The thought of your car having an electrical problem feels complex. You’re always hearing on the television how expensive car diagnostic and electrical repairs are.
Rest assured, it’s not as earth-shattering as the media wants you to believe. Quite honestly, common electrical repairs are simple to diagnose (even with a standard multimeter) and sometimes even simpler to repair.
The adage is, the hardest part is finding the problem. The easy part is fixing it.
I will organize this into sections, starting from what I believe is the most simple electrical fault to ones you need to put a little more time into testing.
1. Bad/Weak/Dead Battery
- Lights don't turn on.
- Battery warning light comes on.
- Multiple warning lights come on.
- Engine turns over slowly
This could be the most simple thing to check for. Oftentimes, this is overlooked at first. Especially if a person is convinced, their battery is good. Even if your car started the day before, you always want to start your diagnosis with the battery.
Leaving your lights on or leaving your accessory on too long can drain the battery. Not only this, but some people don’t notice if their car happens to start cranking longer to get the car started, or their crank from the starter is slower due to a battery going bad.
Because their car did ultimately start, they weren’t alarmed by those battery going bad symptoms.
How to test a car battery
To test the battery, you want to follow these steps:
- Turn your multimeter to DC volts and check that the battery is in spec (12.6v-12.8v) This spec in itself can be a bit tricky. This spec is for a fully charged battery. If you were cranking your car beforehand, you might get a reading less than this. So, use your best judgment. If you’re unsure, I’d say hook a battery charger up to your battery and get a full charge before you begin your diagnostics.
- Confirm that your positive and negative battery connections are tight and not corroded onto your battery posts.
- Make sure you can see no physical damage or leaking coming from the battery.
- Check the date on your battery. Batteries should be replaced every 5-7 years. Sometimes just changing an old battery is just what you need.
- Once you’ve confirmed that the physical appearance of the battery checks out, you want to hook your multimeter up to the battery and confirm your reading is still in the 12.6-12.8v spec. then, do the same thing with the car running. If you get a reading above 14v but below 15v, then your battery is getting a charge from the alternator. Lastly, you want to shut your car down and then watch for parasitic draw. As mentioned before, your resting car battery spec is 12.6-12.8v. when you shut your car down, your voltage should not drop below this. It’s common to see your battery drop around the 12.4v range. You just don’t want to see your battery get below 12v, especially if your voltage is dropping rapidly.
2. Bad Battery Cables
If you have ruled out the battery being at fault, the next step is to check to see that your battery cables don’t have excessive resistance.
- Take your multimeter and put it in the DC volts setting, and check your reading across the positive and negative battery post.
- Once you have that reading, take your positive lead from the positive post and move it to the connector of the positive battery post. The spec for this test is .2 volts. You do not want your reading to drop less than .2 volts from your battery’s resting voltage. If it does, that doesn’t necessarily mean you have a bad cable. If your terminals are corroded, you can clean them off with a battery cleaner and a wire brush and do the test again. If cleaning the terminals doesn’t change your reading, then you’ll have to move further with testing the cables.
- You can perform the above test on both the positive and negative sides.
- Disconnect the positive battery cable from the battery terminal as well as at the starter. make sure your negative cable is disconnected from the battery, so you don’t arc the starter when removing the positive cable.
- Turn your multimeter from DC volts to Ohms. Stick one lead on one end of the removed cable and one end at the other. (you should use alligator clips, so you have your hands free) You should read no more than .6 ohms. If you’re in spec, try wiggling the cable with your meter still hooked up to the cable and see if your reading changes drastically. Your reading should not fluctuate much. If it does jump excessively, the cable needs to be replaced. You can do this test on both the positive and negative cables. Note: the negative cable is usually connected to the car’s frame. So don’t look for the end of the negative battery cable at the starter.
3. Blown Fuses
When diagnosing an electrical problem, your fuses are one of the first things you should check. During your test, you shouldn’t just check the fuse with the multimeter.
You should always pull and inspect the fuse itself. A fuse can still make contact, even when broken if the fuse isn’t totally blown. This causes intermittent electrical issues.
The following steps are what you should take when testing a fuse:
- Pull the fuse panel cover off the fuse panel and confirm the fuse you need in the circuit you’re testing. If your car didn’t start, you would check the fuses associated with the starting circuit.
- Take your multimeter, and check for battery voltage at the fuse.
- Pull the fuse out with a pair of needle-nose pliers (whether you have voltage or not) and inspect the contacts. Make sure they are not loose or burnt.
- Inspect the contact within the plastic piece of the fuse for burnt or broken connection.
- Also, make sure that the amp reading matches up with the OEM recommended value that is in your service or operator's manual.
4. Spark Plug
Spark plugs are a crucial maintenance item that you need to look at just as closely as you do other high-priority maintenance items. (I.e., air filter, oil change intervals)
Without properly maintained spark plugs, you can experience symptoms that are common with many automotive electrical issues. So it is important to keep up on your maintenance to rule out the spark plug as the cause faster.
But, if you suspect your spark plugs may be the culprit, the following steps will cover all your bases.
- Make sure all of your spark plug wires are intact and secure on your plugs and distributor cap.
- Pull the wires off (one at a time, so you don’t confuse which spark plug they correspond with), remove the spark plug, and confirm the electrode is intact and doesn’t have excessive carbon build-up.
- Obtain a spark plug gap measurement tool and confirm that your sparkplug gap is in spec with OEM standards.
- Next, is to grab your multimeter and place it to the ohm reading setting and take one test lead to one end of the plug and the other test lead to the top of the exposed electrode. The electrode actually runs within the sparkplug from the end; you can see when you remove it to the end that is inserted into the spark plug wire.
- A normal reading for an automotive spark plug is between 500 and 3000 ohms. As the sparkplug ages, the ohm reading may decrease. This is normal. However, if your reading is higher than 5000 or O.L on your meter, then the sparkplug needs to be replaced.
When your alternator is starting to go bad, you can notice:
- Dimming lights
- Your battery light on the dash illuminating
- Difficulty starting or car stalling
- Your battery continues to drain down after only being stopped for a few hours.
- Headlights are dim or flickering.
- Random electrical failures.
- Car stalls or has difficulty starting.
- Battery dies while driving.
A failing alternator has many symptoms. Thankfully they are often obvious. So if you are new to being a mechanic, these symptoms are easy to trace.
The job of your alternator is to recharge your battery. When you start your car, the cranking amps from your battery need to be replenished.
The alternator test is similar to the bad battery test.
- Hook your multimeter up to the positive and negative posts of the battery.
- Confirm that your resting battery voltage is between 12.0v and 12.8v
- Then, start your car and do the same test. At this time, you should be reading about 14v. This shows that the alternator is charging your battery. If you’re not getting a similar reading and your battery has excessive draw once the car is shut down, then you know the alternator is no good. Make sure that your drive belt is in good order; if it is missing ribs or stretched out, this will cause your alternator to not perform how it should.
All these tests are performed with a multimeter because multimeters can be relatively inexpensive and can perform multiple functions. However, there are tools out there that will test your battery, starting system, and charging system all in one.
These tools are nice but are not always accurate. It’s better to know how to perform all these tests with a multimeter so you can get the most accurate results.
If you are stil having problems after checking the battery, cables and alternator the next step is to read fault codes from the vehicle. Use an all-system OBD-II scanner to read fault codes from the various modules. Most scanners, can run a full system scan and show all the codes that are present. The fault codes will often point you in the right direction.