I was going to suggest a charging problem or battery cable connections, but your dandy observation of the hot cable clinches it. The cable's clamps should be cleaned and tightened. When multiple cables are hot, or the one is hot over its entire length, there is too much current for that cable to handle, either because it's too small in diameter or something is drawing excessive current, but when just one tiny area is hot it is always due to excessive resistance at that spot. Because the battery cables carry very high current at times, the resistance that's causing all the trouble can be WAY too small to measure, but it shows up as a voltage drop and as heat.
A common trick to identify the high resistance, (when you don't have the luxury of that clue you found), is to use a cheap digital voltmeter to look for the resistance. As I mentioned, it is way too small to measure, (the meter leads will have much more resistance than you're trying to measure, but the meter leads don't have to carry hardly any current), but you CAN measure the RESULTS of the resistance.
Put one meter lead on the battery POST, and the other lead on the cable clamp. They are the same point in the circuit, but because of that tiny resistance, when a helper cranks the engine, that heavy current will cause a voltage drop to occur. THAT's what can be measured with the voltmeter. You want all the voltage to be dropped across the starter motor where it will do work. The limit is 0.2 volts for any mechanical connection where a cable is bolted to the starter, where it's bolted to the battery, or where the other cable is bolted to the engine and to the battery. You really won't need to do this test if cleaning the cable clamps solves the problem.
If that cable still gets hot, suspect the wire is corroded under the insulation, but the problem area will still be identified by the hot spot.
Thursday, March 18th, 2010 AT 10:58 PM