ‘on’:

https://www.bxclub.co.uk/forum/viewtopi ... 3d468d4935

[Jaba]

It can be just corrosion under the earth contact strip. You will see what I mean when it in your hand. This is a GOOD fault in the winter time, much better than it failing to turn on at all.

[Kitch]

Cheer Kitch- is that the little box of tricks attached to the fan itself?

Mounted inside the fan motor housing, yeah.

Aha, this post from the archive implies I have a failed transistor due to the symptoms being permanent ‘on’:

https://www.bxclub.co.uk/forum/viewtopi ... 3d468d4935

Ah, so it's a part of the resistor pack?

[RobC]

If I’m reading the old tech post (and circuit diagram) correctly I don’t think there are any resistors- it’s a transistor inside the fan housing.
/pedantmode

[white exec]

Iirc, originally a module with a single (Darlington) power transistor, mounted on a metal heatsink.
Originally located in the heater/steering column area, but later moved to the blower (scuttle) area, for improved airflow cooling. Suffers from exposure to weather.

[rutter123]

Then disintegrates into dust.
They are still available new but quite pricey.
I'm sure someone in the know could make a replacement with a few components.