One reason I am adverse to clamping to the frame is, (even though is is safer), as you pointed out, it is either painted or possibly corroded. Additionally it is frequently difficult to get a clamp connection of sufficient area to pass the required amps due to the contouring of the clamp. Take a look at a typical clamp connection and you will see it often is only contacting at the high points, not enough area to pass that starting current. I have re-contoured my jumper cable clamps to make better contact.
Quote:
Originally Posted by TimMullen
It doesn't matter which one is connected to "frame". The purpose is to make the last connection to a place other than the negative battery post, because as you make that connection, you are going to get a spark (and when you disconnect too). Although unlikely, it is possible that one (or both) of the batteries will vent hydrogen gas and it is possible for the spark to ignite it and cause an explosion. By making the last connection to the "frame", it moves the spark away from the battery and lessens the chance of an explosion (which is already pretty small).
Now, the reason that I have been putting "frame" in quotes is that what you really want to connect to is a good ground. If you are in fact connecting the jumper cable clamp to a painted frame surface, you aren't' getting a good electrical connection. You want to connect to a sizable chunk of bare metal. I find that the best connection is usually to the alternator housing or it's mounting tabs. It is bare metal that is solidly connected to the engine block to provide an electrical path for the electricity it's generating. Clamp to that, or to one of the engine lifting "hooks" on an engine to get a good electrical connection. If that isn't easy to do, 99.9% of the time you can just connect to the battery posts without a problem - but if there are any funny smells, or it's in a closed area (like the boot of the car), make sure you ventilate it first to let any possible explosive fumes dissipate.
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