Reverse light switch differences

[Matt Cole]

Any electronics engineers on here? My reverse light doesn't work correctly at the moment. The reverse light gearbox switch on a 5 is normally open until put in reverse then puts power to the light. On the megane gearbox switch it works the other way, the switch is closed until in reverse.

This means my reverse light is perminantley on until I put in reverse.

Is there any way to wire it simply to work?

[Ian S]

You can't swap the switches?

You could use a relay but it would be on all the time, draining some current. Maybe that would be OK, provided you arranged the feed to it to only be on while the engine is turning the alternator.

As you're only operating one bulb maybe a small relay with a lower current coil will suffice.

What the wattage of the bulb? Lets say 14.8 Volts, as with the engine running it can be in that area these days. 20 Watts(?) for the bulb. Watts = I x V. So 20/14.8 = I = 1.35 Amps DC.

So a single pole, 'double throw', miniature relay with a 12V DC coil rating and a 2 Amps or so rating for it's contacts should do it.

Just use the gearbox switch to switch off the feed to the relay so it's normally open contacts then close and turn on the reversing lamp.


To do it with electronics is a lot more difficult unless someone happens to know of a module that can do it over the temperature range needed for car use, ie, -10°C to +50°C. Transistors flow more current more as they get hotter and may not work at both temperatures. I found this out with the charge temp gauges and I had to abort the transistors switching, that worked at room temp but not at 50°C, and use a (relatively) clunky big relay to dim the LED display.

[Matt Cole]

Cheers Ian. A comprehensive and detailed reply.