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Thread: lights/power/voltage drops

  1. #1
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    lights/power/voltage drops

    Ughh

    I'm getting pronounced dimming of all lights (headlight, dash, you name it) when anything else draws power-brakes, signals, fan. The van actually shut off the other night when I turned the blower on, so I can't ignore this any longer. (That's a lie, I certainly can, and might.)

    The headlights stay dimmer with foot on the brake when the van is off, but return to normal in a second when its running. I have LED headlights (and that's the only place I have LED, to my knowledge) so the first thing I did was remove them and replace with the halogens. No change, so I put the LED back in. I checked it with the van off and key on, no change, so its apparently not the alternator (right?). I was due for a new battery anyway so I bought one, no change. I've cleaned and replaced the battery terminals and cleaned the ground on the alternator and the one inside the right front fender, no change. I ran a new battery cable from the battery to ground, no change.

    I know there are a dozen or two more scattered ground locations, but this is a rust free van so they should be good. I think this must be wiring somewhere. But I have little patience for endlessly chasing ghost after ghost. If it wasn't threatening to turn off the engine or cause a police pullover I wouldn't waste another minute on it.

    Being a recalcitrant and unapologetic electric incompetent, I would need some ideas on what to check next, and how to check it. I have not been able to find another Previa that had this problem. It seems to be a problem with some later 4runners, but no solutions were found. They have accepted it as "normal" supposedly caused by brake booster.

    It may have gotten worse since I replaced my blower, but I drive the van so little except when traveling I'm not sure. It is wired the exact same wonky way its always been wired.

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    Re: lights/power/voltage drops

    I would think the next suspect is the alternator itself. Go somewhere (Autozone, etc.) where they can hook up a charging circuit analyzer. The battery isn't particularly good at dealing with any sustained current draw by itself - so if you had the ignition on but the car wasn't running, and saw the headlights dim when you pressed the brakes or turned on the fan, that's par for the course.
    "If you find yourself holding a sledgehammer or a crowbar, it's time for a break."

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    Re: lights/power/voltage drops

    I did have the (old) battery and alternator tested at 2 different parts stores, though I can't say exactly what tests their device did. The battery showed a loss of cranking power but it was charging properly. I don't know if this covered the specific testing you are thinking of but I would guess so?

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    Re: lights/power/voltage drops

    No idea then. I don't what they test at the parts stores - if it's just voltage at the battery, that's not great. The gold standard is some kind of load test where a drain is deliberately placed on your system to see if it can provide the spec'd amps.
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    Re: lights/power/voltage drops

    I think they just check the voltage output, but we did put it under load with all electricals turned on

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    Re: lights/power/voltage drops

    Added resistance from anything along the path from battery + through loads then to chassis ground is a problem. Anything that has a higher resistance than normal can assume a larger share of the total voltage than normal, leaving a lower-than-normal voltage on the thing you want powered.

    Let's say your lights are dim when just powered from the battery at 12V because other parts of the current path have a high resistance and have too much of the total voltage across them . Turn the van on, and the alternator increases the battery "+" to around 14V (in order to charge it). That additional voltage divided across that current path now give the lights enough voltage "share" to brighten up. Turning on the fan, lighting the brakes, etc. might pull the battery "+" voltage down somewhat, lowering the lights' voltage share and dimming them again.

    Maybe also turn your attention to the wiring out of the battery + terminal into "fusible links" and fuses. There could be corrosion/broken/fraying wires & connectors there, upping the resistance and reducing the voltage and current available to most everything. I've seen discussions here about the Vans but can't recall anything specific about the Previa. It's likely to be a PITA to deal with - I haven't had the pleasure yet. It might explain your engine conk-out as well.

    Measurements of the charging system aren't going to address that "downstream" electricity flow to devices past the battery unless they also measured current draw rather than just voltage at the battery.
    "If you find yourself holding a sledgehammer or a crowbar, it's time for a break."

  7. #7
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    Re: lights/power/voltage drops

    The fusible link connections all look clean, but a couple fusible link "filaments" (if that's what they are called) including the MAIN are severely corroded. Not sure if that could cause the problem but they definitely need to be replaced.

  8. #8
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    Re: lights/power/voltage drops

    Nope. No change with different links. I also disconnected the radio and blower to see if they were involved, nope and nope.

  9. #9
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    Re: lights/power/voltage drops

    Something easy to try - the power for the headlights runs from a fusible link, through a Head Light Relay, through (right & left headlight) fuses, to the headlights, then to ground through the combo light switch on the left stalk. That relay could potentially have developed a high resistance, thus "robbing" the headlights of some voltage. That relay - the third across the top row under the dash cover - is the same as the heater relay to it's left. Swap 'em and see if that indicates the headlight relay is going bad.

    Not sure why that would affect the instrument panel lights if that's the problem, but it's a quick experiment. Might as well jiggle that stalk switch a bit if the headlights still dim on the off-chance that's the culprit.
    "If you find yourself holding a sledgehammer or a crowbar, it's time for a break."

  10. #10
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    Re: lights/power/voltage drops

    Will check those out, just saw this curious comment on a Craigslist Previa "dent in the tailgate was causing the battery to drain, so we put a cut-off switch on the battery, and that works well."

    00M0M_gqXn0AR0bjm_0CI0t2_600x450.jpg

  11. #11
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    Re: lights/power/voltage drops

    Drat. Tried the fusible link switch, no go. I found some sketchy connections on one headlight wire, but rewired those and no change. Well maybe it helped reduce resistance a little, only time and a night drive will tell. It still dims, I know that.

