Originally Posted by Dave9
You ought to post vehicle specific issues in a forum specializing in that vehicle, then you are most likely to come across fellow owners who know things like whether it has a battery saver feature that reduces idle current and if so (probably does), what the time-out period is for that feature.
Audios mentioned a half hour. That's a good start but may not be long enough. I know some vehicles do have one that is 40 minutes or longer. For this reason I would go conservative and assume an hour in the following steps:
1. Disconnect alternator to battery wire at alternator, engine off. Use a meter to make a series connection between alternator and wire to measure current, first in amp range on meter. There should be zero current flow with engine off. If it reads zero, next switch meter to mA range and measure again. There should still be zero current. Anything higher is a sign the alternator has a fault.
2. An initial current draw test from the battery won't necessarily tell you much. The presumption is you have multimeter leads in series with positive cable and battery positive. Negative cable to battery negative will also work and is arguably a bit safer than having a loose positive cable flopping around in a (grounded) engine bay. The multimeter needs a reliable way to maintain contact with the battery leads one way or the other for the duration of the test. You can either keep the battery cable connected for an hour (as mentioned above) and manually hold the multimeter probes on both the battery cable and the battery terminal while you remove the cable from the terminal (if you accidentally let either meter probe lose electrical contact then the test must restart with another hour wait time), OR you can use multimeter probe tips that have claws to keep contact with the battery terminal and the battery cable so they are mechanically disconnected from each other but remain electrically connected through the multimeter for the hour wait. Probe tips with claws are highly desirable because they allow more reliable results and only one person to do #3 below.
3. After an hour wait you should see your long term current consumption. If it is above roughly 50 mA (even that is on the high side), it is time to grab a fuse diagram and start pulling fuses to see which circuit has excessive drain. This can be done by one person but is much faster if a second person is looking at the multimeter display while the first person is pulling and reinstalling the fuses one at a time.
4. Once you find a subcircuit with excessive power draw at the 1 hour point, it is time to look at wiring diagrams unless you want to just throw money at shotgun repairs. Using diagrams you can use a meter to probe specific circuits on each fuse one at a time, or if shotgunning I would start with checking the relays, either swapping in new ones or if the vehicle uses multiple relays of the same type (very common) you can swap relays from one position to another to see if it resolves the drain. Try to swap fuses and relays for any computer modules last as some (body computer or main ECM especially) can reset the hour waiting period.
5. Keep in mind that if you measure no excessive current after an hour, it might just be a bad battery. Consider how old it is, check its fluid level if not maintenance free, and of course you can pull it and have it tested. There are some testers that are mobile but you take what you can get. If the battery is bad to the extent of causing this then you may also see an excessive level of self discharge after charging, even with it hooked up to nothing.
Since you are talking about a time till dead of 2-3 weeks, this could take a while to track down if it's just the battery itself. You should assume it might settle down to around 12.6V fairly soon after charger disconnect but after 2-3 weeks it shouldn't have dropped much below that if not connected to anything. Unfortunately this test is impractical to most people because it means leaving a battery lying around out of service for so long, I mean to prove it as a good low self discharge rate. It could fail with excessive discharge within a much shorter period, in a few hours.
Excellent and well-written post!
