Hi, I have a 2003 Skoda Fabia Estate 'Classic' and have been checking my rear lights.

 

I have noticed that the lowest bulbs in the rear holders on both sides are Dual Filiment bulbs - rear Side Light and rear Fog Lamp - however, when I switch the rear Fog Lights on only the Right (driver's) Side comes on?

 

I have checked the bulb on the left side by swaping it over with the right side and it is OK, both filliments working.

 

So, is this normal that only the rear Right Fog Lamp comes on?  The second filiment (Fog Lamp) in the left side being redundant?

Yes it's normal to have just the one rear fog light

Just wire a link from the RH side to the LH side to enable both rear fogs.

 

DB

Thanks for the replies, just nice to know it's correct.

 

Thanks

DWS

Most cars have only one fog light, something to do with seeing where the offside of the car is etc etc, how ease is it to wire both up to work? ive seen people make 5 bulb break lights using the fogs too?

See post #3

 

DB

http://www.briskoda.net/forums/topic/265504-dual-rear-fog-light-how-to-guide/?hl=%2Brear+%2Bfog#entry3136003

Do NOT wire in two fogs.

They were designed to run 1.

1 is enough to see in the fog without dazzling other road users or being mistaken for brake lights

BUT - with two - there's two choices- one fails and with two , car has an extra one , or ,car behind gets more warning

As above, its normal. If they were meant to have 2 fogs they would come out the factory with 2.

if people behind you mistake two rear fogs for brake lights, perhaps they'd slow down

With two rear fogs fed from the same fuse and wire, the fuse is much more likely to blow, leaving no rear fogs lit.

If you uprate the fuse, the wire is no longer protected to the right degree.

With two bulbs lit, the current in the single feed wire will be virtually double, so the voltage drop along the wire will be almost double, so neither bulb will be quite as bright as when there was a single bulb.

 

It's a great idea...

Both my rear Fogs work perfectly OK and bright on both my cars and have done for the last 10 years with no fuses being blown.

 

So I suggest you ignore the above post.

 

DB.

Seems odd to me that the rear left lamp unit was designed with a double filiment bulb holder and bulb fitted to it by the factory in that position with the intention that one of the filiments would be redundant, why didn't they just design a single filiment holder in that position?

 

Actually, thinking about it I might have answered my own question here, the double filiment bulb holder and bulb on the left side can be brought into play in markets with left hand drive cars are sold.  

It will be a universal part for either left or right hand fitting. the only left or right handed part would be the wiring loom.

 

DB

Seems odd to me that the rear left lamp unit was designed with a double filiment bulb holder and bulb fitted to it by the factory in that position with the intention that one of the filiments would be redundant, why didn't they just design a single filiment holder in that position?

 

Actually, thinking about it I might have answered my own question here, the double filiment bulb holder and bulb on the left side can be brought into play in markets with left hand drive cars are sold.  

Its also cost cutting, its more economical to design a generic part used for both sides and then fit a generic bulb than it is to create 2 bespoke parts with 2 separate bulbs. If it cost only 2p more per unit to create 2 separate bulb carriers, multiplied over the hundreds of thousands of cars produced, thats a lot of money to waste!

if people behind you mistake two rear fogs for brake lights, perhaps they'd slow down

 

Ummm, Flip that argument around. The reasoning is that people will see the foglights (and growl, grit teeth, spit blood as it's not foggy!!!!, just drizzly) but will then MISS the brake lights coming on. Or at least not notice them as quickly. Where the foglight(s) are wholly separate from rear clusters, that's maybe not an issue (but so 1980s)

With two rear fogs fed from the same fuse and wire, the fuse is much more likely to blow, leaving no rear fogs lit.

If you uprate the fuse, the wire is no longer protected to the right degree.

With two bulbs lit, the current in the single feed wire will be virtually double, so the voltage drop along the wire will be almost double, so neither bulb will be quite as bright as when there was a single bulb.

 

It's a great idea...

looking at the fog lamp wiring it's standard wiring. One lamp @  21w/12v = 1.75 A. Two lamps = 3.5A, on at least 5A wiring. Lowest fuse fitted = 5A , so it wil fail on short term usage at 10 A, and possibly on long term use @ 7.5A, a lot more than the wiring or current used . But I think the foglamps are on at least a 10A circuit. Might be worth circuit analysis before ...........................

Thanks for doing the hard yards on the calculations.

 

Couple of questions though:

Your 1.75A per bulb figure suggests a dc resistance for the bulb of 12/1.75 = 6.86 Ohms (V/I=R).  I've got a new-out-of-the-packet 21W bulb here that I'm measuring at just about 0.5 Ohms; why do they differ so much?  Is it something I haven't considered? My bulb seems to have the possible capability to pull around 12/0.5 = 24Amps (V/R=I).  What could be going on?

