Dual filament headlight (strange voltages)

[Robby Villabona]

Hi,

I was just measuring voltage drops on electrical components on my ’97 Civic AT and noticed that when my headlights are on and in low beam, I’m measuring about 13.5V (anywhere from 0.3 to 0.5 V lower than voltage at the battery terminal) on the corresponding fuses in the under-dash fuse box.

The strange thing is, I’m seeing about 0.3V (relative to ground) as well on the high beam fuses. When I switch to high beam I measure something like 0.2V on the low beam fuses. When the headlights are off they’re both 0.0V.

I’m also seeing the same thing on the corresponding wires on the combination headlight switch — 0.3V on the high beam wire if low beam is on and around 0.2V on the low beam wire when the high beam is on.

Is this normal? Or is this a sign that one (or two) of my headlights aren’t well-grounded and current is passing through the filament that is “off” looking for a path to ground?

What started me on this investigation is my alternator is having a hard time keeping my battery fully-charged with headlights on in really bad stop and go traffic, especially when I’m stepping on the brakes.

I also tried investigating my brake light switch to check for voltage drops. And so like any normal brake switch, the “output” is 0.0V when the brake pedal isn’t pressed. However when I turn on the headlights (which also turns on the tail lights) I again measure ~0.2V on the brake switch output. I figure it’s connected because the tail lights and brake lights (except the high mount brake lights) are also dual filament bulbs serving both purposes. Is this normal?

Topic

[zero]

Measure your voltage running with loads on and see if the alternator can keep up.

[Robby Villabona]

@DaFirnz,

Yes, I’ve done those tests (among other things) and the alternator keeps up. With all accessories on I get about 13.5-13.8V charging voltage at the battery post. I’ve also done voltage drop tests on the charging system and it’s running fine with negligible voltage drops.

It’s when I’m driving at night in really bad stop-and-go traffic that I begin to run into issues. At some point in the drive when the engine’s sufficiently warmed up I will see my charging voltage drop momentarily down to 12.5V and my idle down to 500-600 rpm before it goes back up to about 13.5 when (presumably) the ECU raises idle. It’s not so bad that my battery discharges — it’s just bad enough that after driving my battery is just around 75% charged (between 12.4-12.5V). And so driving at night with AC on in the rain makes it even worse. When I drive in the daytime with the AC in full blast I always finish the drive with the batter fully-charged (12.6V).

What I haven’t done (due to lack of proper tools) is to test the amperage output of my alternator to see if it’s maxing out. However before I go about replacing it with a bigger one I’d like to make sure that the electrical components of my car are good.

And so that led me to that observation of seeing some small voltage on the circuit that’s supposed to have zero volts when not engaged (e.g. high beam circuit having 0.3V when low beam is on, but is zero when both lights are off). I was wondering if people here see the same thing and if that’s normal. From what I know about electronics, it shouldn’t be (right?). I see it either a sign that the headlight switch is “leaking” voltage or that at least one bulb in the park/head/brake lights circuit has a bad ground and is making the lighting circuit draw too much current. Or maybe there’s an explanation for what I’m seeing to be normal — just don’t know what that is.

[twiggy]

Because you mentioned that it’s worse in the rain, have you checked the alternator belt to make sure that isn’t slipping?

[Robby Villabona]

I mentioned it’s worse in the rain because of the added load of the wipers. But belt tension’s ok.

Actually it’s really just the H4 (dual-filament) light bulb voltage behavior I’m curious about — if what I’m seeing is normal. It’s a question that’s separate from if my alternator is good or the right size.

I’m beginning to suspect that maybe it’s just normal. The bulbs aren’t dim so it’s probably not a bad ground. The “normal” explanation I can think of is that since the filament is wound as a coil, the current in the one that’s ON is probably creating a magnetic field that’s inducing current on the nearby coil that’s OFF.

[twiggy]

If both filaments share the same ground somewhere, when one filament has current flowing through it, the ground voltage seen at the bulb will show a small voltage due to ohm law (V=IR) and the small wiring resistance, the ground on the unlit bulb will show the same 200-300mV you are reading (because they probably share that ground). You would probably measure 200-300mV on both terminals of the unlit bulb because the filament has little resistance and whatever switch or relay controlling the bulb is in a open state.

[Robby Villabona]

Yes, you can actually see those filaments within the bulb housing sharing the same ground. 200-300mV seems quite high for voltage drop at ground, right?

Until I can find a friend with a car with dual filament light bulbs to verify (it so happens I can’t find one), I’m tending to think this is most likely magnetic induction at work rather than resistance at ground. The reason being I’m seeing it across bulbs that are on three separate grounds — it just seems weird they’d all have ground issues of similar voltage drops, not to mention their brightness appear normal.

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