Reverse lights switch

 The MK II MGB was fitted with reversing lights to comply with new regulations in 1968. These were 2 square lights located in the rear panel between the taillights. They were powered by wires running down the driver side loom that also fed the rear lights, indicators, number plate lights and fuel tank sender and pump feeds. There is a simple switch fitted to the driver's side of the gearbox that makes continuity when engaged by moving the gearstick into the reverse position. This switch is the same as the overdrive switch which is also located on the gearbox. These switches can play up and in the case of the reverse lights means they either stay on all the time, or don't come on at all. Of course, if they don't come on, always check the bulbs and rear wiring loom first, especially the earths, as it's much easier than getting under the car. Once you've eliminated these it's time to get the car up on ramps (or stands) and get your hands dirty.

The switch is a very simple Lucas plunger switch which has a ball end plunger that physically pushes two contacts onto two contacts that are attached to the wires from the loom. In the case of the reverse lights these are both green wires and carry 12V to the lamps. The faulty switch is almost definitely a stuck plunger and/or dirty contacts. Both can be fixed using the original switch with no new parts.

First pull the wires off the switch. It doesn't matter which terminal you attach what wire to. The switch just makes continuity between the 2. Now undo the switch which is normally only hand tight. If it is tighter than that you could try needle nose vice grips or I made a 1 1/8" spanner (see below) for the occasion. With the switch out give it a good clean and test the plunger mechanism. If the ball is stuck, it's time to pull it all apart and clean it inside and out. The metal body of the switch has been punched to hold the plastic part that has the 2 contact tabs. Just hold the switch carefully in a vice and using a hammer and punch, punch these folded over parts back out to free the plastic part.

Be careful as there are springs inside this so once the plastic part is free pull it all apart on the bench and evaluate what's going on. If the ball plunger is stuck solid, which is common, use some contact or parts cleaner and gently tap the ball into the switch body. The ball plunger will (hopefully) slide all the way out and drop out of the body. Now clean the plunger and body. If there's rust and dirt like mine, use parts cleaner and fine wet n dry to clean the shaft and socket. The plunger shaft has an O ring so be careful not to destroy this. With both parts clean reinsert the plunger and move it backwards and forwards making sure it doesn't stick. Now clean the springs and contact patches well with the same method. Once everything is clean and the plunger is moving freely reassemble everything doing the reverse of what you just did. With the plastic part seated back in the metal body just recheck your plunger is moving freely and if so, bend the body of the switch to hold the plastic part back in.

 

Now before you slide back under the car put your multimeter on the 2 tabs with it set to beep. Push the plunger and your multimeter should beep with the ball in and not beep when it comes back out. Your switch is now working. Refit the switch into the gearbox and test the lights are working as they should. If your loom was OK and your switch was working with the multimeter your reverse lights should now light up when you put the car in reverse. If they don't light up there's another issue. While the car is up, check your gearbox oil and give the area a clean and check for anything suspect. If it all looks good put the car back on the ground and go for a drive. 

If you have trouble removing the switch from the gearbox, you'll need to make yourself a spanner to get it out. I just ground down an old spanner to suit by taking it out to 1 1/8" and making it thin enough to get into the slot.