rossman Posted September 7, 2014 Share Posted September 7, 2014 So, the problem is when I go to connect the negative terminal on my battery it arcs every time. Big spark, lead spattering off the battery terminal, ignition off, key out. I assume this means I have a short somewhere. Here is a little history. It's a 1972 240z with extensive electrical modifications, build page here. The car was running, no arcing, no electrical issues. I pulled the dash to install an aftermarket speedo and tach, neither of which are connected to power at the moment. I verified that all the passenger side connections have the same color wires on both sides of the connectors. I pulled all connectors on to the stock fuse box and disconnected power to it. I haven't messed with anything under the hood except removing the old battery and reinstalling a new one. One thing I noticed is that some of the under-dash wire connectors are not obvious where they connect. I'm worried I have a ground connector connected to power. I'm looking for tips to track down the short, like where I should start and how to proceed and if there are any common culprits. Thanks for any help! Quote Link to comment Share on other sites More sharing options...
NewZed Posted September 7, 2014 Share Posted September 7, 2014 Use a meter and check for shorts from the positive cable connections to ground. The starter lug that the positive cable is connected to and the alternator charge wire which is usually connected to the same spot. One of those two has to be grounded. Then narrow it down from there. If it's the starter, easy, if ti's the charge wire you'll have to check the voltage regulator and the various loads along the way. All you can do is keep checking with the meter until you find that short circuit. Sounds like a big one though, and since the fusible link doesn't blow I'm guessing starter. Then there's always the reversed connections. Happens to many. http://www.atlanticz.ca/zclub/techtips/wiringdiagrams/240z/1972_240z.gif Quote Link to comment Share on other sites More sharing options...
rossman Posted September 7, 2014 Author Share Posted September 7, 2014 NewZed - Thanks for your reply. Essentially the only thing stock about the electrical system is the lighting wiring. I've replaced all the fusible links with maxi fuses. I put my DMM on the 10A setting then put the terminals inline with the negative battery terminal and the DMM internal fuse popped immediately. Luckily I have a spare! How can I test for the short without making an electrical connection to the battery? Continuity test? I get 10 - 30 milliohm resistance between ground and the alternator/battery positive terminal. Alternator ground to other grounds have no measurable resistance; which makes sense. Quote Link to comment Share on other sites More sharing options...
NewZed Posted September 7, 2014 Share Posted September 7, 2014 By "shorts" I meant continuity. There shouldn't be any continuity from the end of the positive cable to the end of the negative cable (ground), when the key and everything else is off. You don't even need the battery installed to do the testing. Set the meter for ohms/resistance. Quote Link to comment Share on other sites More sharing options...
Miles Posted September 7, 2014 Share Posted September 7, 2014 By "shorts" I meant continuity. There shouldn't be any continuity from the end of the positive cable to the end of the negative cable (ground), when the key and everything else is off. You don't even need the battery installed to do the testing. Set the meter for ohms/resistance. Good idea. Leave the meter hooked up and go back to the wires you most recently played with and unplug the connectors one at a time. Place the meter where you can see it as you unplug connectors/wires. Just be methodical and do one thing at at a time. Quote Link to comment Share on other sites More sharing options...
rossman Posted September 8, 2014 Author Share Posted September 8, 2014 Thanks guys. I'll work on it later this week. I'd bet money there is a connector reversed or damaged wire insulation as both the fuse box and all the connectors at the steering column are all disconnected. Quote Link to comment Share on other sites More sharing options...
rossman Posted September 13, 2014 Author Share Posted September 13, 2014 (edited) I assume there should be 0.0 ohms between the positive and negative terminals on the alternator. If that's true then I found the problem. Edited September 13, 2014 by rossman Quote Link to comment Share on other sites More sharing options...
rossman Posted September 14, 2014 Author Share Posted September 14, 2014 Maybe I should rephrase that as a question... Should there be any resistance between the positive and negative terminals on the alternator? My DMM is measuring ~15 MOhm. I'm using as CS130 alternator if that makes any difference. Quote Link to comment Share on other sites More sharing options...
NewZed Posted September 14, 2014 Share Posted September 14, 2014 Yes there should be. A lot. 15 megahoms is a lot. If there wasn't the battery would discharge immediately. Quote Link to comment Share on other sites More sharing options...
rossman Posted September 14, 2014 Author Share Posted September 14, 2014 I thought it would be an open circuit. Quote Link to comment Share on other sites More sharing options...
rossman Posted September 14, 2014 Author Share Posted September 14, 2014 I unplugged everything and that is the only source of resistance less than open circuit. I'll do some more testing and reconnect everything tomorrow. Quote Link to comment Share on other sites More sharing options...
NewZed Posted September 14, 2014 Share Posted September 14, 2014 I would have left everything connected, removed the battery, confirmed that you had low resistance from the positive cable to the negative cable (confirming the short), then disconnected things one by one until the short went away. Quote Link to comment Share on other sites More sharing options...
rossman Posted September 14, 2014 Author Share Posted September 14, 2014 That's what I did must have missed it. I dunno. Quote Link to comment Share on other sites More sharing options...
NewZed Posted September 14, 2014 Share Posted September 14, 2014 (edited) I didn't want to complicate the process but it could be a relay powered circuit that's shorting. That's harder to find, since the relay needs power to cause the short. Your description of molten lead makes it sounds like you have a big wire shorting though. Looks like a tough one. One thing you can do is to put a light bulb, like a bulb in a tail light socket, in between the end of the positive cable and the battery positive. The light bulb will allow enough power to flow to cause the short, but not enough to burn anything up. If the short is there the light will light up. If it's a relay, you'll hear the relay click when you connect the power. I think that you'll get enough current through a small bulb to activate a relay. Even if it's not a relay the light will tell you if the short is there. One possible way to figure it out. Edited September 14, 2014 by NewZed Quote Link to comment Share on other sites More sharing options...
rossman Posted September 14, 2014 Author Share Posted September 14, 2014 Thanks NewZed. I found the problem! I knew it had to be something simple and it was. The positive and negative battery posts are backwards. Autozone sold me the wrong battery and I didn't verify it. hahaha! Quote Link to comment Share on other sites More sharing options...
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