Headlight Mod

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Headlight Mod

Post by Toby_HDi »

I have been doing some investigation into this and I am now going to do it. I may even relay my sidelights as well.

Am looking for a factory look so will try to place everything in the relay boxes and the wiring along the harnesses already there...will let you know how it goes.

I do have a question however....Where do I buy the stuff? I have looked around everywhere and the most obvious place - maplin - doesn't do the right relay as far as I can see.

Hopefully once I've got the items I'll get on a do it and maybe do a write-up with photos as I don't think this has been done yet


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Post by rory_perrett »

Have a look at http://www.vehicle-wiring-products.eu/V ... mepage.php used them for all my kitcar stuff a few years back.

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Post by red_dwarfers »

I did manage to get a 40 amp 12v automotive relay from maplin but not thick enough wire.
I got my relays from our local south west based 'Kevin Cooper' automotive factors that sell everything! :lol: A lot more helpful and chatty than my local GSF grumps :lol:

My 40 amp wire was found within the 'chav up your car with massive speakers' section of Halfrauds. Its advertised as 42Amp speaker wire, but its actually written '42Amp power cable' on the wire. I think 3 mts of this was £7.

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Post by Toby_HDi »

Thanks for the input. One thing I was looking for was preferably finding the relay that has 2 87 outputs rather than just the one. I did find one on VWP but it is only rated to 30A.

Other question for those who also like the look of a factory finish is where can I put the relays and fuses. I'd like to get them in the relay box just behind the battery but there would seem to be no room in there. Any ideas where to locate them?


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Post by CitroJim »

Toby_HDi wrote:I'd like to get them in the relay box just behind the battery but there would seem to be no room in there. Any ideas where to locate them?
Put them in the ECU box Toby, plenty of room in there. Alternatively, they can be mounted in the spare relay holes in the engine fuse box between the battery and ABS block.

On mine, one relay (for the offside) is in the ECU box and the other (for the nearside) is in the engine fuse box. Rather neat and it keeps wire runs short and nearly invisible.

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Post by Toby_HDi »

I've looked in said fuse box but there would not be enough room for all the relays. I would want to keep them together. I think therefore they would need to go in the ECU box and then run the feeds to the battery along the ECU harness to the battery

Jim, did you earth your relays back to the battery negative post or use a beefy earth strap somewhere else? Also as I would be going for a tidy finish, would the existing connectors cope with the extra juice? This way I wouldn't need to do any rewiring inside the headlamp. I could just splice into the harness just before the plug that goes into the back of the lamp unit.

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Post by JamesQB »

If you fancy a challenge, you could always pop to your local electrical shop and buy some 10mm twin+earth solid core. Great fun.

On a serious note, such shops should be able to get you some veryheavy duty relays. We use all sorts at work for electrical jobs and get it all from our local CEF.

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Post by Toby_HDi »

Thanks again for advice.

Taken a look at VWP and they seem to have the stuff I need. Think I will put them in the ECU box. They have various bases too. Just need to work out how to secure it all in now.

On another note. Do I need exactly the same type of relay for the heater blower and rear window heater circuits? Might do those at the same time and prevent my ignition switch burning out rather than fixing it when it does.


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Post by bxman »

you can fit the relays inside the headlamp box itself .
It does mean using 4 relays in total, one for each bulb,
I ran a 60amp wire through a hole that I drilled in the housing split it to feed to each relay .used the existing feeds to power the coils, earthed them back to the bulb holders and linked the outputs up to bulbs with 4 inch links not much room for them to rattle about and they should be dry enough.
picked up the feed from the spare fuse holders next to the radiator fan supply; the buzz bar has provision for 4 60amp fuses.
Just a single wire and the existing plug carries everything else.
It's certainly worth the effort, it brings the light level up markedly and I have yet to beef up the earth.

neglected to mention no need to cut or alter any of the existing wiring. it can be returned to standard in seconds.
Last edited by bxman on 21 Nov 2008, 22:45, edited 1 time in total.

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Post by Toby_HDi »

Hmmm not a bad plan. However, I was thinking about having the relays in the ECU box with the wiring somehow going through where the existing connector is as I doubt that the connector can be used in this mod. I was thinking of maybe using some kind of rubber bung or even a plastic cap with holes for the wires which would fit in where the connector is (should I find it can actually be removed. Any idea if they can be?

