Boot light not working (electric gremlins again)

[dvance]

Seems like the electric gremlins have not passed up my car . Since I bought it the boot light has not worked, so decided to have a look.

Took out the whole assembly. Seems it's a simple thing with a diode and a bulb. The bulb reads a bit over 1 Ohm in resistance, so should be working. The diode checks out with junction voltage of about 0.6 Volts. Should be good as well. The voltage supplied to the light is 2.2V instead of the required 12 (the bulb is rated at 12V 10W). Will appreciate any ideas why that might be so? The only thing that I can see happening is that the line to the boot is bridged somewhere along the way and is leaking voltage.

Don't really want to go to a garage and pay for hours of labor to just get the boot light fixed

 

[Slimy]

What are you measuring it against, IE what is your ground? Are you going across the connections to the bulb or going from the wire to the chassis? If it gives 2 volts across the wires, and 12 volts when connected to ground, that suggests a dodgy earth.

Why is there a diode? That seems unusual for such a basic connection.

 

[Muttley]

I presume we are talking about a Mk.2 Toledo, and not the Alfa 147 your profile says you drive . . .

No diode required for a filament bulb, and there isn't one there as standard. Sounds like it's been modified for a LED replacement bulb, and there's a resistor in the wiring somewhere.

 

[Slimy]

No diode required for a filament bulb, and there isn't one there as standard. Sounds like it's been modified for a LED replacement bulb, and there's a resistor in the wiring somewhere.

Ah yes, I like your thinking . And wiring modified for an LED array would never allow a standard filament bulb to work.

What do you reckon, strip it all back to plain wiring and retry?

 

[dvance]

Hmmm, 2.15V across the terminals, as the bulb or fitment's not grounded to the chasis anywhere. Did not think to measure against the chasis will do that. Will post a pic tomorrow of what the whole thing looks like so you guys can tell me if it's anything near what you've got.

Yeah it's on my V5 20V

I also wondered why there would be a diode there. Still, why the 2.something volts? And where would that voltage divider (i.e. resistor) been fitted to muck up the voltage?

EDIT:
So I've tried measuring with respect to the body, but well, it's all painted so that should not work?

 

[Muttley]

You can find a handy earth point on the 12V socket, or if you don't have one, the rear light cluster. The earth point is behind the nearside boot lining panel.

 

[dvance]

Can you please elaborate on the position?

 

[dvance]

Hey guys, took a picture with my phone (horrible). Drew over it to show you the circuit. This bit works if hooked up to a proper 12V supply (as you might expect).

Could not find the ground point, but could not really get the side carpeting off as I am unsure how to unclip it and don't want to damage the clips. Pretty sure there's a ground on the rear light cluster, but have no idea which one it is. Help

 

[Muttley]

Well, now that you've shown us the picture I had to go and look at mine.

It has a diode in the same location (looks like a 1N4004). It isn't shown in the wiring diagram and I've no idea what it's there for.

My light fitting drops 11.6V across it (car not running). So there's definitely something wrong with yours.

The boot light switch is in the negative side of the circuit. So the first thing to do is check you're getting +12v to the bulb. If you haven't got a 12V "cigar lighter socket" in the boot (nearside boot wall) then get the rear light cluster out and use the surround of either bulb holder as your earth reference. If you're getting 12V to the bulb then the fault is likely to be with the boot switch. This is part of the lock mechanism, and the very devil to get at. Seat do not list the switch separately, and say you have to replace the whole lock if the switch fails.

Try to find the electrical connector on the boot lock: I've never done this, but I expect you'll have to take the boot lid liner off. The wires should be brown/black and plain brown: brown goes to the earth connection in the boot and brown/black goes to the light. If you short the connector out, you should see 12V across the bulb. If you do, and the bulb lights, then the boot switch is faulty. If you don't then I'd guess the boot wiring is faulty, either to the earth point or from the 12V feed.

 

[dvance]

Cheers, will have a through look tomorrow, it's been raining all day so no chance of poking around. I'll try to take the rear light cluster out to use as reference point. No boot lighter socket unfortunately, or could not find it .

I think the diode is there as a precaution just to make sure nothing feeds back to the car if something goes wrong with your light.

 

[dvance]

Had a weird experience tonight. Took off the whole boot light fitting, and took the car out later on. When I pressed the brake pedal, the right indicator light on the dash would illuminate to about 50% as would the actual indicators. When the right indicator was on, and I pressed the brake, it would blink faster than usual. I think the right brake light was not working at that time as well.

I got back home and I put the boot light fitting back in, and voila all the problems went away. Any ideas?

 

[Slimy]

That's definitely an earthing fault, possibly on your right cluster. It sounds like it's using the bulb fitment as a ground loop instead of going to the chassis, so it's stopping the boot light from working and when its removed its trying to use the other bulbs in the cluster as an earth.

Check the whole cluster is earthed.

 

[dvance]

Will do, though it would have been useful to have a circuit diagram so I have an idea what to measure where

 

[dvance]

Right so I had a proper poke around yesterday. Seems the light problems I had after removing the boot light were due to a loose connection in the light cluster, so that seems to not be troubling me anymore.

Took proper measurements against ground of the boot light pins. Ground give about 9V and the live pin gives 12V as supposed. Now, I went ahead and took the same measurements with the boot closed and open i.e. with the light supposed to be on and off, and they were exactly the same. This leads me to believe it's the boot-lock switch that's not working properly. Would anyone be kind enough to wiggle in the back of their Tolly and possibly give me the measurements they have on a proper running light? Mind you it's a bit tricky doing it with the boot closed .

 

[dvance]

Seems it's just me talking to myself, but anyway -- found this:
http://uk-mkivs.net/forums/p/17995/170708.aspx

Will give DIYing it a try this weekend

 

[Slimy]

Sorry mate, I was planning on giving this a go for you but I'm afraid even the missus can't convince me to go outside when it's torrential rain!!

I'll try it in the morning if it's dry, I'm on bedtime story duty very shortly...

 

[dvance]

No worries! I am pretty sure it's the boot switch as when i unlock my car with the remote and just open the boot, the car re-locks itself fairly quickly while the boot is still open, which leads me to believe that it's not aware that it actually is open. I have the gut feeling that I'll have to splash out the 30 quid that the new boot latch costs to get it to work right

 

[Muttley]

Just take the bulb out of the boot light fitting and buzz for continuity between the earth connection on the light cluster and the earth side of the boot light.

But it does sound like your boot lock switch has failed

 

[dvance]

Well, good idea, but that's how I measured the voltage on the boot light leads, so I doubt they'll be connected i.e. ground on light cluster to ground on boot light gave me something around 9V at both open and closed boot.

What I really want to try is shorting the connection from the boot lock switch to see if it's really the switch, unfortunately I need to get to it first to do that.

 

[AP_V5]

If its re-locking with the boot open it almost definitely has to be the boot switch, this also means if you unlock just the boot and open it I'm guessing the alarm won't sound either so def something you want to sort.