brokenrinker Posted May 30, 2018 Share Posted May 30, 2018 I'm working on a '35 Chrysler Airflow. I've switched it to 12volt and installed a GM 3 wire alternator. I noticed that when I measure resistance across the disconnected battery terminals I get about 6000 ohms. I disconnected everything but the alternator. When I pull the two wire plug out of the side of the alternator the resistance across the battery terminals increases to the Meg ohm range but doesn't drop to zero. Any thoughts? Link to comment Share on other sites More sharing options...
TerryB Posted May 30, 2018 Share Posted May 30, 2018 For all intents and purposes megohms is zero. Link to comment Share on other sites More sharing options...
brokenrinker Posted May 30, 2018 Author Share Posted May 30, 2018 fair enough but with the alternator plugged in it's at 6k? Link to comment Share on other sites More sharing options...
TerryB Posted May 30, 2018 Share Posted May 30, 2018 I would recheck the wiring to make sure everything is correctly connected. Certainly there has to be a path from the battery to the alternator for the battery to charge. A 6k ohm resistor connected to 12v produces a current flow of 0.002A. 1 Link to comment Share on other sites More sharing options...
brokenrinker Posted May 30, 2018 Author Share Posted May 30, 2018 The alternator circuit has a diode between the ignition switch and one of the terminals on the alternator. Evidently it's there to prevent the engine from continuing to run even after the key had been turned off. Could that have something to do with it? Link to comment Share on other sites More sharing options...
joe_padavano Posted May 30, 2018 Share Posted May 30, 2018 (edited) So just out of curiosity, are you troubleshooting a problem relating to the charging system or creating one where one doesn't exist? Seriously, take a look at the attached wiring diagram. If you are measuring resistance between the disconnected + battery cable and the battery terminal, note that there is a direct path to ground through the resistors in the regulator (R2 and R3). Edited May 30, 2018 by joe_padavano (see edit history) 2 1 Link to comment Share on other sites More sharing options...
brokenrinker Posted May 30, 2018 Author Share Posted May 30, 2018 (edited) I'm glad you're curious. I was creating a problem where none existed. Your schematic explains it. Everything works fine, charging etc. I have done a ton of wiring work and I happen to be standing next to the battery cables holding a multimeter. I checked resistance across the two leads expecting to see an open circuit. When I saw some resistance (current flow) I got confused, which is a common occurrence with me. Thanks! Edited May 30, 2018 by brokenrinker (see edit history) 2 1 Link to comment Share on other sites More sharing options...
Frank DuVal Posted May 31, 2018 Share Posted May 31, 2018 Just for education, zero ohms is a circuit with no resistance, or a perfect conductor. Megohms is approaching infinity ohms. So an open circuit is described as infinite ohms, not zero ohms. Drop to zero means no resistance, a "short", not an open circuit. 1 Link to comment Share on other sites More sharing options...
ArticiferTom Posted May 31, 2018 Share Posted May 31, 2018 Just dirt , grease and moisture on the top of a battery will give you a reading . Link to comment Share on other sites More sharing options...
Bloo Posted May 31, 2018 Share Posted May 31, 2018 7 hours ago, joe_padavano said: So just out of curiosity, are you troubleshooting a problem relating to the charging system or creating one where one doesn't exist? Seriously, take a look at the attached wiring diagram. If you are measuring resistance between the disconnected + battery cable and the battery terminal, note that there is a direct path to ground through the resistors in the regulator (R@ and R3). Is that schematic for the 10si/12si? Link to comment Share on other sites More sharing options...
joe_padavano Posted May 31, 2018 Share Posted May 31, 2018 6 hours ago, Bloo said: Is that schematic for the 10si/12si? Yes 2 Link to comment Share on other sites More sharing options...
37_Roadmaster_C Posted June 1, 2018 Share Posted June 1, 2018 Hi Broken You have now joined the exclusive club for those of us that have to much technical experience and way to much time on our hands........ Also, Thanks Joe. That schematic will be very helpful to those of us that want to find the real answer for GM alternator failures . Link to comment Share on other sites More sharing options...
joe_padavano Posted June 1, 2018 Share Posted June 1, 2018 10 hours ago, 37_Roadmaster_C said: Also, Thanks Joe. That schematic will be very helpful to those of us that want to find the real answer for GM alternator failures . In that case, you might find this Service Bulletin from Delco Remy useful. Yes, it only covers 10SI, 15SI, and 27SI, but the 12SI is essentially the same as the 10SI. 2 Link to comment Share on other sites More sharing options...
Ltc4748 Posted June 2, 2018 Share Posted June 2, 2018 The easiest way to check for a battery drain is to take the ground terminal off the battery and connect a test light between the battery post and the terminal. The light should not even glow a barely visible dim red color. To check if you have it connected correctly and that the test light is good open the door or turn something on, the set light will glow brightly. This can even be used on new cars with computers but there are other steps you must follow. If the light is glowing and everything is off disconnect one thing at a time until the bulb goes out. Best of luck Link to comment Share on other sites More sharing options...
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