 

I notice that you haven't used circuit analysis to demolish my voltage drop/brightness argument, why's that?

With two rear fogs fed from the same fuse and wire, the fuse is much more likely to blow, leaving no rear fogs lit.

If you uprate the fuse, the wire is no longer protected to the right degree.

With two bulbs lit, the current in the single feed wire will be virtually double, so the voltage drop along the wire will be almost double, so neither bulb will be quite as bright as when there was a single bulb.

 

It's a great idea...

 

Two Bulbs take 3.6 amps,cable rating will be about 12 to 16 amps, original fuse is 10 or 15 amp.The voltage drop is next to nothing.Your eyes will not be able to see any difference in brightness in one or two bulbs.There is a lot of leeway,fitting two bulbs will be fine.

Edited 15 October, 201312 yr by AndyPandy

Fuse is 7.5A, according to Haynes.  See my previous post regarding current calculations.

 

"Your eyes will not be able to see any difference" : Maybe, maybe not, forgive me if I don't take your word for it.

 

"The voltage drop is next to nothing": How do you know, have you measured it?

Thanks for doing the hard yards on the calculations.

 

Couple of questions though:

Your 1.75A per bulb figure suggests a dc resistance for the bulb of 12/1.75 = 6.86 Ohms (V/I=R).  I've got a new-out-of-the-packet 21W bulb here that I'm measuring at just about 0.5 Ohms; why do they differ so much?  Is it something I haven't considered? My bulb seems to have the possible capability to pull around 12/0.5 = 24Amps (V/R=I).  What could be going on?

 

I notice that you haven't used circuit analysis to demolish my voltage drop/brightness argument, why's that?

 

 

Fuse is 7.5A, according to Haynes.  See my previous post regarding current calculations.

 

"Your eyes will not be able to see any difference" : Maybe, maybe not, forgive me if I don't take your word for it.

 

"The voltage drop is next to nothing": How do you know, have you measured it?

Here are the calculations to answer the above two posts,I liked the challenge,takes me back to Technical college many years ago

 

A bulb filament when hot,has a higher resistance ,a resistor is made with constantin,which is an alloy which does not change its resistance value when heated. Ohms Law  states that the resistance in a circuit ,can be found by dividing the voltage across the circuit in your case 12volts,by the current flowing in your case 1.75 amps,giving an answer of 6.86 ohms. In answer to your question about voltage drop,the resistance of the wire and fuse is very low say 0.1 ohm ,which drops only  0.175 volts in its length,so there will be 12 v-0.175v across the bulb ,equals 11.825v with one bulb ,and 11.65v with two bulbs.If your eyes can see the difference between 11.825v,and 11.65v ,they are better than some of my voltmeters!!

Edited 15 October, 201312 yr by AndyPandy

You're right on the money with respect to the bulb resistance.  About 14 times as resistive when hot compared to when cold.  So when you switch on the lamp, there's a brief surge of much higher amps than the continuous running current.  This sort of thing is often described as an 'inrush' for obvious reasons. It's important for the rating of switch contacts and fuses associated with this sort of circuit, even though it's brief.

 

[slight aside] Interestingly Skoda's Manual in the glovebox mis-describes Fuse 37's rating as 5A, where what's actually fitted is 7.5A, which agrees with Haynes's wiring diagram and table of fuse values.  Doubly interesting is the fact that it's the only fuse in the whole panel that's 7.5A.  One would think that if it really didn't make much odds, they'd've rounded it up to 10 or down to 5, like most of the others are.

 

Having stuck a 4mm plug on the end of a longish bit of wire earlier, I was in a position to measure the 'standard' voltage drop when I got home, with my test meter leads now able to span the distance from fusebox to rear cluster.

With my extra long test wire stuffed in with the bottom pin of the rear cluster connector, and my other probe on one or other side of fuse 37, I fired up the engine and switched on the rear fog.

Voltage drop from end to end of the wire; including fuse = 0.262V, not including fuse = 0.243V.  Bigger than I expected TBH.  I haven't yet linked across to try it with the second bulb added, but probably will in the next week or so.  Bearing in mind that brightness of an incandescent lamp is approx. proportional to voltage squared, I'm now more confident than I was that doubling this voltage drop will be perceptible in terms of brightness. Will check this too next week.

 

HTH.

If your fussy about each bulb having the same brightness,(voltage) you could always move the supply from the fuse to the centre rear of the boot,and feed each bulb from it!

No, that's not the issue. The thing is that neither bulb will be as bright as the single one was, so following drivers will need to get closer before they see your car. :wonder: