A few weeks ago, my lights started turning off. I'd come back later and they would work fine.

I thought it was the lighting column stalk, so I put in a temporary switch taking over from the stalk in providing an earth. However, they continued switching off.

Then, I happened to open the bonnet (at the front) and the headlights switched on (I'd left the light stalk in the on position). All the lights worked fine but when I closed the bonnet, the headlights' relay clicked the lights off again; I heard the distinctive click.

The under bonnet light switch seems to be the culprit as I the lights went off when I depressed the switch nipple, simulating a closed bonnet.

I'm not that good at automotive electronics and can't understand how a switch can cause the lighting relay to switch off.

Can anyone explain why this is happening?

2 Answers 2


This sounds like a ground problem, not a switch problem. The under-bonnet light and its switch are not the culprits, but a clue.

Not having a wiring diagram for your vehicle in front of me, I can't tell you anything about the wiring layout. However, it appears that the under-bonnet light is providing a ground path for the headlight relay in lieu of its proper ground path. When you press the under-bonnet switch plunger (not "nipple"), you are opening this ground path and the relay goes off. The same thing should happen if you remove the under-bonnet bonnet bulb itself.

What you need to do is find the headlight relay and trace its ground path or look at a wiring diagram to determine the ground path, then correct any broken/intermittent connectors/wires/attachments. It could be as simple as tightening a ground lug, or as difficult as re-wiring the relay.

  • Hi Scrappy, Thanks for your help, v useful. I have got a wring diagram that I can e-mail to you. I can't see a way of attaching it to this message, unless you know how? Otherwise can I e-mail to you? Aug 2, 2016 at 16:32
  • Glad to be of help, Chris. It would be helpful to state the year/make/model/options (and any modifications relevant to wiring) of your vehicle, then we can see if the wiring diagram is already out there on the internet. I would be normally be OK with having you email it to me, but given that there are is no private messaging function here, I don't want to put my email out for all to see.
    – Scrappy
    Aug 2, 2016 at 18:12
  • Hi Scrappy, I understand about not giving out e-mail address. The car is a European spec 1973 Ferrari Dino 246 GTS. No electric windows, emission controls or air con. Here's a link to the wiring diagram: Aug 3, 2016 at 9:30
  • Sorry here's link: unevoiture.com/manuals/doc/… Do you need a legend? Aug 3, 2016 at 9:33
  • Sorry forgot to say that I'm not using electronic ignition - just normal coil and mechanical points Aug 3, 2016 at 9:34

If the car has an automatic light option like nearly all cars do it really sounds like you're gonna have to replace what they call a headlight switch for some reason in parts stores. It looks like a little receptacle box that a wiring harness plugs directly up to. For some reason that part can effect every light on the car. Hope that helps.

  • Hi Trigga, Thanks for your response. The car is a 1973 model so hasn't got the automatic light option - I'm assuming you mean the option you get on modern cars that automatically switches the lights on when it starts getting dark? Aug 2, 2016 at 16:35
  • Yes. And that same little terminal can also prevent your taillights from coming on even when using the switch manually.
    – TRIGGA
    Aug 4, 2016 at 15:10
  • Italian auto electrics!! Why did they make things complicated back in the '70s (-; Aug 6, 2016 at 22:41

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