I'm reading a voltage drop in my trailer wiring and wondering if it's
Here's what I've found:
Flat four plug from vehicle reads 12.5V from ground pin to tail light pin
without the trailer plugged in.
Flat four plug from vehicle drops to 10.5V from ground pin to tail light
pin when the trailer is plugged in.
Light fixtures on trailer read 9.5V from when testing both "socket ground
to socket hot" and "ground-screw-into-frame at rear of trailer to socket
Resistance between trailer's flat four ground wire attached to trailer
tongue and the ground at the sockets and the ground-screw-into-frame at
rear of trailer reads about 2.5 ohms.
The side marker lights have no ground wire, using the self tapping screws
that hold the markers to the trailer shell as the ground. The rear lights
use a ground wire, with the ground provided by a pigtail to a self tapping
screw into the trailer frame.
What would cause a 2 volt drop right at the plug when the trailer is
plugged in and then another 1 volt drop at the fixtures?
I would start by giving the plug a good cleaning. Without a load except for
the meter, there will not be any noticable voltage drop at all. In a 12
volt system, it only takes a small ammount of resistance to have a large
If the lights only pull 1 amp and you have only 1 ohm of resistance, you
will loose 1 volt. You mention 2.5 ohms, If you zeroed out all the
resistance of the meter, that is where you are loosing most of the voltage.
Going back to that 1 amp load, (which is probably not as much as you have)
that 2.5 ohms will drop 2.5 volts under the load of the lights.
I would make sure that the wires going from the battery or switch to the
plug are good and tight with no corrosion on them. Then clean both plug and
socket and make sure the ground wires are tight and don't have any corrosion
The wires should be put on with lugs, but if the wire is just wrapped around
the screws, that would be a good place to start.
On Friday, August 16, 2013 4:04:16 PM UTC-4, Ralph Mowery wrote:
That's for sure. IDK how many lights he has, but just 12 watts worth
of lights would be 1 amp, giving him that 2.5 volt drop. On the other
hand, if he has voltage drops without any loads on at all, then
he must have some partial short somewhere.
Ralph provided a pretty good answer, so there is no need to repeat what
he already stated.
I will add, however, that I noticed a sizeable drop on my 5-way to 4-way
adaptor, which I ascribed to a typical voltage drop due to
semiconductor(s) (diode and/or transistor) in the adaptor.
The way I dealt with this was to ditch the incandescent bulbs that I
previously had on the trailer for LED units, which have no problem
lighting up nice and bright, despite the lower voltages (the
incandescent bulbs were really dim, and I didn't feel safe driving with
them in such a condition).
Si junction drop is considered to be .6V-.7V but diodes can easily
drop over 1V at higher currents. Perhaps you're thinking about
Schottky diodes? They're closer to .3V at very low currents and
perhaps .7V at higher currents.
Since I'm running flat four to flat four, there are no electronics involved
in my situation.
The LED route is where I'm headed which is why I was checking out the
wiring, just making sure everything was OK. I'm looking at LED replacement
that will plug into my existing bulb socket.
This 1157 replacement bulb says it will run at 7.5 - 14 V
The marker units say they'll run at 9 - 16 V
However this complete unit, which plugs into the 1157 socket, doesn't list
a voltage range.
I'm guessing the complete unit will be brighter than just the bulb, and I'm
guessing it'll run at the 9.5 V I have at those fixtures, but I'll have to
call the vendor to find out for sure.
I'm assuming that LED's have one brightness. In other words, any voltage
above the minimum produces the maximum brightness, correct?
LED's another benefit is it is fractional second faster coming upto full
brightness. In panic braking situation it can mean a difference for
avoiding the accident. LED is either on or off voltage is applied.
No; you can make an LED barely come on, or you can run it with too much
current and it will get brighter than specified. Too much current and
you burn it out (heat death), in an amount of time that depends on the
excess amount of current. There is a threshold for them to begin to
come on at all though, at which point they are still dim, while an
incandescent has an infinite amount of dimness (down to zero).
There do exist current-limiting circuits for LEDs; I have one in my
headlamp that keeps the LED at a constant state until the battery drops
down below the threshold needed to maintain the illumination level.
As far as taillight assemblies go, my experience is that the unit I got
works great at the 9.5VDC that I got from my original converter (the new
one puts out 11.3VDC). I'm sure it would be brighter if I fed it 12VDC,
but it put out more than enough light at 9.5VDC, and far more than
incandescents at that voltage (which were barely discernable), which is
why I switched.
On Fri, 16 Aug 2013 23:39:29 +0000 (UTC), DerbyDad03
Not technically. The more current through a LED the more light - but
good LED LAMPS use driver circuits that control the current. This
makes your assumption relatively close to true - for quality lamps
(but those are not terribly common on the market - you pay your money
and you take your chances.
Better ones that have a proper driver, yes. Don't use LED replacement
"bulbs" instead use dedicated LED lighting assemblies. The latter are
far superior in every way, are likely available as sealed units (in case
you are talking about a boat trailer where the lights may be submerged
when launching) aren't significantly more expensive, and are fully legal
for road use.
This or something similar should work well
Peterson, Grote, etc. are also good brands to look at - whatever your
local boat supply, truck parts store, etc. has in stock. Any of those
manufacturers should have a selection of surface mount, flush (snap in
gasket) round, flush oval, whatever you need.
I can't use the type of tail lights that you linked to. My tail lights are
built into the rear doors, which is why I provided the link to this
It replaces my current lens and plugs into the 1157 bulb socket.
Unfortunately, it's one of the few LED products on that website that
doesn't list the operating voltage range. I'll be giving them a call soon
to find out.
As a side note, the kit you linked to includes those Quick Splice
connectors. Those belong in the same container as back stab receptacles -
the trash container.
oh yeah, if you're going to splice, use proper solder and heat shrink,
or else proper plug in connectors... I just ASSumed that nobody
actually used those things.
can you post a pic of your current taillights? I bet there is a drop in
LED full fixture replacement available unless they're real odd, which is
really the best way to go... they'll last you another decade at least.
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