Posted by Sherman in Idaho [108.162.246.24] on Monday, September 19, 2016 at 04:11:40 :
In Reply to: Re: 6 volt to 12 volt posted by Rich n [162.158.78.159] on Sunday, September 18, 2016 at 22:12:57 :
In a pinch, you can jump a 6V truck from a 12V one. The 6V truck probably has no 6V electronics on it, so that's not an issue. Turn off the lights so they don't burn out. The 6V gauges won't like it, but if you're quick about it and follow this method exactly, you won't burn them out. The key is to ONLY supply 12V to the 6V truck WHILE it is cranking. Hook the jumper cable that is ungrounded (normally positive) on the good truck to the same polarity (pos or neg) on the dead truck. If the dead truck is positive ground, you'll be clipping to ground on that truck. That's okay. So long as the trucks aren't touching, you can jump a 6V positive ground truck from a 12V negative ground one. Next clip onto the other terminal (usually negative and grounded) on the dead truck. Now it gets trickier. Have someone turn the starter switch on the dead truck to "start". If it tries to crank a little, that's fine. NOW, with the dead truck trying to crank, clip the remaining lead to the grounded battery terminal or a very solid ground on the good truck. Expect noticeable but not frightening sparks. I prefer not to pit my lead battery terminals so I clip to the frame or engine block, but any ground will work.
At this point, the dead truck should crank vigorously and start. Try to disconnect that last-connected jumper cable as soon as the truck starts and at the same time as the person holding the ignition switch to "start" lets up on it. Timing won't be exact, but if the gauges on the 6V truck are only seeing 12V for a second or so, it won't hurt them.
This method works because under cranking load, especially when pulling a lot of current through the number cables, the actual voltage at the dead truck is right around 6 volts, due to the internal resistance of the battery and the resistance of the jumper cables and clips. Once you let up on the starter of the dead truck, most of the load disappears and the voltage will jump back up to 12, which is why you need to disconnect immediately.