Windscreen Wiper Self-park Problems

This article was prompted by an enquiry from Colin Goldsworthy, who recently acquired a Magnette with wipers that didn't self-park. He was struggling to understand the electrical wiring that allows the self-park to work:

“The wiper works fine but stops as soon as the wiper switch is turned off.  I have taken these motors apart on previous Magnettes but it was a long time ago and, although I have spent several hours thinking through the logic of how it operates and have dismantled the motor, I can’t get my head round how the wiring should be for the self parking function.  Mechanically it is easy – when the wiper switch is turned off, the motor continues to run until the cranking arm pivot on the driving wheel reaches the contact inside the circular plate, presses against it as it slides over it and breaks the contacts stopping the wipers in the parked position.  But the wiring doesn’t seem to make any sense to me.  The previous owner has fitted a new complete wiring loom on the car and I have checked the two wires to the terminals on the base of the motor (marked 1 and 2).  The all green one is live feed when the wiper switch is switched on and the other is an earth.  The wire that runs from one of the terminals inside the motor to the contacts inside the adjustable circular plate must surely be an earth as the contacts are riveted to the casing.  In which case, I can only see that the self-parking feature can work only if there is a second live feed to the motor that by-passes the wiper switch (i.e. permanently live), otherwise the motor couldn’t keep running to self park the wipers after the switch has been turned off.  Also, of course, when the wipers are correctly parked, the contacts inside the circular plate are being held apart, so the fact that the motor starts up when the wiper switch is turned on must indicate a separate earth as well.  In short I can only see that the self-parking feature can work if there are two live feeds (one switch controlled and one permanent) and two earthing connections.But the wiring doesn’t allow for any of this and the wiring diagram in the workshop manual is no use as this shows just simple single live feed to the wiper motor and a single earth. I know I must be missing something, but for the life of me I can’t see what it is.  I think I vaguely recall some 30 years ago finding out how the self parking feature worked electrically and thinking “that’s clever” but I can’t see it now.” 

After more head scratching, the penny dropped and Colin finally worked out the solution:

“I’m feeling pleased with myself as the answer came to me while I was pondering it all and it was confirmed by John Shorten when I saw him last week to collect a few more parts.  The key to solving it was when I realised that the lead from the wiper switch to the motor should not be a live feed as one would suppose but an earth from the motor through the switch!  There is only one live feed required and this is a permanent one straight from the A4 fuse (the one that becomes live when the ignition is on).  There is a second earth through the self parking contacts via the earth wire on the motor casing to the inner wing.  You connect the wire from the wiper switch to the external terminal directly above the internal terminal that has the wire soldered to it that runs up to the self parking contacts inside the round plate. The Magnette workshop manual is completely wrong as this shows a live feed from the wiper switch and a single earth but John (and Peter Martin, who I also called on last week to collect yet more parts) confirmed that the wiring diagram in the Wolseley 15/50 and the Oxford Series II and III manuals show the correct wiring set up in all its glory.  John actually keeps an Oxford Manual next to his Magnette one. He showed me the wiring diagram which confirmed the set up.  Peter also gave me a copy of a hand drawn wiring set up for the 15/50 which tallies with the Oxford. (To see this drawing, click the link at the bottom of this article). So the way it works is that, with the crank arm stopped over the contacts in the self park “box” (so no earth through them) and no earth through the wiper switch, the motor is dead.  Pulling out the wiper switch provides an earth so with the permanent live feed it springs to life.  When the wipers are switched off it continues to run as there is still an earth through the self park contacts.  As soon as the crank passes over the self park contacts the second earth is broken and it stops.  To achieve this I ran a discreet short black wire from the A4 fuse point (I ran it behind the fuse block) black taped it round the back of the main wiring loom and then direct to the motor which is only a few inches away.  I disconnected the live feed to the wiper switch and replaced it with an earth to a suitable point under the dash.  Hey presto it now functions perfectly.  One other thing I should mention (which I recall I had to do on one of my previous Magnettes) is that I had to replace the external wire that runs from the earth terminal inside the base to the self-park contacts.  It’s odd because, just like my previous one, it looked fine but wasn’t working.  So this short wire should be replaced if things still don’t go right.  It’s easy to forget to run the wire through the external plate on the side of the motor before soldering the new wire.  I had to redo mine!”

The only other point to mention is that the circular plate carrying the contact-breaker rotates a bit, changing the relationship between the position of the rack and the point at which the cut-out opens. This means that you can finely adjust the park position of the blades by moving the plate with a bit of trial and error. If your wipers over-sweep and hit the screen rubber, the chances are that there is wear in the rack and the wheel-boxes. Here’s a good tip that will save you money: you can compensate for the wear by rotating the wheel-box spindles 180 degrees. With the wipers parked, remove the arms and make a paint mark that touches both spindle and housing. Then detach the rack conduit from the motor and carefully withdraw the rack. With the rack withdrawn, rotate the spindles by a half turn and reinsert the rack. All being well, once the rack is fully home, the marks on the spindles and their housings should be opposite each other, not adjacent. And, more importantly, there should be a lot less play in the wheel-boxes. Thanks to Colin and also to Peter Martin, who provided the wiring diagram. If you would prefer a two-speed wiper system and are prepared to lose a bit of originality under the bonnet, then it can be achieved using a later-style Lucas 14W motor. 

Windscreen Wiper Wiring Diagram 

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