Boot/trunk light in the 60s saloons

In a Mk10 it now works. After I tapped the bulb assembly lightly with a screwdriver it came back to life. With a small mirror on a long handle, I could see it turn off as the lid was near the lock position. The bulb gets very hot, and gives good light in the dark, but we dont want it to remain lit when driving, with headlights on.
I think it has a on/off device built in which is activated depending on the angle of the lid. Something which could easily fail after 55 years and very little exercise.

If this is like the one on an S Type it is a mercury switch in the base of the socket. They are known for going bad from some internal corrosion (?) to the mercury. Often a sharp bump with ones fist will turn it on. Maybe wire it to be on all the time with the headlamps on but use an LED bulb that draws very little current and creates no heat?

Not sure that LED bulbs create no heat.

You’d expect so, but I recently replaced the interior light festoon bulbs with LEDs and they came with huge heat sinks on the back and they do get hot. Not as hot as the old ones, but hot enough to hurt a bit.

It may be that I bought cheap rubbish, but if you look on ebay you’ll see that almost all of them have heat sinks.

If you’re thinking of replacing the switch I’d look at magnetic proximity ones as there are no moving parts

Perhaps the ones You’ve bought are advertised as CAN-ready or so, meaning that they have an extra resistor on the back just to increase current draw to regular bulb level, so modern car CAN bus can’t detect a burnt bulb.

  1. Piotr is right I think with the CAN-bus resistor
  2. LEDs create a little heat, it shortens their almost indefinite life span, but there’s always a resistor involved, and with a big resistor to fool the computer it will need to shed some heat
  3. If it is a mercury switch, and it will be, it has been used into the 70s. The problem is that they have been unreliable from the start. You can find a good one as replacement.
  4. Some say that passing high current through the switch so it gets really hot can fix it indefinitely, just don’t melt it- reasoning: corrosion from arcing or so
  5. Alex Cannara made a mercury switch circuit that is hidden in the boot lid and does the switching; you can either use a mercury switch or a steel ball switch.
  6. the reason it failed is likely that the lamp draws too much current, so fit a led replacement or better, transistorize it
  7. skip all that and find a (non-CAN) LED, or a CAN LED because you don’t really have to worry that much, and it’s probably wired to the lights anyways
  8. or install a switch, either on the trunk lid or manual, and call it a day.

David