4 ms·

It's fairly straightforward if the car is unmodified, as everything should be fused. Multi-meter between positive terminal and positive lead and pulling each fu

by rozap 4y ago

It's fairly straightforward if the car is unmodified, as everything should be fused. Multi-meter between positive terminal and positive lead and pulling each fuse to see when the drop goes away will isolate the circuit. Then you've narrowed it down pretty far and can go from there. The horrible thing is if the car is modified, and you discover an un-fused circuit.

sokoloff 4y ago
I’ve had unknown draws before and many times they’re not constant draws. A constant draw is easy and quick to find. A draw that only happens when some module or other wakes up is a lot harder to find.
- flibbityflob 4y ago
  As an example, my 2017 Corolla runs the fuel pump for a while 5 hours after shutdown to test fuel system pressurization. I didn’t even know until I recently started parking it inside and I finally heard it running.
  - Arrath 4y ago
    I had to sleep in a work truck once and every hour or so it would modulate the shutters in the air vents to refresh the air in the cab or something. The click and buzz whir kept waking me up, it was quite annoying.
  - rootusrootus 4y ago
    Yeah that's pretty normal across different manufacturers, and you only hear it if you park in a quiet garage and happen to be in the garage at the moment it turns on to do it's thing. It's testing the seal of the fuel cap to make sure you aren't just venting gasoline fumes.
    - Scoundreller 4y ago
      And usually it only runs in a certain temperature range. Winter climate cars may get several months between a test.
  - officeplant 4y ago
    I learned about this with my 2013 Scion iQ. I was living with my brother who didn't allow weed smoking in or near the house so I would go hotbox my car. Weirded me out the first time I'm sitting there in silence smoking a bowl and suddenly fuel pump noises. Which were easier than normal to hear because I took the back seats (the only sound deadening in the back) out of that car so I'd have room for a full size spare tire instead. With my current car (2020 Kia Sportage) I've noticed the head unit is waking up before I even start the car. As soon as you unlock the car I can see my flash drive light up with activity as the head unit gets ready to pickup music from where it left off.
- gdavisson 4y ago
  I recently watched a YouTube video from someone who'd tracked a nasty intermittent parasitic draw [0]. The problem only happened after turning the ignition on & back off (so the standard test of disconnecting the battery & reconnecting via the ammeter wouldn't show it), and the draw cycled between 3.6A and 0.3A. Also, after turning the ignition on & back off there were a lot of (normal) transient draws (from things like the dome light that don't turn off immediately). Even with the problem circuit disconnected, it drew around 6A (!) immediately after the ignition was switched off, dropped to 0.4A after about a minute, and sat at that level for another 9 minutes before dropping again to 0.06A. That means if he hadn't waited ~10 minutes per test, he'd have been chasing draws that were actually normal. Combine an intermittent fault with intermittent normal behavior, and you've got a troubleshooting nightmare. [0] https://www.youtube.com/watch?v=rVScppKsfHs https://www.youtube.com/watch?v=rVScppKsfHs
- falcolas 4y ago
  Yes. I had a Dodge Charger where the parasitic draw wouldn't really become problematic until 3 days after you re-attached the battery. At which point it would be so bad it would discharge from full to empty overnight. It could even out-draw a trickle charger. I took it to several mechanics who couldn't find anything - because they'd unplug the battery to add a meter into the circuit. They weren't willing to wait 3 days afterwards to come back to it. I ended up disconnecting the battery overnight. Worked fine then. Traded it in; I feel a bit bad for the next buyer.
everybodyknows 4y ago
> Ammeter across the battery terminals and pulling each fuse to see when the drop goes away How does this compare with the way presented in the article based on measuring resistive voltage drop across each fuse in turn?
- rzzzt 4y ago
  Ye olde ammeters were secretly voltmeters with a low-value but high-precision shunt resistor in series with the circuit to be measured. Current passing through the resistor causes a tiny voltage drop, which is measured by the meter. As the voltage is proportional to the current, the scale painted on the cardboard behind the needle did the actual conversion. In the author's setup the fuse takes the shunt's place; its resistance is apparently a known value that can be gathered from a datasheet.
  - deleted 4y ago
    [deleted]
  - ilyt 4y ago
    > Ye olde ammeters were secretly voltmeters with a low-value but high-precision shunt resistor in series with the circuit to be measured. "ye olde"? That's how every typical one works, till you go to clamp meter > its resistance is apparently a known value that can be gathered from a datasheet. that's... optimistic
    - Nextgrid 4y ago
      > that's... optimistic The absolute current value probably doesn't matter - in fact I very much doubt a hobbyist-level multimeter is even capable of accurately measuring the (fractions of?) millivolts across a fuse. It's more important as a boolean signal - "is there current being drawn on this fuse?", and then usually even those fractions of millivolts will generally be enough to make your el-cheapo multimeter register 1mV and tell you something is drawing current, where as a fuse with no voltage drop across it at all will reliably show 0mV (just like it would if the probes were shorted) on even a cheap meter.
mlyle 4y ago
> Ammeter across the battery terminals I know what you're trying to say... but this reads like very dangerous advice and you may want to revise. Ammeter between a battery's terminal and the car's electrical system. Also note that you'll probably create sparking when doing this, and if the battery has recently been charging this can be dangerous (hydrogen outgassing).
- johnwalkr 4y ago
  I think the mistake may have actually been writing ammeter instead of voltmeter. Ammeter is not safe as general advice as a typical one won’t be anywhere near large enough capacity for working with a car battery and should come with a lot of disclaimers about how to hook it up. Measuring voltage drop would work but only for fairly high loads.
  - mlyle 4y ago
    > Measuring voltage drop would work but only for fairly high loads. Yah, that doesn't really make sense for small parasitic loads. > Ammeter is not safe as general advice as a typical one won’t be anywhere near large enough capacity for working with a car battery It should be fine with the car not "on", but the general level of danger of doing anything near a car battery requires caution.
    - duskwuff 4y ago
      > Yah, that doesn't really make sense for small parasitic loads. If your multimeter has a sub-mV range -- which a decent meter should -- you can measure a voltage drop across the battery ground lead. I've successfully done that to diagnose a battery draw in my car.
      - peteradio 4y ago
        Exactly, you are measuring a change, so its the precision of the meter not the absolute V. Maybe easier to measure on a nearly dead battery?
voldacar 4y ago
Ammeter in series, not across the terminals! Unless you are trying to weld something :]
- AlexandrB 4y ago
  Realistically, you'll just pop the fuse in the ammeter. If it's a nice ammeter the fuse might be pretty expensive.

