Voltage Drop Question

 

2006 Honda Shadow 600

I noticed that when I apply front brakes or rear brakes, my headlight and
front marker lights dim slightly.  The only way I noticed it is that when I
pull into my carport at night (which is actually uphill), the headlight is
hitting a target just a few feet ahead and I use the brakes to stabilize the
bike while I turn off the engine and then engage the clutch again (so it
doesn't roll backwards).

The power loss is very slight, probably about 5%, and you wouldn't notice it
unless you were looking for it.

I used to work designing auto electrical systems, and I know that one sizes
copper to get maybe a 5% power loss -- using larger wires just costs money.

Based on what I know about Ohm's law and vehicle wire size economics, I'm
going to guess that the behavior is completely normal and I don't need to go
fishing any further.

Are there any dissenting opinions?  Does anyone think I need to go fishing?

Thanks.
--
David T. Ashley              (dta@e3ft.com)
http://www.e3ft.com           (Consulting Home Page)
http://www.dtashley.com       (Personal Home Page)
http://gpl.e3ft.com           (GPL Publications and Projects)

 


Sounds pretty normal. I have an onboard LED voltage indicator
and can look at it and watch the effect of a turn signal blinking
on and off. The most interesting place to look for voltage drops
is in the charging system. Any large drop on the main + or - path
such as a crappy ground or cable connect would definitely be
worth tracking down. Otherwise, as long as the battery voltage
remains good, I wouldn't worry much.

 



Mine is/was much worse. I was getting only 8.7 vdc at the headlight with the
engine at high idle (4K) on my old KZ900.  I found the biggest share of the
resistance in the headlight switch.  It appears the Japanese like to run the
entire machine through the ignition switch.  As they get older, I understand it
is normal for some corrosion to begin in each and every electrical joint or
contact point.  After using the wiring diagram to track through every
connection, I was still unable to clear the resistance under load.  I ended up
installing a relay system for the headlight with headlight power being directly
drawn from the battery through a fuse.  I used the original switch circuit to
power and control the relays.  For the first time in 30 years, this thing has a
decent headlight.  Time involved = about 30 minutes, Relays, connectors and
wiring <= $15.  The big difference between mine and later models is mine has the
headlight controlled by the rider instead of being controlled by the ignition
switch.  I think yours may already have a relay that turns the light off when
the starter is engaged.  You can still go downstream of that to install a relay
setup to power the headlamp.  I suspect they are still using the ignition
circuit to provide power instead of a separate power feed.  It makes a hell of a
difference.  I still get some slith dimming at idle but, the headlamp is no
longer affected by the brake or turn signals.

Lugnut

 


That is quite a horror story.  Two things surprise me about it.  #1)Current
tends to bake off impurities on the contacts, and #2)That represents a lot
of power loss at the switch ... like "meltdown" power loss.  1 volt at 10
amps is 10 watts across the switch ... that should be close to flame city.


You must be quick.  It would take me longer to install a relay.  Cramped
space, splices and all that.


For a $5.5K bike, one doesn't get sophistication like relays.  I happen to
know this because I just replaced the starter switch on the right handgrip.
The headlamp circuit goes ALL THE WAY UP THE HANDGRIP WIRING AND THROUGH THE
STARTER SWITCH.  No relay in there.


I've got plenty of brightness.  It was just the small drop that caught my
eye.


You've got a lot of energy to correct an aging electrical system like you
did.  I'd just buy a new $5.5K bike.

Dave.

 

David T. Ashley wrote:

Maybe I'm not educated enough to understand why this is surprising.

If you have 3 lights on a circuit and then flip a switch that adds 3
more, wouldn't you expect the original 3 to be less bright?  I would
think most of the resistance would be the filaments in the additional 3
lights rather than the wiring of the bike.

Also, the effect may be less when your engine is above some rpm.  The
headlights on my Bandit are quite dim at idle, and perk up as I blip the
throttle.