You can't use that common sense test light in series with a battery cable. There are computers trying to go to "sleep mode" which can take up to 20 minutes. Until then, it is normal to see a current drain of up to three amps. Your test light adds too much resistance and limits current flow so some computers won't be able to perform some self tests. They will reset and try again leading to a pulsing test light.
You can insert an ammeter between the battery post and cable but you must also bypass it with a jumper wire. If you start out with it on a 2 amp scale, you could blow the meter's internal 2 amp fuse. If you start out on the ten amp scale and wait for the computers to time out, that scale won't give you anywhere near the needed accuracy so you have to switch to a lower scale. That means removing a lead and placing it in a different jack, or it means turning the switch. Either action creates a momentary open circuit and wakes up the computers for up to another 20 minutes. To prevent that, you must use the jumper wire to bypass the meter any time you move a lead or turn the switch. Once you're on the new scale, remove the temporary jumper so all current goes through the meter.
Once the computers are timed out, unless the manufacturer tells you differently, 35 milliamps, (.035 amps) is the industry standard maximum allowable drain. Some manufacturers now specify up to 50 milliamps because there are so many unnecessary computers hung onto every part of the car. Each one has a memory circuit that constantly draws a little current. 50 milliamps won't even come close to lighting up a test light but it can be enough to drain a battery in a couple of weeks.
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Tuesday, January 10th, 2012 AT 11:45 PM