How to easily diagnose common automotive electrical problems

By Carl Heideman
May 26, 2024 | electrical, Restoration, electrical problems | Posted in Shop Work , Electronics & Electrical Systems | Never miss an article

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When it comes to the electrical system on a car, a lot of people shake their heads in despair. Even experienced mechanics can shy away from electrical work for fear of the unknown. While some components do use computers and advanced circuitry, solving most electrical problems just isn’t rocket science.

Using simple tools and some common sense, it’s possible to diagnose and solve nearly all electrical problems quickly and easily. Among these tools are a test light, a volt meter and a good understanding of electrical basics and common problems. However, when you go out to tackle an electrical problem, remember that the generalizations in this article may not apply to your car-you’ll be wise to consult a service manual for its specific electrical information.

Golden Rules and the Basics

There are several golden rules for solving problems with automotive electrical systems. Corrosion is the biggest reason for failure. Wires do not fail. Grounds, connections, and individual components fail regularly. And in most cases, it is far better to repair a faulty factory circuit than to rewire it.

Corrosion is electricity’s biggest enemy. Battery terminals, fuse blocks, sensors, switches, connectors, and grounds are likely to fail because they are corroded. Cleaning or replacing these connectors will repair a great percentage of electrical problems.

With most cars, the body and frame serve as one of the “wires” that feed each circuit. Usually, the car body and frame serve as the negative side of each circuit (the ground), and the positive side of the circuit is fed with a wire. If a device is not properly attached to the body or frame, or the attachment point is corroded, the circuit is compromised and will not function properly. The first thing to do when a circuit fails is to make sure it is grounded properly.

Individual wires do not fail. Insulation may crack or burn off, but the wire will still conduct electricity. The only time a wire will fail is if it is physically damaged, cut or broken. Damage can usually be detected by following along the wiring loom and looking for cuts. If the outside of the loom isn’t damaged, it is safe to assume the wires inside aren’t damaged, either. If you suspect a bad wire, read on-wires don’t just go bad, but connections do.

Connections at the ends of wires fail regularly. Sometimes, they break or come loose. Other times, they corrode. Factory wiring harnesses usually do not hide connections under tape or other wrappings. When tracing a problem, follow the harness and verify that each connection is clean and functional. Switches, sensors, light bulbs, and microprocessors are all electrical components that are susceptible to failure. Components with moving parts, that generate or receive heat, or that are exposed to water or other corrosives are the most likely to fail.

In almost all cases, it is better to repair a factory circuit than to rewire around it. Adding circuits for new accessories is one thing, but don’t change the way one left the factory. Most factory electrical systems, including much-maligned Lucas systems, were carefully designed by trained engineers and work perfectly well until corrosion or component failure sets in. Shadetree mechanics who wire around factory circuits usually do so out of a lack of understanding. They often take shortcuts or make mistakes which can be dangerous (read as “will burn up a car”).

Tools Needed

Given these common problems, a 12-volt test light is an essential tool to diagnose and trace an electrical failure directly to its cause. A quality test light looks like an awl with a light bulb in the handle and a wire sticking out of the top. It should have a sharp point and an alligator clip at the end of an 18- to 36-inch-long wire. These lights are available for under $10, so every toolbox should have one.

Another helpful tool is a voltmeter. Voltmeters come in two flavors, digital and analog. Each type has advantages and disadvantages, but either works well for diagnostics. Pick the type you prefer in the cost range you can afford.

Voltmeters are usually combined with other measurement features. One typical combination is the Volt/Ohm meter, which includes the ability to measure resistance of a circuit in ohms. The other typical combination is the engine diagnostic meter, which usually will measure dwell (for points ignitions), current (amps), and include a tachometer function. Volt/Ohm meters are available from under $20 to much more. Engine diagnostic meters are more expensive-plan on spending at least $50, and much more for a high-quality unit. If you have to buy just one meter, start with a cheap Volt/Ohm meter.

Get a few pieces of jumper wire with insulated alligator clips on the ends. It’s a good idea to include an inline fuse in the jumper wire in case of mistakes or problems. A few wire brushes and a battery post cleaner should round out your electrical tools.

