Diagnosing Car Electrical Problems

Most car electrical problems, like the problems in any electrical circuit, can be traced to the power supply - in this case, the battery.

Note:  See my companion articles on Mysterious Electrical Gremlins and Understanding the Check Engine Light.

On online car forums, you often see postings about car electrical problems. And you may hear questions about car electrical problems from friends and acquaintances. In almost every case, the scenario is the same, and the cause the same, and yet people end up spending 2-3 times more money than they have to, in order to fix the problem. As I will note in a subsequent blog entry, people are often irrationally afraid of repair bills, and as a result end up spending hundreds, if not thousands of dollars more in repairs, in an attempt to avoid a repair.

The typical car electrical problem starts out as follows:

"I went out to start the car this morning (boy was it cold today!) and when I turned the key, it just clicked - nothing. It was fine yesterday, so it can't be the battery. I had a friend jump-start the car and it ran fine after that, so it can't be the battery. The car ran fine all day and started OK after work so it can't be the battery. I went to the mall and when I came out, it did the same thing again! I hads someone jump start the car and it ran fine, so it can't be the battery. There must be a short circuit somewhere draining the battery! Can anyone help me?"

The common thread on these postings or inquiries is always the same - the questioner immediately dismisses the battery as the problem, when in fact it is often the most likely problem. Once they put these blinders on, the problem becomes "mysterious and unsolvable" merely because they irrationally discard the most common solution.

Car batteries are much better today than they were even a few years ago. I've had batteries last as long as 9 years in a car, which is amazing. Only a few years back, the "60 month" DieHard car battery from Sears was considered the top of the line. Technology has improved over time.

But car batteries can fail at any time, even when brand new. And although they can last as long as 9 years, typically most fail in the 5-7 year range, if not before. And cheaper batteries might not last even that long.

The failure mode of car batteries appears sudden, which is something that confuses people. Most folks' experience with batteries is based on flashlight or other consumer product batteries. A flashlight battery fails slowly over time, getting dimmer and dimmer, until one day, it is a mere flicker and then goes out. Consumers think that this gradual failure mode applies to car batteries as well, which is why they dismiss the battery as the problem when the car "suddenly" fails to start.

A new car battery, fully charged, should be putting out about 13.5 Volts. Yes, it is a 12-volt battery, but if it actually is at 12 volts, it is partially discharged - actually mostly dead. At 11.5 Volts it may not start the car. Below 10.5 Volts it is basically dead and may cause all sorts of weird things to happen in the car (car alarm going off, etc.).

So the level of charge and voltage is not linear. It is not like the battery is half-charged at 6 Volts and fully charged at 12. More like half charged at 12 volts and fully charged at 13.5 Most consumers don't get that, and if they are smart enough to put a voltmeter on the battery, wonder why the car won't start when the battery is showing 11 volts, which they think of as 11/12ths charged. 11 volts is pretty much dead.

The other thing consumers fail to realize about car batteries, is that they don't have a lot of depth of charge, and this depth becomes shallower over time. As a car battery ages, it may hold little more than a surface charge. There is enough charge to crank over the engine and start the car - once - before the alternator kicks in and recharges the battery, which in turn will now have just enough charge to re-start the car.

On the first cold day of the year, where the engine needs to crank a little harder or longer, or the first time the driver leaves the dome light on for a few moments too long, this surface charge isn't enough to start the car, and "suddenly" the battery appears to fail. "Gee, it worked yesterday," the owner thinks, applying his flashlight battery experience to the situation, "it should work today!"

If you have a fine ear for such things, you may notice that, just prior to failure, the starter motor is cranking a little slower than before. But the change is very gradual and most folks don't notice it in time. On the other hand, you will notice how the starter motor spins much faster once you replace the battery, as the contrast is noticeable. But it is not like in the movies where the car cranks slower and slower - at least anymore. Modern starter motors stop cranking at all, once the voltage drops below 11.5 or thereabouts.

Thus consumers are so quick to dismiss the battery as the source of their troubles, as the car doesn't crank at all, but merely "clicks". The consumer thinks this "click" is an indication of some sort of mechanical trouble, and often blames the starter motor. However, when battery voltage drops down to about 11.5 volts or less, there is not enough juice to turn the starter motor. The clicking you hear is the starter solenoid pulling in, a sound you don't normally hear as it is normally drowned out by the sound of the engine turning over. But chances are, if you hear a "click" you have a dead battery.

So how to you diagnose and fix these problems? As I noted in the header of this piece, in any electrical circuit or device, the most common failure mode is in the power supply. So when diagnosing problems in any electrical circuit, the first thing an Engineer does is check the power supply. In this case, the battery.

With the car off, your battery should be at 13.0 to 13.5 volts, once it is charged. If it won't hold a charge, well, end of story, buy a new battery. Anything from 12.5 to 13.0 Volts is suspect, and if you are having problems and the battery is of any age at all, I'd replace it. Below 12.0 volts, your battery is likely shot.

