[alfa] re: Relay in headlight circuit

[Joe Elliott <jee@xxxxxxxxxxxxxx>]

E-code headlights with standard bulbs won't draw any more current 
than the POS American headlights that they replace.  However, if you 
buy non-standard bulbs that are stamped with a higher wattage than 
the stock ones, (e.g. stock is probably 55/60 watts for both American 
and European lights--you could get 85/100 watt bulbs if you wanted 
to) then you will draw more current and risk killing your headlight 
switch prematurely, which is costly, dangerous, and sometimes a pain 
in the ass to repair.  That said, Alfa headlight switches don't last 
forever, so it's wise to add relays regardless of what headlights you 
use (although I STRONGLY recommend European lights in any car, 
especially something like an older Spider in which your best chance 
at surviving an accident is probably avoiding it).

As for relays, the relays have four terminals.  You wire your 
headlight switch (switches, actually) to switch the relays (one for 
high beams, one for low beams, unless you're anal like me and you put 
left and right high beams on separate relays in case one fails). 
Then you wire the relays (other two terminals on each relay) to 
switch power to your headlights the same way the column switch(es) 
once did.  Voila.  As for placement there are basically two 
approaches.  One approach is to put the relays under the dash and tap 
into all the existing headlight wiring.  First, ground the coil 
circuits on the relays.  Relays pull next to no current (on the order 
of mA) so you can ground it anywhere with weeny wire.  You take the 
big fat +12V wire that goes to the headlight switch(es) and use it to 
power the relays' switching circuits (this is where the headlight 
current is going to go, so it needs to be a big fat wire going to the 
relay, but the switch(es) no longer need a fat wire--cut and splice 
accordingly.)  Now you're left with the wires that run from the 
switch(es) to the headlights, and two open terminals on each relays. 
When you cut those wires you'll have one loose wire to go with each 
open terminal.  Just make sure you connect the low beam wire coming 
from the switch to the coil circuit of the low beam relay, and 
connect the remaining terminal on the relay to the wire that goes to 
the low beams.  And do the same for the high beam switch and relay. 
If you take this approach, you dont have to run much wire, and you 
retain the original fusing of the headlight circuitry, which is also 
a plus, unless you have an old BMW with no fuses on the headlights. 
The other approach is to put relays under the hood and connect the 
existing headlight wires to the relay coils.  Then you run new wire 
from the battery/alternator to the relays' switching circuits, and 
from there to the headlights, which you ground somewhere (maybe the 
original ground).  The benefit of this approach is that you can use 
heavier-than-stock wires for the headlight current, to accomodate 
higher-wattage bulbs (adding relays saves the switch from those 
currents, but you can still melt your wiring if you put that current 
through the stock wiring).  The downside is that the wiring generally 
requires more work, and it becomes your responsibility to make sure 
the headlights are properly fused.  The fusing presents a dillemma 
because you want your fuse to be "upstream" of the relay in case 
there's a fault with the relay or its connections, but you also want 
the headlights on each side of the car to be fused independently so a 
fault in one headlight doesn't leave you with no headlights in a 
dangerous situation, and the wires don't branch off to the individual 
lights until "downstream" of the relay.  One solution is to use 3 
fuses instead of two for each circuit, but that's not very elegant, 
and each one of those connections is an opportunity for an annoying 
dirty connection in a couple of years.  My solution was to use two 
relays for my high beams, one for each side of the car (two fuses and 
two relays is less inelegant than three fuses and one relay, in my 
opinion, and provides the added safety of redundant relays).  I did 
my low beams using the first method (relay under dash, retained stock 
fusing), so the clutter under my hood is limited to the high beam 
circuit, and the redundant relays don't look too silly.  (I would 
have done both circuits using the first method, but I chose to run 
higher-wattage bulbs in my high beams, necessitating heavier wiring.)

I hope that makes some sense.

Joe Elliott
'82 GTV-6


At 1:42 PM +0000 6/15/04, alfa-digest wrote:

> Date: Tue, 15 Jun 2004 08:50:12 -0400
> From: Forza <headroom@domain.elided>
> Subject: [alfa] Relay in headlight circuit
>
> Hello Again....  I have installed Bosch E4 headlights...  Fred D (still
> miss him) once told me if I used them I should install a relay to protect
> the column switch from burning out from the extra current drain...    If
> you have made this mod or know of it please advise as to parts and location
> of installation...
>
> Many thanks
> Gabriel Forzano
> 86 Spider Veloce - Rosa
> South Florida
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