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Interior lights - fade out or just turn off?

Z

zcacogp

Active member
Chaps,

A vague one this. On your 944, do the interior lights fade gently when you close the door, or do they just turn off?

I have a dim (see what I did there?) memory that the interior lights on my S2 faded when I bought it, but once the alarm was fitted they just turned off. Which is what they do now. They do have a delay on them (when you close the door they remain on for a period of time, and after a while they turn off. If you turn the ignition on then they turn off immediately.) They also turn on when the alarm is 'disarmed' and the doors unlocked, so they are on before you open the door.

Is there a light fade circuit built-into the car that the alarm is over-riding? Or is my memory playing tricks on me? I've printed out the wiring diagram and there is nothing on that that helps, but such functionality would probably be built into a relay and not explicitly mentioned on a wiring diagram.


Oli.
 
Fading gently cannot be a relay which is either on or off. Just wire a capacitor in parallel with the bulb.
 
Ian,

No, but such functionality can be (and often is) built into a relay, to allow an output to rise gently or fall gently. OK, strictly speaking that isn't a relay, but it is built into the 'relay modules' which are plugged into the fuse box.

A capacitor would be the simple option, but it would need to be a pretty big one if it is to offer any duration of fade. A small circuit with a MOSFET would be a more elegant solution (and is what I may end up doing), but if the functionality is already there I'd like to find out why it isn't working.


Oli.
 
Yes I see. If you just want the bulb to fade gently then I don't think a capacitor would need to be massive for an incandescent bulb?

I could calculate it or I could arse about in the shed later. What's the bulb - 6W festoon?
 
Hmmm. Yes, it would probably be 6w. Maybe 7w. But to give a fade of 5 seconds or so the capacitor would need to be ... well, chunky.

Bit of calculation (tell me if I go wrong):

P=7watts (bulb size)

And E = P T (Where T = time)

So E = 7 T

Capacitor theory says that E = 0.5 C V^2

So 7 T = 0.5 C V^2

So C = 14T / 144

Which, roughly says that C = T / 10

So if you wanted a 5 seconds fade time (assuming linear fade), you'd need a 0.5Farad capacitor .... which is pretty big - non? (Or have I got my calculation wrong somewhere?)


Oli.
 
I think your calculations are probably sound but the fade of an incandescent bulb is not linear as the resistance changes as the temperature decreases as the bulb fades.

I wish I hadn't started thinking about this now....[8D]
 
Yup, R will decrease as I decreases.

But P will remain the same, which is the relevant bit, so that bit holds sound.

I think however that that assumption is the problem; as the thing charges (or discharges), the voltage across it will change. So plotting V against T is not a straight line, and therefore the V^2 bit will fall over quite badly.

Either way, I think the required energy from the capacitor will mean you need quite a large capacitor, which will be both expensive and large (and therefore hard to put in the light fitting.) I'm thinking something about the size of a matchbox (or possibly bigger.)

A FET (or MOSFET) would be a more elegant solution. With a much smaller capacitor on the base.


Oli.
 
Perhaps memory fails me but I could swear some of my cars have and some haven't?! My early lux for example I could swear I remember fading but the red turbo I owned definitely went off as soon as the door shut.
 
Ben,

Hmmm. Interesting, thanks. Interior fading lights are something that Porsche have had for a long time (I think they featured on the 914/6) so I'd suspect that they would be on the 944's. And if they did fade on your Lux then it sounds like it's not a 'higher-spec-models-only' thing.

I wonder whether there is some common alarm fitting mistake that causes the interior light dimming functionality to be lost. I am sure that mine stopped working when I fitted the alarm, and the alarm does have a connection into the door pin circuit (to detect both that the doors are closed when the alarm is 'armed', and to set the alarm off if a door is opened.) Given that the door pin circuit is part of the interior light circuit then I'd suspect the fitting of the alarm more and more.

I presume your turbo does have an alarm?


Oli.
 
My 1991 S2 had interior lights that just went out immediately and I wanted them to dim gradually after closing so I fitted an interior light from a Mk2 Golf GTI which has this function built into the light unit, it fit directly into the existing hole in the roof lining and most of the cables fit directly except one which needed a smaller connector job took about 1/2 hour to do and cost £0.99 from eBay.

As it is in series with the lamp in the boot this also dims gradually.
 
ORIGINAL: david924s

My 1991 S2 had interior lights that just went out immediately and I wanted them to dim gradually after closing so I fitted an interior light from a Mk2 Golf GTI which has this function built into the light unit, it fit directly into the existing hole in the roof lining and most of the cables fit directly except one which needed a smaller connector job took about 1/2 hour to do and cost £0.99 from eBay.

As it is in series with the lamp in the boot this also dims gradually.
Click to expand...

Ding[:)][:)][:)][:)] top marks. I no longer care about investigating this and will be taking this advice the moment I get a 944 on the road if it doesn't dim already.

Paul I think both the lux and the red turbo had an alarm but the lux had a clifford alarm installed by me by a guy who was very very good. he removed all previous alarm wiring before installation (of which there was much) so I guess if done right it 'shouldn't' interfere.

Note it is possible I have just forgotten the light going out immediately in the lux and it is also possible that a previous owner had stuck a mk2 golf light in when the original unit had failed as the car was not the kind to have genuine parts installed!
 
Well I was baffled by the equations and was thinking sod that I'm just going to be happy with it going out straight away, after the past 2 post think I'll be having a go at this [:D]

Do all Mk2 Golfs have the dimmer built in with their courtesy lights?
 
What about charge up times of the capacitor say if you close your door, open it briefly and close it again?

What about something like this?

http://www.maplin.co.uk/Module.aspx?ModuleNo=3315
 
Dammit, you lot just made me put my shoes on and go outside. I HAD to check what my 1990 944S2 does. All interior lights go instantly on/off with the door open/close. Simplez, and suit me fine. [:)]
BTW - anyone know why my engine bay light doesn't work? I've checked the bulb and it's good.
 
The engine bay light only comes on under certain conditions - can't remember which. I think the lights need to be on and the key in the ignition or something like that - i.e. it is not like your courtesy light that just comes on with the opening of the door.
 

ORIGINAL: sawood12

The engine bay light only comes on under certain conditions - can't remember which. I think the lights need to be on and the key in the ignition or something like that - i.e. it is not like your courtesy light that just comes on with the opening of the door.
Click to expand...
Ahh, I'll check tomorrow. My slippers are too comfy to remove at this late hour. Thanks Scott.
 
Iirc the car has to be on sidelights for the bulb to come on when the bonnet is opened.
 

ORIGINAL: Eldavo
Iirc the car has to be on sidelights for the bulb to come on when the bonnet is opened.
Click to expand...
Yes. And the thing is used so infrequently it quite often has a bad contact on it somewhere, or the bulb is blown.


Oli.
 
not directly relevant cos I cant remember either - however I got some LED festoons of ebay - much nicer light than the old bulb and less battery drain.
Tony
 
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