Morris Minor Owners Club

Hi,
Mark Evans posted up a problem he has with a non-functioning heater knob on Facebook, I thought I'd repost my reply on here, in case it's of help to others and to provoke more discussion if people have more to add from this forum:

Mark Evans post: Hi so the heaters in our Mog dont seem to work when I turn the nob does anyone have any ideas what I can look for to try and mend them or do I need a new heater unit
Thanks. Mark

Hi,
I take it you mean the Rheostat, which you turn the black knob on the front of your heater, from the off position, to increase the fan speed?
If so, I too have found these to be temperamental, my Series 2 has a simple on / off switch fitted instead.
I made up a detachable 'vent closer', to direct (hopefully) more heat up to (clearing) the windscreen on cold mornings.
Picture below.
Best wishes,
Mike.

Best wishes,
Mike.

As has been said many times the heater switch (Rheostat) is counter intuitive. One click to the right from off is maximum. The further it is turned the slower it will run.

The heater fan is a simple electrical circuit with a supply, a switch, a load and a return. All components need to be functioning correctly. Fault finding is simple and methodical. No, a complete new heater should not be required, but one or more components might be needed, and basic maintenance will help.

Is the car +ve earth as original, or converted to +ve earth? This will not affect the testing per sae, just which wire on a meter is connected to earth.

Are all wires connected properly (including earth) and terminals clean?

With the ignition on but the heater off, is there 12v on the supply wire? If no, investigate the supply.

Turn the heater switch on. Is there 12v on the feed to them motor? If no, then the switch is faulty and should be replaced.

With power off, does the motor spin freely when flicked by hand? Has a bearing failed? Is the motor dirty and in need of cleaning? Depending on the unit fitted, are the brushes OK? Does the motor run OK if supplied with power directly from a reliable source?

Internally, rheostats generally comprise a spiral of resistive wire, across which runs a brass(?) blade, connected to the knob.
If you've isolated the fault to the rheostat, I'd suggest first squirting some switch cleaner into it, and sweeping the knob back and forth.
Only if that doesn't work, think about dismantling it (I've no idea how easy that is .). Or get a new one (again, no idea how easy )
And while investigating in situ, don't leave the ignition turned on longer than necessary. Depending on where the contact breaker points are parked, the coil may become a 50W heater and - without its usual airflow - suffer damage.