Morris Minor Owners Club

...no, moderators, this does not need moving to "Off Topic" ....it's an electrical prob for sure and I'm not too good with electrikery. Switch the heater motor on and it blows out hot air. Switch the windscreen-wipers on as well (cos it's snowing) and the blower stops. What's the likely cause as there doesn't seem to be anything common to the two functions according to the wiring diagram in Haynes. It is probably the first time that I have tried to switch both on together since last winter though the car is used fairly regularly. And it's a bit chilly out.....

Perhaps the wiring has been messed around with at some point? Autoelectrician may save you a lot of time and grief.

They share a common earth - so the heater fan is probably 'finding earth' through the wiper motor - until you switch on the wipers. So check under the dash for the heater earth wire which should be attached to the body on a tag.

Back in early 2013, I changed my windscreen washer to electric and had to change the wiper switch. If memory serves, the heater motor was using the earth on the back of the wiper switch (as indicted above). Cannot remember how I wired the heater earth with the new switch. So if you have the same sort of arrangement, you may have the heater wire on the wrong terminal of wiper switch.

Not sure if this will help, but may be worth checking were the wires to the heater go as suggested above.

The heater and wipers share some wiring and are switched earth. The Workshop Manual is not very clear as it shows the heater and wipers as two separate unconnected units

The wiring diagram for early cars shows the wiper being earthed directly to the 'E' terminal on the control box, and for later cars there is a note on the wiring diagram stating that the wiper is still earthed at the control box, but not shown as such on the diagram.

If original wiper earth is not good (but 'OK' for one motor) and the heater earth has been connected into the wiper earth, that could be the source of the problem. Probably easiest to run a separate earth from each switch (if wired with 'switch to earth') directly to the body?

Has the car been converted to alternator? It is not unknown for the wipers to stop working after the redundant control box has been disconnected, not realising that there is an essential earth connection that was not redundant!

Right, thankyou folks, sounds like a weekend standing on my head, cleaning terminals and replacing earths. I will report back!


hopefully the weather will remain warmer and dry cos I'm going to be outside...............

[quote="IslipMinor.] Probably easiest to run a separate earth for each directly to the body?[/quote]
The wipers and heater are earth switched so any wiring must be connected to the switch then earthed

Ehhh? Just make sure the body of the heater has a good earth connection - and all will be well.

Mike,

Yes - I have edited the original to clarify that.

I have also removed the assumption in this post about heater motors being 'switched to earth' - see the next post.

Just make sure the body of the heater has a good earth connection

Roy, That comment made me think, is the heater actually 'switched to earth'? It is not the 'normal' way of wiring a switched circuit.

Looking at the factory wiring diagrams, it does not look like it is! The early rheostat and the later toggle switch are both in the supply side to the heater motor and the other side of the motor goes to 'earth' - i.e. a conventional 'switched supply', not 'switch to earth'. Items 18 (switch) and 19 (heater motor) have the wiring as:
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I will be taking ours out tomorrow, as part of the complete strip down before the respray next month, so will have a good look at how it is wired - this may not be how it was when the original 'round' heater (now a Mini cored fresh air unit) was retro-fitted by the supplying BMC dealer in 1960 though!

The wiper is definitely 'switched to earth', because the park circuit is wired in a similar way, and breaks the circuit to earth when parked. The wiper switch simply overrides the park circuit with a parallel circuit to earth.

In the diagram below, the green (G) feed from the A4 fuse first goes to the wiper motor (to the right of the number 51), from the motor a black/green (BG) wire goes to the switch (to the left of the number 51) and finally a black (B) wire from the other side of the switch to 'earth' (on the control box, not directly to the body):
[frame][/frame]
The commonly used Lucas 14W 2-speed wipers are wired in the conventional 'switched supply' mode, which is part of the complication when converting from the standard single speed Minor wiper.

I don't know about early round heaters - but certainly later heaters have the switch on the supply feed to the fan motor - and a black 'earth' lead that obviously has to be connected to a good earth. And I believe this connection is shared with the wiper motor earth.