With the car running, by the way, the voltage should be 14.0 to 14.5 Volts, which also confuses a lot of consumers. A battery won't charge unless the voltage applied to it is greater than its basic voltage (otherwise current won't flow). So to charge a battery that is nominally 13.5 volts, you need 14 to 14.5 volts to make things happen. If the car is less than 14.0 volts when running, you may have an alternator problem - but more about that later.

Most auto parts stores or auto centers will test your car battery for free - either in the car or with it removed. Such testers will test the battery under load as well, and provide a more complete diagnostic than mere voltage level.

However, car batteries are cheap (usually less than $100) and if they are more than 4 years old and you are having problems, it is a good idea to just replace the battery to eliminate this as a potential problem source in your diagnostics. This is particularly true if you live in a cold climate and winter is coming. If your car has an older battery and winter is on its way, it might not be a bad idea to buy a new battery, just so you don't get stuck somewhere on a cold night. It is cheap insurance and less costly than calling a tow truck.

You can install a battery in a car yourself pretty easily, but with modern cars, there are some considerations to take into account. Some car radios (such as in BMWs) have anti-theft codes that need to be entered after the power has been removed, so make sure you have these codes before removing power to the car.

In many cars, self-learning computer systems may have to "re-learn" themselves when you put in a new battery. So, for example, in my Ford pickup, the transmission will shift poorly for the first 50 miles after installing a new battery.

Most modern cars will also set off the check engine light if voltage drops below 10.5 volts, as "low voltage" may be one of the error codes. Moreover, low voltage may trigger spurious error messages as well. If your car has been jumped a number of times, chances are, you'll get a CE light, which will need to be reset.

Other flaky things may happen if your battery is disconnected. My BMWs, for example, set off the brake and ABS lights when power is removed. Once the car rolls a few feet, the lights reset themselves. There are some folks who "transplant" batteries into a car, using a jumper cable or charger, to keep power to the car when the battery is being replaced, to avoid problems. The only problem with this approach is that the positive (+) battery cable is "hot" when disconnected and can spark if allowed to touch ground.

And speaking of which, when disconnecting a car battery, always do the negative cable first. When connecting, do the negative last. If you disconnect the positive first, you may shock yourself if you are touching the positive terminal (with your wrench) and your body touches a ground on the car body. Similarly, if you connect the negative first, when you connect the positive, you increase the chance of electrocution.

Thus, if you are at all uncomfortable or unfamiliar with the process, have a mechanic replace the battery. People who are not handy with tools should avoid the process.

The second problem with the "it can't be the battery" people have is that oftentimes their battery problems morph into alternator problems. It has happened to me, so I know. My truck sat for long periods of time between use and when I went to start it one day, it wouldn't start. I jumped the battery and it worked fine. Over a period of a month, the car stereo and other electronics pulled down an older battery to the point where the truck wouldn't start.

The problem was, once I jump-started the truck, the alternator had to pick up a huge load to recharge a totally dead car battery. For the old generators and alternators of the 1950's and 1960's, this was not a big problem. But today, with weight a paramount concern for carmakers, alternators will not put out sustained high currents, without overheating. A modern alternator may be rated for 100 amps or more - but only intermittently, not continually.

As a result, my truck battery went dead again. And when I bought a new battery, the problem persisted. The new battery appeared not to be holding a charge. I checked the system voltage and realized that my alternator wasn't charging. In fact, the windings had melted together and shorted out. (It should be noted, that when it comes to "there's a short somewhere", the alternator is usually the culprit. A shorted alternator can drain a car battery, even with the ignition off).

Lesson learned, I bought a rebuilt alternator. On the box were explicit instructions NOT to try to charge a dead battery with the alternator - they just can't handle the load! I was lucky, others are not as lucky. Some folks continue this route, running a brand new battery dead again and again, and then smoking new alternators one after the other. Several alternators and batteries later, they sell the car, convinced it has an "unsolvable" electrical problem.

You see, in addition to being basically dead at 11 volts, a car battery is not a deep-cycle battery. As a result, you can destroy a brand new car battery (or severely shorten its life) if you leave the lights on and run the battery totally dead a few times. Even a 2-3 year old battery can be toast, if it has been totally discharged more than a few times.

So the bad battery leads to a smoked alternator which in turns allows the new replacement battery to go dead, which in turn smokes a new alternator, and so on.

(Any by the way, an alternator, unlike a generator, requires a battery in the circuit in order to work properly. If you disconnect a battery from a car while it is running, you will likely "fry" the alternator in short order - like immediately.)

To avoid this problem, fully charge a car battery before installing it. Test the alternator as well as battery (again, most shops will do this for free, or you can check the output voltage as noted above). If you must jump start a car, leave the cars connected for a while to allow both alternators to charge your battery. Don't run your car for a long period of time, but rather shut it down after 15 minutes to let the alternator cool. But avoid jump-starting if at all possible.

In modern cars, jump-starting can cause all sorts of problems. Electrical systems as noted above, can do flaky things when voltages go low. Jumping can cause spikes in voltage which are just as bad. And every time you jump-start a car is an opportunity for someone to reverse the cables, which causes all sorts of bad things to happen.

