In the meantime, there had been sporadic rain on my trip down to Hertfordshire in the TR7, and this showed up a problem which I had not previously observed, quite possibly because this was my first trip out in the rain. What happened was that when I turned the wipers off, they continued sweeping. Sometimes they would stop after half a dozen uncalled for sweeps, sometimes it would take 30 sweeps or more. Flicking the wiper switch on and off might or might not have helped – if the wipers stopped after one of these flicks of the switch, it was probably mere coincidence rather than cause and effect. The problem was all the worse because of the intermittent nature of the rain.
Simon Hebditch at Robsport advised that there was a plastic parking switch on the side of the wiper motor that kept the blades moving after you switched off until they were in the park position, then a contact would be broken that stopped the flow of current. Sometimes that switch worked loose, and because the contact could no longer be broken, the wipers would just keep going.
Back at base, I located the wiper motor tucked up inside the nearside front wing by the base of the screen, partially hidden from view by the battery and screenwash reservoir. Removing these obstacles cleared access, and the plastic switch was indeed loose. I made the mistake of thinking that the cam which acted on the switch was located on the gear wheel inside the motor, and so I moved the switch towards this as far as it would go. After refitting the battery and pouring water on the screen, activating the wiper stalk showed that they did indeed now stop on command, but that they parked quite a long way up the screen. So I removed the wiper arms, repositioned them on the spindles and tried again. This time they parked where I wanted, but when I turned them on again, the wipers first dropped down even lower and off the bottom of the glass before moving only halfway up the screen and then returning to rest.
Back to the long-suffering Simon Hebditch, and he advised that the switch needed repositioning. That is when I realised my mistake – the cam that cuts off the parking switch is on the inner drive cable that takes forward and back motion from a shaft on the gear wheel to the wiper spindles. That means moving the switch towards or away from the centre of the gear wheel will alter the parking position. So I repositioned it halfway along the adjustment slot, tightened it down and tried again. This time it was spot on.