First of all, how much drain do you have? Second, your test method is part of the problem, so third, tell me what the symptom is that you're trying to solve.
Since the mid '90s most cars have at least one computer that will draw high current for up to 20 minutes after stopping the engine. You need to wait for it to go to "sleep" mode before any current drain testing is done. Why the engineers saw fit to do that is beyond me, but it's a fact of life for almost all cars now.
The next issue is putting the test light between a battery post and cable clamp worked fine in the ''70s when we didn't have computers. The only things we could have to cause a drain was a stuck trunk light switch, glove box light switch, or here and there a sticking mechanical voltage regulator. Those lights drew half an amp and would cause the test light to glow plenty bright to be seen. Today, with all the computers, Chrysler says a good, fully-charged battery will crank the engine fast enough to start after sitting for three weeks, and to do that, the maximum allowable drain is 35 milliamps, (.035 amps). That is now the industry standard, unless the manufacturer specifies differently. Cadillac allows up to 50 milliamps. There is no way that puny current is going to light up your test light. In fact, other than the really expensive ones, those bulbs can require as much as half an amp to light up full brightness. It can take 100 - 200 milliamps to make the bulb's filament glow enough to be seen. That means you could have a 100 milliamp drain, which is unacceptable, but the test light would not light up.
The bigger problem has to do with the method itself. Many computers that need to time out can draw as much as three amps for up to 20 minutes. Anything that interrupts the current flow will start the cycle all over. Also, you can't get three amps through a test light, so the computer may never time out.
The best way to do this test is with an amp meter with an inductive pickup that clamps around a battery cable. Unfortunately, most of them are for testing generators and starters where a tolerance of a few amps is acceptable. Those are extremely inaccurate at low current values, and they can't read values less than an amp. That leaves the best alternative as the cheap little digital volt-ohm-milliamp meter.
Those meters will read accurately down to one milliamp and up to ten amps or more, but there's more problems. Most of these meters use a common ground test probe jack and a common positive jack for all of the lower current ranges except for the ten-amp scale which uses its own positive jack. Other than that ten-amp scale, the highest range is usually a 2-amp range. If you connect the meter between the battery post and cable clamp, like you did with your test light, and you have a computer that draws three amps for 20 minutes, you'll blow the meter's internal 2-amp fuse. You need to start out on the ten-amp range, and that means using the ten-amp current jack on the meter.
So now you're reading, lets say, 3 amps. After 20 minutes the computer has gone to sleep mode and current has dropped. The ten-amp scale doesn't give you enough accuracy so you need to switch to the 2-amp scale. That means pull the test lead from the ten-amp jack and put it in the regular jack. You just created an open circuit for a few seconds, and that's all it takes to wake the computer up and start the 20-minute sequence all over, plus, your 2-amp internal fuse just blew, ... Again.
The secret to this test procedure is to use a clip lead / jumper wire to connect the battery post to the cable. That in effect shorts out the meter so no current will flow through it to be measured. Set the meter up on the ten-amp range, then remove one end of that jumper wire. Now current has to go through the meter.
The fun starts once the computer has timed out and current has dropped. First, put the jumper wire back in place, THEN you can do anything with the meter and there will never be an open circuit, so the computer won't wake up again. Set the meter to read on the 2-amp range, but there's more to the story. On this range there is no decimal place of accuracy so we'd like to switch to the 200 milliamp range. All volt-ohm meters use a "break-before-make" switch, usually a rotary switch, meaning as you turn the knob, it breaks the connection to the 2-amp range then it makes the connection to the 200 milliamp range. That very tiny gap is enough to create an open circuit as you turn the switch, and that will again wake up the computer. Rats. All you have to do is reconnect the jumper wire, THEN switch to the lower range, then remove the jumper wire. This will let you get down to the lowest range possible so your reading will have as much accuracy as possible.
Now if your current flow is excessive, you can get an idea of where to start looking by the value of that current, and whether it is pulsing or changing. Pulsing current can indicate a circuit that is trying to turn on, and that is going to be done by a computer, especially if it's a low value, like under 100 milliamps. A steady drain of close to half an amp is typical for a 194 "peanut" bulb like you'd find in a glove box, vanity mirror, or license plate lamp. A tail light draws around 3/4 amp, and a brake light draws close to one amp.
When you have a drain higher than 35 milliamps but not real high, ... Say 50 - 70 milliamps, look for something electronic that has a memory circuit that is always live. That includes the radio, Engine Computer, instrument cluster, and the majority of the many other computers. Sometimes a defect will prevent the unit from turning off completely so it will draw more current than normal for its "off" state, but not as much as when it's turned on.
On some cars the wiper's "park" circuit remains live when the ignition switch is off. That's so the wipers will park where they won't freeze to the glass, even if they were running when you turned the ignition switch off. There were a lot of problems with the park switch inside the wiper motor gear assemblies years ago, and it was fairly common to find one where the main gear didn't quite turn far enough for that switch to turn off fully. There wasn't enough current flow to make the motor spin, but it could draw excessive current.
If you see a drain of a couple of amps for maybe ten to 15 seconds, then it drops down for a little while before it comes back, feel in the inside fuse box for a hot circuit breaker. Auto-resetting circuit breakers are used for safety items like wipers and head lights. That is done so if an intermittent short occurs while driving, there's a good chance of the breaker resetting and you can see again long enough to safely stop the car. I had this problem on a conversion van many years ago. It turned out to be the upholstery for the passenger's seat got caught in the switch for the power seat. The seat was moved all the way up but the motor was still trying to run. Since a stalled electric motor draws very high current, it popped the circuit breaker, and the current drain dropped to an acceptable level. Once the breaker reset, current went way up again. Those circuit breakers work on heat build-up, so even a minute after one trips, it will still feel hot, even if it has reset.
The common procedure is to pull fuses to see which circuit is responsible for the drain, but you have to watch that you don't pull the fuse for any computer that will wake up again. Also be aware that safety system computers have two power feeds, each with their own fuse, and you have to pull both fuses for them. They do that so if one fuse blows, you have the other circuit to run the warning light. Air bag and anti-lock brake computers are the two most common for this. You'll want to have the interior lights or switches set so you can open the door without turning those lights on, then you can pull fuses in the inside fuse box. Those usually are not where you'll find computers' fuses. It's best to block the door switch to keep the interior lights off. Almost all cars today have to have a computer to turn the interior lights on, and if you have a dash switch to keep the lights off, opening a door still activates a circuit in the Body Computer. Even though that computer doesn't turn the lights on, it might be running a self-test on the seat belt switch, or it's turning on the wireless receiver for the ignition switch, (yes, I know that doesn't make sense), or it's turning on some other circuit in anticipation of you starting the engine. All of those actions will cause an increase in current flow that is normal, but you may not recognize it as such.
Given the car's age and mileage, consider broken and frayed wires between the door hinges, especially if you have a circuit that isn't working, like the power mirrors or locks. Speaking of locks, some cars' interior light switches are built into the door latch mechanisms now. To block them so the interior lights don't come on, just use a screwdriver blade to push the latch closed while the door is open. Just don't forget to pull the handle to open the latch before you try to slam the door shut!
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Monday, September 5th, 2016 AT 12:55 AM
