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Re: Tips for Finding Short? Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Wed, 3 Sep 2003 07:20:53
In Reply to: Tips for Finding Short?, Dan W, Wed, 3 Sep 2003 06:17:40 Members do not see ads below this line. - Help Keep This Site Online - Signup |
I'll bet it's not an electrical drain, but it's worth checking.
Method-
First, get a decent meter that reads current. One that goes up to an amp, at least.
Remove ground cable from battery. Jumper ground cable to negative post, and put the meter in parallel with the jumper. Wait at least 1 minute, then remove the jumper. You will now be reading the quiescent current drain. A normal current drain is in the 10-25 milliamp range (clock, radio, possibly alarm). A healthy battery should be able to supply 20 ma for weeks.
OK, so why the trick with the jumper? When you disconnect the battery, then reconnect it, a bunch of things turn back on - for example, the dome light, but also ECU, etc. That's why you put the jumper on - it takes the big current instead of the meter. That's also why you wait a minute before taking the reading - to let those things charge up, and for the dome light to time out. Remember, if you open a door, you'll turn the dome light back on.
OK, what if you see more than 20 ma, more like an amp or so? Then, yes, start pulling fuses. Remember, if you stick a fuse back in, you may very well see the current go way up for a few seconds to a minute. If the current is too high, you can pop the fuse in the meter. That's where the jumper comes in.
I'll bet that you'll only see 20 or so milliamps, and that unless the dome light is stuck on, you don't have an unusual drain. Yes, it's possible, but typically not the problem.
Two possible problems - a bad alternator/weak voltage regulator, or a tired battery. With everything hooked up, start the car and use the voltmeter to measure the voltage at the battery. With no load (headlights off, fan at low, etc), you should be seeing voltages in the mid 13 volt range. With a large electrical load, (Headlights on, fan on, defroster on), the voltage may dip into the high 12 volt range, but no lower than 12.6 volts.
If you're seeing lower voltages, then the alternator isn't charging the battery properly. It might be the alternator, but more likely just a tired voltage regulator. To be exact, the brushes built into the VR get worn and don't make good contact. Replacing the VR gets you fresh electronics and new brushes.
If the voltages are right, then chances are the battery is just tired. This is especially true if it's been discharged a few times. Car batteries aren't designed to be completely discharged, and every time they go through a discharge cycle, they lose life. The sign of this is the ability to be charged, but they lose the charge very, very quickly. So if you have a battery that's a few years old and has gone through some discharge cycles, then chances are you just need a new battery.
The last thing to do is wash the battery. What? Make up a weak solution of water and baking soda, and wipe the battery clean - top and sides. Why? Batteries outgas when they charge, and some of that outgassing has acid vapor. This condenses on the sides of the battery, and combines with dust and dirt to make a weak conductor on the outside of the battery. Wiping it clean with something that neutralizes the acid removes this. I don't think this self-drain is an issue, but there's nothing wrong with a squeaky-clean battery.
I'll bet you just have a weak battery, and second place falls to a worn VR.
posted by 192.249....
, Wed, 3 Sep 2003 07:20:53 <-- Viewing This Message
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