I've been encouraged to put this up because there are probably plenty of light switches lying around in parts boxes that could be fixed.
There are two problems that arise with these switches:
1. One of the parts of the lighting circuit may not work,
i.e. the tail lights may not work but the headlights and parkers do, etc.
2. The dash light dimmer doesn't work.
How the main switch works:Down one side of the switch there are three contacts: from the back of the switch to the knob end, they are the contacts for:
1. Front parking lights
2. Tail and number plate (this has a bridge to the dash light dimmer control) No matter what position the switch is in (except "off"), this terminal is live.
3. Headlights.
Inside the switch there is a slide with two contacts.
When the switch is pulled to its 1st stage (parking lights) contact is made with terminals 1 and 2. When the switch is pulled fully on, contact is made with terminals 2 and 3.
If the contact point for, say, terminal 2 is corroded or dirty, then terminal 2 won't work, while terminals 1 & 3 will work (and so on with the other terminal contacts).
RepairRemove the switch from the dash. Carefully prise the 4 corner tabs back from the fibre board to which the terminals are attached, being careful not to damage the edges of the fibre board as you'll need the board complete to enable reassembly. Don't bend the tabs back any further than necessary to avoid breaking them off.
Lift off the fibre board. You'll see the slide with the "pimples" that make the electrical contact. Note the position of the slide so it can be replaced in the same position. Clean the slide and the contact "pimples".
Clean the contact points on the underside of the fibre board. (I found mine were really dirty and stopped the electric circuit.)
Reassamble the switch - and all phases should work.
Dash light dimmerWhile you've got the switch out, inspect the rotating section that operates the dimmer. It has a rotating contact that runs over a resistor coiled wire. As the contact moves over the coil, electrical resistance increases or decreases, thereby adjusting the brightness of the dash lights.
If the resistor coil is still intact and complete, carefully clean its surface and the surface of the rotating contact point. (I used a points file to do this.)
If the resistor wire is broken, or if the dash lights still do not work, do this:
When you removed the switch from the dash, you will have removed the grey wire attached to the dash dimmer control section of the switch. It leads to the dash lights. When you replace the switch in the dash, connect this grey wire to the
middle one of the set of three contacts on the other side. This is the contact that leads to the tail/number plate lights. When the light switch is on, this is always live, so the dash lights will always be on (but not dimmable).
(Thanks to Ace for this tip)Note that when the headlights are on, the parkers are off. In modern cars, when the headlights are on, the parkers are also on, so that if a headlight globe blows, there will still be some form of light on each side of the car. If you want the parkers "on" in this way, I can't see any problem with connecting the white wires (a pair of them that are originally connected to the first terminal) also to the centre (tail light) contact point.