    I'm down to about my last straw. I'm going to open up the tailgate and inspect all of my wires, sockets and bulbs in there. Not because of the dent guy above, but because I remember having a weird electrical issue once where something in the cab wasn't working and Fred on the Yahoo group steered me to checking all of the lights and sure enough a faulty taillight socket or bulb was causing it. I see little chance of that being the case here, but maybe I'll get lucky and find something.

  12. #12
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    Re: lights/power/voltage drops

    Wayne, I know you said ground points must be good because it's a no-rust van, but there are only 7 ground points (A..G) in three groups: Engine (ABC), Interior (DE), Back (FG). Maybe we could look for functional tests to exclude them as possible failure points.

    First test would be the headlights, because it's where you first noticed the problem. The left and right headlights have different ground points. Can you shine the headlights against a wall (or have someone look at them) while you step on the brake to operate the brake light switch? It would be interesting to know if they dim the same amount, or differently.

  13. #13
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    Re: lights/power/voltage drops

    By the eyeball test I would have to say the headlights dim equally when applying the brakes.

  14. #14
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    Re: lights/power/voltage drops

    If it was me, I'd want to make absolutely sure the alternator is good, especially since your original post said pretty much any significant load on the electrical system was causing the lights to dim, not just the brakes. Internally the spinning part of the alternator creates a varying voltage that's then altered into a fairly steady voltage by some circuitry. IF there's something wrong with that circuitry, the voltage read by a meter on "DC" volts (more or less an average of what's coming out of the alternator) might read OK, but wouldn't reveal that the alternator's output was actually varying too much. That would be seen by setting the meter to "AC" volts and seeing if the variation in voltage was over around 0.25-0.5 volts. Current is another practical test as the alternator is supposed to adjust its current output depending on the load (the manual specifically calls out measuring current draw out of the alternator of around 30A with the heater fan on high and high beams (obviously less with LEDs)).

    I would imagine a dedicated battery/alternator tester would detect any of this, but I've never seen one in use outside of Youtube.

    Unless the lights didn't dim when they were testing the charging system and you turned on a bunch of stuff?
    "If you find yourself holding a sledgehammer or a crowbar, it's time for a break."

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    Re: lights/power/voltage drops

    Quote Originally Posted by Previologist View Post
    By the eyeball test I would have to say the headlights dim equally when applying the brakes.
    Hmm... looking again at Dan's comments prompted another thought: If you remove the STOP fuse (2nd row, 4th from left, 20A), does pressing the brake pedal still dim the headlights?

  16. #16
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    Re: lights/power/voltage drops

    No. It does not dim with that fuse removed.

  17. #17
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    Re: lights/power/voltage drops

    OK, with the brake circuit still disconnected from power, does anything else that draws power dim the lights, like the fan on high (turn up the one in the middle AC part too) + interior lights + radio? With the van running at least a bit above idle, so the alternator's on and running at a decent rate. You had mentioned in the beginning it wasn't just brakes that caused the dimming.
    "If you find yourself holding a sledgehammer or a crowbar, it's time for a break."

  18. #18
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    Re: lights/power/voltage drops

    Also, since you're kind of being drawn into the mysteries of electrical theory despite your reticence, consider one of these:
    "Southwire 16030A Autoselecting CAT IV Multimeter"
    I bought one for most of my family members as they're supposedly very easy to use with fewer manual settings required. (Sure, they're still in the blister packs years later except for one used when I was visiting to help fix a lamp, but they don't deliberately monkey with the Dark Arts as you do). Plus, you get to be seen looking at the display, slapping your forehead and exclaiming "1.21gigawatts ?!?"
    "If you find yourself holding a sledgehammer or a crowbar, it's time for a break."

  19. #19
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    Re: lights/power/voltage drops

    Quote Originally Posted by man_btc View Post
    OK, with the brake circuit still disconnected from power, does anything else that draws power dim the lights, like the fan on high (turn up the one in the middle AC part too) + interior lights + radio? With the van running at least a bit above idle, so the alternator's on and running at a decent rate. You had mentioned in the beginning it wasn't just brakes that caused the dimming.
    Yes, those still cause dimming with the STOP fuse pulled, whether off or with engine running above idle.

    I want to clarify my comment about other things causing dimming. The brakes always had the most dimming effect on the headlights, followed (or perhaps equaled) by the front blower. The mid-blower has a less pronounced dimming effect.

    The other effects are less, like when I turn on the blinkers the headlights will sort of pulse brighter and dimmer in time, but its not as pronounced as the dimming effect from braking or the front blower.

    Also other things will dim certain dash lights, but its hard to compare the magnitude of that to the headlights. I just noticed that it can happen. If I have the ignition on, the turn signals will pulse the dash dummy lights ever so slightly, for example. And that still happens

    I have a couple of cheap multimeters, one digital one analog.

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    Re: lights/power/voltage drops

    Well that all makes sense: larger loads (like stoplights and blower high) causing larger voltage drops.

    I have a couple of cheap multimeters, one digital one analog.
    Good - let's use the analog meter. (I hope it's not auto-ranging.) Do you see the connection in the battery fuse block between the 100A ALT link and 50A AM1 link? Can you get the common (low) side of the meter on that point, and the high side of the meter on the battery + (the terminal that the fuse block is mounted on)? What is your meter's lowest DC volts scale?

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