The high current test shunt is second only to the multimeter in usefulness in testing motorcycle charging and electric starting systems. For starting systems, the shunt is used to determine the condition of the starter motor. For charging systems, the shunt provides one of the easiest ways to measure charging performance.

Starting Systems
An import motorcycle’s starter uses about 1000 watts of power every time you press the button, an air-cooled Harley-Davidson almost double that. Given a 12 volt battery, the import starter therefore sucks up about 85 amps of current (1000 / 12 = 83), and the Harley 150~175 amps. Turn the equation around and you can see that electrical power is the product of volts x amps (12 x 83.3 = 1000). The interesting thing is, the starter doesn't care in what combination it gets the two -- i.e. more current and less voltage or more voltage and less current -- as long as it gets the total 1000W. Understanding this basic fact is the key to diagnosing starter trouble.

Suppose for example you use the shunt to measure the starter's current draw, and the import starter's current requirement is up dramatically, say to 120 amps. That's a tip off to a problem, and the problem is not enough voltage. The starter is sucking up more current in an attempt to make up for the lost voltage, thereby getting the required watts. (It will get its required wattage one way or another, remember -- just think watts = volts x amps.) There are two reasons for this apparent voltage loss. First, the starter may be worn and therefore less electromechanically efficient, causing the voltage it receives to do less work. Second, the battery's voltage may be dropping dramatically under severe load, meaning the battery itself is "toast." Either way, the starter attempts to substitute for the lack of useful voltage by pulling in more current, providing us a very useful clue to system condition. To tell whether the battery or the starter is the problem, temporarily replace the battery with a tested and known good battery. You could even jump your car battery over. The second battery, if it is in good condition, will not drop voltage excessively, and therefore, if the starter still acts voltage-deprived by drawing too much current, it's because it is mechanically inefficient (worn), and not because the battery is weak. If however the starter's current draw is now normal, the original battery is the problem. Make sense?

Charging Systems
It is always best to check your charging system's output in amps, not volts. Charging is an input/output affair. The charging system's performance depends not only on the battery's charging input, but also on its output, that is, the vehicle's electrical loads. When we speak of the vehicle's electrical loads therefore, note that we never speak in terms of voltage, because that is constant, but always in terms of amps. For example, if an electrical accessory draws 2 amps, we know that the charging system must replenish the battery that same 2 amps, or the battery will soon be discharged and unusable. Thereofore, when observing charging system behavior, we alway compare the battery's input with its output, and that is possible only by measuring current flow. Not that you can't check charge with a voltmeter -- you can. But you aren't going to be seeing the whole picture, and you are therefore likely to mis-diagnose. Here's another way to look at it. Imagine a bucket under a water faucet. Water is rushing into the bucket, and as result, the level in the bucket is rising. Which of the two demonstrates electrical activity more clearly -- the torrent of water or the rising water level? The torrent, of course. It’s raw current, whereas the water level is merely the result of that current. The same is true when checking the charging system. When you check it with amps, you are seeing the actual movement of electrical current into the battery. When you check it using volts, you see only the delayed result of that current’s progress. Your information is second-hand, and therefore more susceptible to misinterpretation.

Using the Shunt
To use the shunt, simply unbolt the negative cable at the battery and attach the shunt in its place. Clip one end of your shunt to the negative terminal, the other to the unbolted cable. Then attach the black meter lead to the battery side of the shunt, and the red meter lead to the battery cable side. Don't attach the meter leads to the shunt's wire, but to the shunt's alligators clips, and don't even think about doing anything to the positive cable. Set your multimeter to read DC Volts (NOT amps), on a scale under 1 volt, or its smallest voltage scale. On digital meters, this is often 300mV, while on analog meters it will probably be 0.25 or 0.50 Volts.

For the starting system check, remove the ignition fuse or otherwise disable the ignition system so the vehicle will not start. On Hondas, this means simply turning the engine stop switch to "off." Crank the starter. The reading will peak momentarily, then stabilize at a lower position. If your analog multimeter has 0.25 as the smallest DCV range, look for a "250" on the far right of the meter face and count backward to the needle position. Likewise, if the setting is 0.50, look for a "500" and count back to the needle position. On some meters, it may be necessary to mentally add a zero. Digitals will read directly. A steady reading (for import machines) between 75-85 amps is normal, more spells trouble.

To test the charging system, connect the shunt as above, except clip the red meter test lead to the battery side of the shunt, and the black test lead to the battery cable side. When the kewswitch is turned on, the meter will read backward -- this is normal. After starting the engine, let it warm up for a few minutes. Then increase the engine speed to about 3,000 rpm (depending on the vehicle), and read the meter. The result should be between 1/4 and 1/3 of the battery’s amp/hour rating. Smaller vehicles therefore will indicate 1~3 amps, and larger vehicles two to three times as much. If you get this much current, the charging system is outputting properly.

The high current shunt works through a simple but elegant application of Ohm's Law. It has exactly 1/1000 (0.001) ohm of resistance. As a result, every 1/1000 (0.001) volt drop across its length equals 1 amp of current. The best part is that the shunt is ridiculously easy to make yourself. Simply solder a Mueller #27 alligator clip to each end of a piece of #8 stranded copper wire. But be very careful about the following three specifics. First, the wire must be #8 stranded copper, available at any home improvement center, and the Mueller #27 alligator clips must be carefully soldered so that the solder does not wick up the wire. This is critical, as it affects the tool’s resistance and therefore its accuracy. Finally, the shunt’s length from clip to clip (not end to end) must be exactly 6 5/8". This is important, as an error of only a quarter inch will throw off the tool’s readability. Coil the shunt after it is assembled to make it more compact.