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Re: How much current headlights

To: "Bill Dudley" <wdudley4@attbi.com>, "Skye Poier" <skye@ffwd.cx>
Subject: Re: How much current headlights
From: "Rick Lindsay" <ROLindsay@Emeraldgrc.com>
Date: Wed, 17 Apr 2002 15:00:36 -0500
> My understanding is that the reason headlights were not fused 
> in MGs was so that a blown fuse would not leave you without 
> headlights at an inopportune moment.

   That sounds like Lucas logic.  I can just imagine the Lucas
engineers...  "Okay mates, how do we want to play this, a 
headlight failure or a melted wiring harness -- and a headlight 
failure?  Right.  Oh ey? Either way, the prats are gonna bite 
a tree.  I say we go for the cheaper and easier total meltdown 
plan!  Right, grab your kit and let's nip off for a pint?"  
   :-P

> Wouldn't the same logic apply to a relay?  Do "modern" cars 
> have a relay in the headlight circuit?  
 
My German and Italian cars do.  In the newer BMWs, the whole
thing is computer controlled.  No, it's not a Microsoft system.
Thank God!

> Since I plan to completely rewire my 74 1/2 GT   I would
> appreciate comments from those more knowledgeable.

   Can't make a statement as to the "more knowledgeable" bit
but I will share ideas and opinions.
   Bill, we are building a historic rally car using a '68 BMW 
1600-2 as the starting point.  I have built much of the electrical 
system already.  In that car, we are using high output, high current 
lighting WITH relays and fuses.  That has three immediate 
advantages; 
   (1) The high current does not have to run long distances 
        (read: resistive circuit) 
   (2) The switching circuitry needs only to be rated for 
        "control voltage" (read: low current).
   (3) No large gauge, high current wiring is in the cockpit.

   Better put, the switches in the cockpit activate the headlight
(and horn and cooling fan and ...) relays only.  Those relays are 
on a circuit panel on the left front fender well right behind the 
radiator bulkhead.  That makes the run to each headlight no 
more than about a meter.  Each of the three headlight circuits 
(high beam, low beam and driving lights) is fused right where 
it leaves the circuit panel.  The control voltage is also fused in 
the cockpit but is on an independent circuit <with only other 
relays>.
   FWIW, in the rally world, the 'low beams' are called 'ditch
lights', the high beams are 'pilot lights' and the driving lights 
are, well, driving lights. :-)
   If you wish further details, circuits, formulas for current,
wire gauge, etc., just drop a line.  I hope this helps...

rick

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