Electrical short in 1997 Neon
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03-19-2019, 09:47 PM
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Electrical short in 1997 Neon
Hi everyone,
A friend of mine asked me to troubleshoot a severe battery drainage in his '97 Neon. He's been chasing it for a couple years now. First, there was an insurance company OBD tracker that he thought was the problem, so he got rid of it. Second, there was an aftermarket alarm system (Code Alarm CSI-200) that we thought caused a short and depleted many batteries overnight. The main symptom is when I place an ohm meter between + terminal of the battery and a chassis connection nearby I get 0ohm i.e. full short between ground and +. As I disconnect the negative battery terminal and measure resistance between positive and chassis-ground I get 0ohm for a couple of seconds and then it starts increasing resistance gradually, into kiloohms in 15-30 seconds, then in a few minutes it gets into Mohm. As soon as I connect the negative terminal back it jumps to 0 ohm immediately and the car alarm goes off. So, I went through every fuse and relay in both fuse boxes, taking them out one by one and measuring resistance between + and chassis and it was always 0 ohm.
I then decided to disconnect the alarm. I could see that only 3 wires of the factory ignition harness were modified. The yellow ignition wire is routed into an ignition relay of the alarm (i.e. no continuity by default). The blue ignition wire is ground and red ignition wire is +12v. The latter two are spliced into (i.e. not breaking factory continuity) to power the alarm module. The blue ground wire also goes to the mentioned relay coil-, where as the +12v red wire goes into the alarm module and a separate wire (orange) from it goes back into the same relay to energize the coil+ so the relay can connect/disconnect yellow wire. I bypassed the relay by connecting the yellow ignition wire together but the car didn't crank. I then bypassed the alarm module by connecting the red + wire and the orange wire out of the relay+ and the car started without the alarm. The short remained! I then wanted to shut off the car and turned the ignition key off but the car kept running. I then disconnected the yellow ignition wire but the car kept running! This is where I got really puzzled and decided to disconnect the relay coil (blue and orange wires) and the car shut off. Measuring the battery+ to chassis was 0 ohm again, with the yellow ignition wire and the alarm relay and module disconnected.
Can someone suggest other troubleshooting methods, places to look, things to check? In my mind with 0 ohms between battery + and chassis I don't understand how the car could run at all. When negative of battery disconnected the short behaves similarly to a slowly discharging capacitor somewhere. A very slow gradual increase of resistance until it's complete discontinuity indicated by my DMM. I would be glad to hear any ideas and/or answer in more detail. Thank you!
A friend of mine asked me to troubleshoot a severe battery drainage in his '97 Neon. He's been chasing it for a couple years now. First, there was an insurance company OBD tracker that he thought was the problem, so he got rid of it. Second, there was an aftermarket alarm system (Code Alarm CSI-200) that we thought caused a short and depleted many batteries overnight. The main symptom is when I place an ohm meter between + terminal of the battery and a chassis connection nearby I get 0ohm i.e. full short between ground and +. As I disconnect the negative battery terminal and measure resistance between positive and chassis-ground I get 0ohm for a couple of seconds and then it starts increasing resistance gradually, into kiloohms in 15-30 seconds, then in a few minutes it gets into Mohm. As soon as I connect the negative terminal back it jumps to 0 ohm immediately and the car alarm goes off. So, I went through every fuse and relay in both fuse boxes, taking them out one by one and measuring resistance between + and chassis and it was always 0 ohm.
I then decided to disconnect the alarm. I could see that only 3 wires of the factory ignition harness were modified. The yellow ignition wire is routed into an ignition relay of the alarm (i.e. no continuity by default). The blue ignition wire is ground and red ignition wire is +12v. The latter two are spliced into (i.e. not breaking factory continuity) to power the alarm module. The blue ground wire also goes to the mentioned relay coil-, where as the +12v red wire goes into the alarm module and a separate wire (orange) from it goes back into the same relay to energize the coil+ so the relay can connect/disconnect yellow wire. I bypassed the relay by connecting the yellow ignition wire together but the car didn't crank. I then bypassed the alarm module by connecting the red + wire and the orange wire out of the relay+ and the car started without the alarm. The short remained! I then wanted to shut off the car and turned the ignition key off but the car kept running. I then disconnected the yellow ignition wire but the car kept running! This is where I got really puzzled and decided to disconnect the relay coil (blue and orange wires) and the car shut off. Measuring the battery+ to chassis was 0 ohm again, with the yellow ignition wire and the alarm relay and module disconnected.
Can someone suggest other troubleshooting methods, places to look, things to check? In my mind with 0 ohms between battery + and chassis I don't understand how the car could run at all. When negative of battery disconnected the short behaves similarly to a slowly discharging capacitor somewhere. A very slow gradual increase of resistance until it's complete discontinuity indicated by my DMM. I would be glad to hear any ideas and/or answer in more detail. Thank you!