By the way didn't realise you were a Malverner - I visit there quite often. :D


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Post by AndersDK »

Thats some pretty hefty components you are calling out there :shock:

Any CE approved headlamp bulb (single filament) will consume a rated power of 55W at the rated bulb filament voltage of 13.25V (makers design requirements).
Thats 110W the pair of headlamp filaments.
According to ohm's law : 110W @ 13.25V calculates to 8.3A total current.

You do not need to design your circuit for more than 3 times that to have plenty of safety margin in the circuit, to take care of the cold filament in-rush current, and the actual bit higher operating voltage and current.
Thats well within a dead standard 30Amp rated relay contact, pr headlamp function (1 dip/dim, 1 main - 2 filaments each).
Your rear & sidelamps would require even less, as they consume less power.

Now for the cable gauge needed :
A total of approx 0.1V finite pure cable loss is the best you can expect, considering standard practical cable gauges used.
But remember this cable loss doubles, as you have both a feed and a return (earth) cabling.
Copper cable has a specific resistance of 0,0175 ohm pr square mm, normalised on a 1m lenght.

Assuming you are doing the best possible installation, you run a single separate cable for each headlamps filament and a separate (from each headlamp) earth return directly to the negative battery pole clamp.
Further lets assume the cable length to first headlamp is 1.5m.
Using a standard 1.5mm2 gauge cabling we then have our cable loss :

Uloss = (Rspecific / area) x Length x Ifilament = (0,0175 ohm / 1.5) x 1.5 x 4.15A = 0,0726V or 73 milliVolts.
The farthest away headlamp may have a cabling length of 2.5m which then calculates to : (0,0175 ohm / 1.5) x 2.5 x 4.15A = 0,121V or 121 milliVolts.

In the feed wiring to each relay contact, you WILL be smart enough to install a safety fuse to cater for fire hazards in the wiring. Thats BTW a legal requirement too and the MOT mechanic will look for it, once he discovers the cable modifications.
A separate fuse for each relay contact set prevents a completely dark front on all possible headlamps, in case of a gross fault.

You will find, that the fuse alone including connections takes care of at least 0.1V voltage loss.
Each relay contact set including connections takes care of another 0.1V voltage loss (at the very best !).
The bulb socket will further eat 0.1V pr connection (you have both a feed and a return).

In total :
Fuse loss + relay loss + 2 x bulb connector loss :
0.1V + 0.1V + 0.2V = 0.4V
The longest cable run loss = 2 x 0,121V = 0,242V (feed + return)
Total loss from battery pole to bulb filament therefore is 0.4V + 0.242V = 0.642V

Assuming you have your alternator and its cabling in very good condition, the charging (engine idle) voltage on the battery will be close to 14.2V
The bulb filament voltage is therefore : 14.2V - 0.642V = 13.558V on the headlamp farthest away from the relay.
The closest headlamp is only some 50milliVolts better, barely seen (if at all) on a voltmeter.

Since the bulb filament voltage is rated at 13.25V you are then still perfectly within your best light output versus lifetime on your headlamp bulbs.

Looking back on the calculations, you will also see that you can not gain anything by using a bulky thunder stereo power cabling for your headlamp filaments.
The cabling is simply not the worst loss component in the circuit considering sensible cable gauge used.

The above sample calculations is also the reason why I (as a former computer worker) uses a dead standard 3-prong PC power cables for light installations in cars.
These 3-prong cables usually are - exactly 2.5m or 1.5m total length - and the gauge on each wire is exactly - 1.5mm2.
And then you have a very handy brown wire for dip, blue wire for main and green/yellow wire for earth in each cable 8)
Such cables are usually a give away as they often comes with a standard US-115V cable connector at no use in our part of the world.

A final note :
If you for any reason decide to do a non-compromise, no-loss perfect connection from the battery to your headlamp bulbs, you will most likely need a wagon load of bulbs.
Because there are then no losses to minimise the in-rush current of the cold filament, which then acts as a flash bulb ... :lol:

Good luck on the project :wink: