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RE: Lighting

To: "'peter harrison'" <>,
Subject: RE: Lighting
From: "Graeme" <>
Date: Mon, 19 Jun 2006 18:29:23 +0930
"My mechanic for my new roadster informs me that the headlights are driven
directly from the  alternator. Doesn't make sense to me since they can be
lit when the car is not running.
I can't glean the veracity oif this from the wiring diagram Is this true??"

Hi Peter,
The headlamps, like any other major electrical load, should be operated by a
The power for the relay is switched via a low current switch i.e. dash or
column switch.
The relay then operates and connects 12V via a high current contact to the
load, in this case the two headlamps.
This high current source is usually from the battery via a fuse, fuseable
link or circuit breaker, whose function it is to protect any wiring from
carrying excessive current.
The alternator is connected to the battery via a current direction sensing
indicator i.e. the amp meter. 
If you operate the headlamp switch with the engine off, the amp meter will
show a current draw but on the "DISCHARGE" or "-" side of the gauge.
If the engine is then started, the amp meter should swing thru "0" and show
a small deflection on the "CHARGE" or "+" side of the gauge.

The "CHARGE" deflection will not equal "DISCHARGE" deflection for a few
The most important reason is that with the engine off the electrical system
is running on the battery which will only be a maximum of 12V, more than
likely, somewhat less than 12V.
When the alternator/regulator is functioning correctly, the electrical
system should be running at 13.8V or thereabouts. 
Thus, at the higher voltage there will be less current draw shown on the amp
meter for a given load. 
This "float" voltage of 13.8V is a designed value to maintain a trickle
charging current to the battery and power the electrical load as it varies
during driving.

If your amp meter, alternator and regulator are in top condition, you can
see how quick the charge circuit reacts to a varying electrical load.

With the engine running, turn the headlamps on and off a couple of times.
The amp meter should give a flick towards the "0" and quickly return to the
normal "+" reading when the headlamps are on.
The quicker it flicks the more efficient your charging system is.
If it dips to the "0" mark but does not recover, it may indicate that the
alternator can not adjust to the extra load. 
Usually the problem can be as simple as worn out brushes in the alternator.
The spring tension on the brushes is insufficient to pass the increased
current required by the rotor thus the generated output is less, thus the
gauge will not recover.
Graeme Suckling 
1965 SP310 
1971 P510 
1972 PL510 
1973 HS30
1993 ECR33 Skyline GTS25t
South Australia.

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