exceptionally retarded headlight question

[Viper21700]

Ok gents, heres the background. Been building a replacement headlight wiring harness for relay use the last couple weekends Ive had off. Got it installed today, for the most part (jeez what a pain that was...) still havent hook the harness into my relays yet.
Heres my retarded question. After installing my harness, I decided to ohm out the harness with the headlights installed to test for shorts that might have popped up. Im getting continuity between the high and low beam leads. so I dug into the harness and pulled the headlights out. with the headlights removed, I checked the full harness with the ohm meter, no shorts. So I scratched my head for a few minutes, then tested the headlights themselves when removed from the harness. Im getting continuity between all three connections on the back of both lights??? how in the name of all that is holy is that? What keeps the low beam from turning on when the highs are on and vice versa? WTF am I missing?

 

[67Dart273]

You are saying.............

That with headlights plugged in........you get continuity beteween the hi /lo beam leads?

NORMAL, LOL


Below is a partial diagram I shamelessly pirated, then hacked up, from the internet.

Your meter is going through BOTH high and low beam filaments. I traced the path through the low-beam lamp.

Actually, in a 4 lamp system as in this example, and with BOTH lamps one either side or even both sides connected, some of the current from the ohmeter also passes from the meter, to the top green wire, through the high beam lamp, down to the common ground, up into the low beam ground connection, and through the low beam filament in that lamp, back down and back to the meter on the bottom wire

 

[Viper21700]

Ok, just had me worried. Spent three weekends in a row over-engineering this harness (namely 10g wire, every connection crimped with non-insulated crimps, soldered, then double wrapped in heat shrink tubing.... I hate wimpy electrical systems roflmao... plus each headlight has its own ground using gold plated audio high amperage ring terminals soldered and attached to a body mount.... like I said over-engineering at its finest) heck my entire underhood electrical system is getting revamped....

This just made me question my sanity... If Im getting continuity between the two lines now, how do the high/low beams work seperatly then?

 

[BillGrissom]

Simple. There are 3 terminals. Hi and lo filaments share a common ground. When you measure from hi to lo terminals, the test current flows thru the hi filament (red lead) to gnd, then from gnd thru the lo terminal (blk lead). When measuring ohms, you must always isolate the element being measured, or at least consider alternate paths.

10 ga wire will support a 500 W police spotlight. There is conservative and then there is way over-kill. I recall the factory used 16 awg (or was it 18 awg). Anyway, some guys think they need 0 awg welding cable for their alternator output.

 

[Viper21700]

nm, just looked at your diagram then figured it out... had to remember that electrical flows from ground to positive, not pos to ground... when the high beams are on via the relay, the potential is there for the lows, but is stopped by the lack of termination at the low beam relay

 

[67Dart273]

nm, just looked at your diagram then figured it out... had to remember that electrical flows from ground to positive, not pos to ground... when the high beams are on via the relay, the potential is there for the lows, but is stopped by the lack of termination at the low beam relay

The direction it flows has really nothing to do with it. Follow the yellow dots. They don't actually go "to ground" they just just use the ground connection between the filaments as an interconnect

I was taught electron flow, "neg to positive" but most of the time you could go either way. The circuit is called that for a reason..........it is CIRCULAR, you must be able to trace back to where you started from.

 

[Viper21700]

now this leads into my next dumb question what is the difference between the high and lows? Is it the position of the element in relation to the reflector?

 

[67Dart273]

now this leads into my next dumb question what is the difference between the high and lows? Is it the position of the element in relation to the reflector?

Depends on the lamp, but for sure filament position, and in many cases wattage of the filament. This gets into theory that I'm not good at. An example might be satellite dishes

 

[KitCarlson]

The filaments differ, the high beam draws more watts, more light.

Similar to tail lights with 2 filament bulbs, they have bright brake and normal tail light.

A good Ohm meter can measure the resistance, however it changes at the bulb heats.

 

[67Dart273]

The filaments differ, the high beam draws more watts, more light.

Similar to tail lights with 2 filament bulbs, they have bright brake and normal tail light.

A good Ohm meter can measure the resistance, however it changes at the bulb heats.

Not always. Some of the old PAR56, that's the old large round "2 lamp" system lamps only differ hi / low by 10 watts. Some of the rectangular lamps are exactly the same wattage.

The primary difference is filament location. That is what changes the beam pattern