Car has electrical short somewhere

Tiny
KIRKMARTIN30
  • MEMBER
  • 1999 TOYOTA CAMRY
  • 90,000 MILES
Car has electrical short somewhere. I have lots of experience tracing electrical problems, but not in cars per se (I am an engineer who worked with factory control systems, and data collection systems). Car sat for appx. 1 month and then when I tried to move it battery was 100% dead. I put volt meter across terminals and got ~0.5V. I then disconnected battery and checked again and got ~4.5V. This told then and there that there is probably a short, as something was bringing both battery cables to same potential. To confirm I checked resistance between cables and got ~2.5 ohms (and if I put the fluke on "ring mode", it rings/shows continuity). To be sure I wan't missing anything, I did same checks on our good car and got readings that I expected (~12.9V accross battery, very high resistance between cables). So I am wondering the following: 1. Why didn't a fuse blow, 2. Is it same to reconnect a battery to see if I can start it (to drive it to a servicer for further diagnosis and repair). My thoughts are 1. Perhaps the short is ahead of all fuses, so none blew (but there may be other more logical explanations), and 2. On one hand the battery was connected until a few days ago. But on the other hand, a dead short means a whole lot of current will be drawn once the battery is there and have strong concerns about starting a fire (or worse.I have seen what batteries can do when the terminals are dead shorted).
Wednesday, January 23rd, 2013 AT 9:38 PM

11 Replies

Tiny
SATURNTECH9
  • MECHANIC
  • 30,869 POSTS
Sounds like you have what's called a parasitic draw draining the battery. Does you meter measure Milli amp and amp draw?Also I want you to have the battery charged and tested before we start looking for a parasitic draw.
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Wednesday, January 23rd, 2013 AT 10:18 PM
Tiny
KIRKMARTIN30
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The battery has been recharged. The voltage was so low that it wouldn't charge with an ordinary charger. I needed to take it to a place where they had a "pulse charger" and it took it over 3 days to come back. But now it is at 12.8V (according to them). I will pick it up on the way home from work. As for my Fluke, I think it has a mA setting but I will have to check when I get home (it certainly measures current, but I am not sure what the minimum threshold is). I will follow up with more info when I get home (and/or tomorrow). Thanks for your help.
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Wednesday, January 23rd, 2013 AT 10:38 PM
Tiny
SATURNTECH9
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No problem also make sure that battery as tested as good before we start.
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Wednesday, January 23rd, 2013 AT 10:54 PM
Tiny
KIRKMARTIN30
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Thanks. I picked up the battery yesterday and measured 12.6V when I got it home (I will check again tonight before proceeding). My Fluke does have a mA setting (there are two places to connect the negative lead for current testing--one labelled '10A' the other '300mA'). My thought as to a next-step is to put the battery back in the car, connect the posititve battery cable, and then put my meter in series with the negative cable and see the current draw. I would start with the 10A setting to make sure there isn't a large amount of current flowing, and then switch to the 300mA setting. I will then report back what I see, and we can try to "zero in" from there. Please let me know if you agree. Or if you had something else in mind.

FYI This may be a very slow process. The car is not needed right now, and my wife and I are extremely busy with our jobs (which involve plenty of take-home work) and 2 little kids. So I may at best be making one or two moves per day (and sometimes none). I hope this is not a problem, as I appreciate your help.
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Thursday, January 24th, 2013 AT 3:11 PM
Tiny
SATURNTECH9
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Your plan is perfect when you hook the meter inline on the negative cable make sure all the doors are closed key is out of the ignition dome light etc are all off. Wait about 10 mins before getting a reading to tell me.
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Thursday, January 24th, 2013 AT 6:17 PM
Tiny
KIRKMARTIN30
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Thanks. I will let you know what I see (in terms of current draw, etc).
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Thursday, January 24th, 2013 AT 8:44 PM
Tiny
SATURNTECH9
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Sounds good keep me posted?
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Thursday, January 24th, 2013 AT 9:19 PM
Tiny
KIRKMARTIN30
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Problem may to be solved. As soon as I connected the positive, and completed the circuit on the negative side (by putting my meter inline) I noticed the dome light come on in the car. I did expect this (and am still confused by it) because whether on or off I would expect the resistance across the light to be high (not the 2.5 ohms I was seeing). Regardless there is no question this was the main draw that resulted in the battery being discharged.

However, one part of the story that I did not tell before is that the battery was only about 2 month's old when it went dead. And the battery it replaced only lasted about 9 months. All of this lead to me thinking that there was a short somewhere, and my poking around with my meter seemed to confirm this. So after I discovered the dome light was on I still decided to see if there was a still a draw. After turning the dome light off (and confirming there were no doors open, etc), I still see about a 20 mA draw from the battery. Does this sound like a problem to you (i.E. That there is a parasitic draw from somewhere)?

Over the course of several days, this does not seem to have had a great effect (as the battery voltage is still above 12.5V). I am thinking that if there is still a draw, it may be so small that it is not an issue as long as I don't let the car sit (i.E. That we drive it at least every couple of days. Or at least charge the battery). Let me know your thoughts. Thanks.
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Tuesday, January 29th, 2013 AT 6:40 PM
Tiny
SATURNTECH9
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20 ma is within specs for a normal amount of draw so if that's all you have you have no worries. Except extended letting the car sit which the battery will die over time from that.
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Tuesday, January 29th, 2013 AT 7:39 PM
Tiny
KIRKMARTIN30
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Thanks. I will be sure to disconnect the battery if I plan to let the car sit again.
But I really appreciate your help with this. I will definitly use this service again, and definitely make further donations!
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Tuesday, January 29th, 2013 AT 10:51 PM
Tiny
SATURNTECH9
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Your welcome thats what were here tell all your friends about us.
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Tuesday, January 29th, 2013 AT 11:30 PM

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