The light in the boot of my car does not light up.
I have checked the bulb and it is fine.
I have checked the voltage at the bulb holder without the bulb and with the ignition key turned on and it is 12V
However, when I put the bulb in the holder it does not light up and the voltage is 0, even with the ignition key turned on.
Where is the fault?
Thanks,
Antonio
First thoughts are grounding issues. Is the earth secure? Are both connections to the battery clean and secure - this should be likely if the car starts but you never know. Then I think we go into deeper issues relating to the loom and then difficulties arise. Has the car suffered a collision and then where? Any wiring connections in that zone?
Good luck.
In our stars?
Well, clearly that 12V is not across the bulb terminals. If one pin has 12V on it to earth, then the earth connection to the other pin must be missing. Check the resistance to earth of the non-12V pin.
In message , asalcedo wrote
Is there a switch in series with the bulb?
How is the bulb turned on? It's very unlikely to stay on all the time. I had a similar problem where the bulb was OK and there was voltage to the connection at the back of the bulb, but no voltage to the contact that the bulb touched. There was a switch on the wiring diagram but I could not find it. I took the bulb holder apart and found a mercury switch inside. The mercury was coated in oxide. I took the mercury out and cleaned it and cleaned its container. The switch worked perfectly after that.
Turning the ignition on is not usually necessary to get the boot light on. There are two places the boot light switch can be fitted in the circuit - either in the 12v feed to the light, or between light and ground. Working out which way it is wired and where the switch is, is the starting point.
If there is 12v at the lamp socket when no lamp is installed, but the
12v disappears when a lamp is fitted - it might indicate there is high resistance at the boot switch and that the boot switch is wired in the 12v supply to the lamp. There are lots of other possibilities though, far too many to list.
A mercury switch - that would make it a very long time ago :-)
It sounds like you have a high resistance connection on the +12v wire, properly at the bulb holder as the boot switch normally switches on the earth wire. Trevor Smith
Many years ago, my father had a Citroen GS which had a mercury switch for the boot light. Being sealed, they are normally very reliable. Sounds like Matty's one wasn't sealed.
With regards to more recent cars, I've had the both the light switch fail high resistance due to dirt ingress, and the earthing of the boot hatch go high resistance through its hinges (which stopped the rear window heater and rear wiper from working). In the latter case, I connected an explicit earth tail between the hatch and the car bodywork, and that was still working fine when I sold the car ~5 years later.
Mistery solved!
It turns out that there is a switch. Quite hidden under some molded parts. It consists of a metal ball that closes the circuit when the boot door is fully open.
The ball was not making contact with the cable terminals. I replaced the terminals and got the switch working fine.
There was indeed no need to turn the ignition key on. The positive battery pole is always connected to the bulb holder. It is the negative pole the one that is switched by the ball. Thus, there is always 12V between the positive pole at the bulb holder and any metal part in the car, which is connected to the negative pole of the battery.
Andrew Gabriel;2117844 Wrote:
I'm not sure what Matty saw. Surely mercury would evaporate over enough time? And I'm really not sure how you clean it. Kind of hard to polish a liquid!
But only Citroen could put in a boot switch that makes the light come on whenever you brake...
Andy
A mercury switch has a curved or bent glass tube with 'contacts' typically at the middle and one end, and obviously containing a bit of mercury. As it's tilted the mercury bridges the contacts to complete the circuit.
The mercury switch was in a 1975 Jaguar XJ6. After about 15 years the mercury was coated in gunk. Fortunately for DIY purposes the switch was easy to take apart and clean. I think I squeezed the mercury through a small hole in a piece of cloth and it became shiny again. The switch was fixed for nothing, and quicker than going for a drive to a spare parts shop, even if the shop had such a device.
The one have in the shed is a straight glass tube with contacts at one end. The tube has to be tilted to at least 30 deg from horizontal to let the contacts be bridged. In actual use they were set to flip up to a vertical position.
I've seen them. What I've never seen is an open one - all the ones I've seen have been a *sealed* tube.
Andy
The Jaguar mercury switch was not an open one, whatever that is. But it was easy enough to unscrew to clean it. A pox on sealed *anything*!
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.