Morris Minor Owners Club

Good evening all!

I've been toiling with this for a while but still haven't managed to fix it - for some reason I can't run both the wipers and heater fan at the same time. As soon as I turn on the wipers the heater fan stops, which in slow traffic can mean a steamy window!

I've tried to check all earth points, but is anyone able to advise where this sounds to be an issue? I'd imagine earth's, but do the wires for these two parts ever meet?

Both the wipers and the heater fan receive their supply via green wires that come from, strange as it may seem, the voltage stabiliser unit on the back of the speedo. This is not the stabilised supply, it just acts as a junction point that is itself fed from a fused supply on the fuse box.

Is the car an original +ve earth or has it been converted to -ve earth? If the latter then it is possible that some wires have been swapped around. It might also be worth cleaning the fuses and fuse box terminals in case there is high resistance and therefore insufficient power to run both wipers and fan.

Thanks! It's been converted to negative earth, so maybe I need to have a root around the back of the Speedo.

I just recently replaced the fuse box as it was causing problems earthing to the body and fuses are all new (although the rhs does show signs of stress and has some burn marks on it).

From my experience as an aircraft electrical engineer, my immediate thoughts are poor Earths or other high resistance connections.

Let me give you a little insight into how these things can work, or rather not....

So you've got 12v supply and 2 or more circuits share the same earth point. I won't go into the actual sums, so please if anyone out there wants to be picky, just live with it

So if the earth point is a bit high resistance, you might drop 1 volt on it, so the, lets say Heater Blower, still gets 11 volts.
So not noticeable in its performance
But when you turn on the Wipers, the same earth point, now with greater current through it from 2 circuits, drops 2 or 3 volts, so the Blower struggles with only 9 or 10 volts, but the Wipers Motor copes with that and they aren't noticeable affected.

So that's a bit of a story using your 2 circuits as the example, just to try and explain how circuits can interact when they share earth points.

Thanks all.

So I had a root around with the glovebox and found something (I think) is quite strange. I took out the black earth wire from the heater (which weirdly was connected via a splitter to a solid green wire) and used another earth point - it didn't work at all.

When connecting back to the green wire (which I assume to be live) it worked again. Now the green wire to the heater is coming from the back of the windscreen wiper switch on the Speedo.

I take it something ain't quite right here? Anyone know the proper setup if so?

*bump*

Do you not have the BMC wksp manual? The respective wiring diagram with colours is shown/described in Section:- N.

Even with the car converted to negative earth the wiring runs will be the same but of course with the current flowing the other way.

See https://www.mmoc.org.uk/Messageboard/vi ... 11&t=28295. Note that the wiper and fan are both fed from the non regulated side of the voltage stabiliser that is on the back of the speedo - assuming that nothing has been modified.

The heater feed is switched with a clear return to earth. The wiper feed is via the motor and earthed through the switch on the dash and the parking switch in the motor. This can cause confusion.