Most cars, by the way, have an "idiot light" or voltage gauge. The idiot light, usually in the shape of a battery, comes on when system voltage is below 14 volts or thereabouts. Thus, when you turn on the ignition, the light comes on, as the car battery is at 13.5 volts. When you start the car, the alternator kicks in, and the voltage goes up to about 14.5 volts, and the light goes off. If the "battery" light comes on, chances are, you have an alternator problem, ironically.

Voltage gauges work the same way, but most are not accurate enough to read a proper voltage level very well. Some older cars have Ammeters, which show the rate of charge. When the alternator is working, it may show a positive amperage, representing the current charging the battery. When accessories are turned on, the amperage may drop or go negative, representing current draining from the battery. Such gauges were interesting to have during the early history of the automobile, when charging systems were fragile. But often the connections on such gauges cause more problems than the good they do. For this reason, I do not recommend "aftermarket" gauge sets, as they add little in terms of useful information and can cause electrical problems down the road.

Presuming you have eliminated the battery and alternator as the problems, where do you go from here? Connectors are the second largest failure mode in a car, and the first connector you should check is the battery terminals. Terminals can come loose (or be improperly installed) or become corroded. Clean and tighten the terminals and see what happens. A neighbor recently had this problem. His mechanic couldn't get the negative terminal to reach, and attached it at an angle. As a result, it was making only intermittent contact and the car would "mysteriously" not start sometimes. loosening the cable so it would reach and then tightening the contact fixed the problem.

Cars left for long periods of time will discharge their batteries. Most cars can sit for about a month before the various computers and radio gear discharges the battery. If you let a car sit for any length of time, a battery tender may be a good idea. These chargers keep the battery topped up and compensate for the minor current drains from all the accessories in the car. I own four of these types of chargers and use them, as I tend to let cars sit.

Note that if you remove a battery from a car, boat, or tractor, you should put it inside, in a heated space, and not on a concrete floor. Batteries can freeze, particularly when discharged, and this freezing usually destroys them. A battery that appears "puffed out" probably froze at some time. Folklore suggests that you should not leave a battery on a concrete floor, although it is not clear whether there is any science behind this. There may be two reasons for this folklore. First, a concrete floor may be cold, and transfer heat away from the battery more quickly, allowing it to freeze more readily. Second, plate material can slough off a battery if it is exposed to vibrations, and a concrete floor may more readily transmit such vibrations to the battery. Whether it is folklore or not, I put my batteries on a wooden shelf in the winter and not in use.

So, you've replaced the battery, tested the alternator, checked the battery terminals, and you are still having problems. Now what? The odds of a real "short somewhere" are actually very small. Wires sitting by themselves and not moving (rubbing or abrading) don't suddenly decide to shed their insulation and short out. Automotive designers are aware of the difficulties of vibration when wires pass through bulkheads and provide grommets and bushings to compensate. And wiring harness portions going to doors and trunks are extensively tested prior to production to insure that they do not break or short as they flex at the joints.

Sometimes errors do occur. For example, in some BMW 3-series coupes and sedans (E36) the trunk wiring harness had a tendency to short out. Most of such cars had a new harness installed under warranty. But such shortfalls are few and far between. These are the rare exception, not the norm.

The easiest way to test for undue electrical drains is to insert an ammeter in line with the negative terminal of the car battery and the ground cable. The current draw should be on the order of milliAmps (check the service manual to be sure) from the various computers in the car (engine, radio, SRS, ABS, transmission, etc.). If it is larger than this, pull fuses one at a time and note the change in current draw. If one circuit seems to be pulling more amperage than normal, you can than trace this one circuit for problems, rather than just pulling at wires willy-nilly. Again, note the precautions above, and make sure if you disconnect power from an electrical device (such as your radio) you won't need a special code to reactivate it.

Aftermarket radios tend to draw more power, even when off, than factory components. Many of these types of radios are installed after the car is a few years old and the battery is getting worn. The combination of an aftermarket radio and an older battery can trigger an electrical problem. And again, the solution is usually a new battery (hopefully replaced before the alternator is shot as well). So often, you hear of these "mysterious short somewhere" pleas right after some youngster put a boom-boom stereo in his old hoop-de.

And speaking of which, some car stereos and nav systems can draw a LOT of power. My new Pioneer AVIC-Z2 is on a 20-amp circuit. The owners manual explicitly states that the radio should NEVER be run unless the engine is running. It draws that much power. And my experience has shown than running the radio even for a half-hour, can pull down the battery enough to the point where the car may not start.

Folks who make a hobby of huge radios and subwoofers (and we all love pulling up to a light next to them, don't we?) often install a separate battery to power the accessory equipment. The standard car battery can easily be drained flat by large aftermarket subwoofer amplifiers and other audio, video, and navigation accessories. If you plan on putting in a bad-ass stereo, you might want to consider an auxiliary battery as well.