When reverse gear is selected the two rear white reversing lights do not come on.
Apply ohms meter across contacts on existing, suspect, switch. Select reverse gear and ohms across contacts should be zero (contacts closed = short circuit). Select neutral or forwards gear, ohms across contacts should be infinity (contacts open = open circuit). If the switch tests OK then the fault is with the wiring, bulbs or fuse. Assuming the reverse switch tests as faulty, replace it. Cost approx £13.00. See cure below.
In my case the fault was a permanently open circuit reverse light switch. The reverse light switch looks like the picture below.
New and old faulty switches
View of switch from above.
The switch is mounted vertically, with the connector uppermost, on the top of the gearbox. It is visible if you look vertically downwards just behind the air filter box. A torch will help.
You can get at it with a narrow hand, using an open ended spanner approx 12 or 13mm across the flats. Ease the switch anticlockwise and then remove by hand. There is a rubber O ring and and it is not overly tight.
The switch has two connectors. Normally open circuit, but closed when the silver ball is pressed in. Inside the gearbox, when reverse is selected a metal plate strikes the silver ball and presses it in. The contacts inside the switch close and the rear white reversing lights should come on.
Take care not to allow dirt to enter the gearbox while the switch is removed. Use a new rubber O ring and smear of oil on the ring, threads and ball. Tighten moderately.
Page started 22 Nov 2017, last amended: 29 Sept 2018