by rozap 4y ago

sokoloff 4y ago

I’ve had unknown draws before and many times they’re not constant draws. A constant draw is easy and quick to find. A draw that only happens when some module or other wakes up is a lot harder to find.

flibbityflob 4y ago
As an example, my 2017 Corolla runs the fuel pump for a while 5 hours after shutdown to test fuel system pressurization. I didn’t even know until I recently started parking it inside and I finally heard it running.
- Arrath 4y ago
  I had to sleep in a work truck once and every hour or so it would modulate the shutters in the air vents to refresh the air in the cab or something. The click and buzz whir kept waking me up, it was quite annoying.
- rootusrootus 4y ago
  Yeah that's pretty normal across different manufacturers, and you only hear it if you park in a quiet garage and happen to be in the garage at the moment it turns on to do it's thing. It's testing the seal of the fuel cap to make sure you aren't just venting gasoline fumes.
  - Scoundreller 4y ago
    And usually it only runs in a certain temperature range. Winter climate cars may get several months between a test.
- officeplant 4y ago
  I learned about this with my 2013 Scion iQ. I was living with my brother who didn't allow weed smoking in or near the house so I would go hotbox my car. Weirded me out the first time I'm sitting there in silence smoking a bowl and suddenly fuel pump noises. Which were easier than normal to hear because I took the back seats (the only sound deadening in the back) out of that car so I'd have room for a full size spare tire instead. With my current car (2020 Kia Sportage) I've noticed the head unit is waking up before I even start the car. As soon as you unlock the car I can see my flash drive light up with activity as the head unit gets ready to pickup music from where it left off.
gdavisson 4y ago
I recently watched a YouTube video from someone who'd tracked a nasty intermittent parasitic draw [0]. The problem only happened after turning the ignition on & back off (so the standard test of disconnecting the battery & reconnecting via the ammeter wouldn't show it), and the draw cycled between 3.6A and 0.3A. Also, after turning the ignition on & back off there were a lot of (normal) transient draws (from things like the dome light that don't turn off immediately). Even with the problem circuit disconnected, it drew around 6A (!) immediately after the ignition was switched off, dropped to 0.4A after about a minute, and sat at that level for another 9 minutes before dropping again to 0.06A. That means if he hadn't waited ~10 minutes per test, he'd have been chasing draws that were actually normal. Combine an intermittent fault with intermittent normal behavior, and you've got a troubleshooting nightmare. [0] https://www.youtube.com/watch?v=rVScppKsfHs https://www.youtube.com/watch?v=rVScppKsfHs
falcolas 4y ago
Yes. I had a Dodge Charger where the parasitic draw wouldn't really become problematic until 3 days after you re-attached the battery. At which point it would be so bad it would discharge from full to empty overnight. It could even out-draw a trickle charger. I took it to several mechanics who couldn't find anything - because they'd unplug the battery to add a meter into the circuit. They weren't willing to wait 3 days afterwards to come back to it. I ended up disconnecting the battery overnight. Worked fine then. Traded it in; I feel a bit bad for the next buyer.

everybodyknows 4y ago

> Ammeter across the battery terminals and pulling each fuse to see when the drop goes away How does this compare with the way presented in the article based on measuring resistive voltage drop across each fuse in turn?

rzzzt 4y ago
Ye olde ammeters were secretly voltmeters with a low-value but high-precision shunt resistor in series with the circuit to be measured. Current passing through the resistor causes a tiny voltage drop, which is measured by the meter. As the voltage is proportional to the current, the scale painted on the cardboard behind the needle did the actual conversion. In the author's setup the fuse takes the shunt's place; its resistance is apparently a known value that can be gathered from a datasheet.
- deleted 4y ago
  [deleted]
- ilyt 4y ago
  > Ye olde ammeters were secretly voltmeters with a low-value but high-precision shunt resistor in series with the circuit to be measured. "ye olde"? That's how every typical one works, till you go to clamp meter > its resistance is apparently a known value that can be gathered from a datasheet. that's... optimistic
  - Nextgrid 4y ago
    > that's... optimistic The absolute current value probably doesn't matter - in fact I very much doubt a hobbyist-level multimeter is even capable of accurately measuring the (fractions of?) millivolts across a fuse. It's more important as a boolean signal - "is there current being drawn on this fuse?", and then usually even those fractions of millivolts will generally be enough to make your el-cheapo multimeter register 1mV and tell you something is drawing current, where as a fuse with no voltage drop across it at all will reliably show 0mV (just like it would if the probes were shorted) on even a cheap meter.

mlyle 4y ago

> Ammeter across the battery terminals I know what you're trying to say... but this reads like very dangerous advice and you may want to revise. Ammeter between a battery's terminal and the car's electrical system. Also note that you'll probably create sparking when doing this, and if the battery has recently been charging this can be dangerous (hydrogen outgassing).

johnwalkr 4y ago
I think the mistake may have actually been writing ammeter instead of voltmeter. Ammeter is not safe as general advice as a typical one won’t be anywhere near large enough capacity for working with a car battery and should come with a lot of disclaimers about how to hook it up. Measuring voltage drop would work but only for fairly high loads.
- mlyle 4y ago
  > Measuring voltage drop would work but only for fairly high loads. Yah, that doesn't really make sense for small parasitic loads. > Ammeter is not safe as general advice as a typical one won’t be anywhere near large enough capacity for working with a car battery It should be fine with the car not "on", but the general level of danger of doing anything near a car battery requires caution.
  - duskwuff 4y ago
    > Yah, that doesn't really make sense for small parasitic loads. If your multimeter has a sub-mV range -- which a decent meter should -- you can measure a voltage drop across the battery ground lead. I've successfully done that to diagnose a battery draw in my car.
    - peteradio 4y ago
      Exactly, you are measuring a change, so its the precision of the meter not the absolute V. Maybe easier to measure on a nearly dead battery?

voldacar 4y ago

Ammeter in series, not across the terminals! Unless you are trying to weld something :]

AlexandrB 4y ago
Realistically, you'll just pop the fuse in the ammeter. If it's a nice ammeter the fuse might be pretty expensive.