Troubleshooting Basics

Before troubleshooting a circuit, check and clean the battery terminals and check all fuses. Make sure the battery is fully charged. If a fuse is burned out, don’t just replace it and think the problem is solved. The circuit affected must be further tested to determine why the fuse burned out. More on that later. After these preliminary checks, use the test light to test circuits as follows:

  1. Check that the test light is working. Attach the alligator lead to a ground, and then touch the pointed end to the positive side of a circuit. The positive battery terminal or a terminal on the fusebox are good test points.
  2. Test the positive lead at the device. Leaving the alligator lead attached to the same ground that was used to test the light, touch the pointed end of the light to the positive connection of the device that is not working. If the light glows, there is either a bad ground or bad device. If the light does not glow, trace the positive circuit that leads to the device using Step 4.
  3. Test the ground at the device. Connect the alligator clip from the test light to the positive lead of the device, then touch the pointed end of the test light to a bare metal portion of the device, to its ground strap, or to a bolt that attaches the device to the body or frame. Ensure that you touch the pointed end of the light to bare metal, as paint or other coatings will not conduct electricity. If the light glows, you have power to both sides of the circuit and most likely the device has failed. If the light does not glow, clean or replace the bolts, nuts or ground strap to the device. A wire brush and/or sandpaper can be very effective for this.
  4. Test the positive circuit that leads to the device. Ground the test light’s alligator clip. Test the light again to ensure the quality of the ground. Working backward from the device, follow the positive wire to its switch, sensor, or source of positive current. Test the circuit at each connection along the way (i.e., each terminal block or snap connector). If the light glows at each connection after the switch, suspect the switch. If the light stops glowing at a connection, clean or replace the connector.
  5. Test a switch, if applicable. To test a switch, check that there is positive power to the switch by touching the positive lead on the “input” side of the switch. If the light does not glow, continue to trace the circuit back to the fusebox or battery. If the light glows, touch the positive lead on the “output” side of the switch. (A manual may be helpful here to show the location of these leads.) Move the switch through its range and see if the light ever glows. If the light does not glow, or glows in the wrong switch position, replace the switch. Occasionally, a switch can be repaired by spraying it with WD-40 or a similar lubricant/corrosion fighter. However, this is usually only a temporary fix.
  6. Test a sensor, if applicable. To test a sensor, microprocessor, or other black box, refer to a manual for testing instructions for that specific item. This is one area where more sophisticated equipment is often needed. Alternatively, replace the sensor or item with a known working item. The latter method is not usually practical, as new electrical parts are generally not returnable and few people keep working spares.

If these steps do not help solve the problem, keep in mind that it could be multiple problems. For example, a device may suffer from a bad ground and a loose connection along the positive side of the circuit. Two or more simultaneous problems are much harder to troubleshoot than a single problem. If you are still stuck, keep reading about common problems and solutions, or consider turning the problem over to an expert.

Common Problems. Common Solutions. Common Sense.

This set of problems and solutions is common to most cars, and dealing with them doesn’t require a lot of specific electrical knowledge, just some common sense. Of course, these are very general and may not work for some specific makes and models. If these don’t work, consult a manual or an expert.

Dead Battery

Charge the battery for at least one hour. Check for clean connections at the battery terminals, starter and grounds. Use the starter to crank the engine over five or six times. Attach a voltmeter to the battery and watch its reading as someone cranks the engine several times. The voltage should stay at 12 volts when the engine is not cranking. If the voltage drops below eight or nine volts while cranking, or the engine won’t crank any more, suspect the battery. If you suspect the battery, and it’s not very old, charge it longer and test it again.

Slow Battery Drain

If the battery is draining overnight or over the course of a few days, some device is still turned on and draining it. To find the cause, disconnect the negative battery connection. Use your test light to jump the negative battery cable to the negative post on the battery. If the light glows, something is turned on. Disconnect fuses and/or circuits one by one until the light goes out.

Trace the circuit that was causing the light to glow to find which device is still on. Dome lights, trunk lights, alternators, and non-factory accessory circuits are common causes of such drains. Radio memories and dash clocks usually are not drains and will not make the light glow for this test. Alternator Over- or Undercharging

Attach a voltmeter to a good ground and a good positive lead. (Usually, the battery works best for this.) With the engine switched off, the battery voltage should read 12 volts. With the engine running, the voltage should read 13.5 to 14.5 volts.

Below 13.5 volts usually signals a non-working charging circuit. Check for a tight belt, and clean connections at the alternator and the battery. Also make sure the engine is properly grounded. Above 14.5 volts usually signals a bad voltage regulator. Either way, the solution is usually a new or rebuilt alternator.

Crank Starter, Everything Goes Dead

Sometimes everything will seem just fine until you crank the starter, then nothing will work, not even the dome light. Starting with the battery terminals, remove them and give them a good cleaning. Then clean the ground strap to the body and to the engine. Then clean the positive connection to the starter. One or more of these connections is corroded. The load of the starter causes arcing at the corroded connection, which weakens the connection. Since these connections are the main power connection for the whole car, they shut everything else down when they get too weak.

Sticking Heater, Accelerator, Clutch, or Choke Cables

What does this have to do with electrical problems? Plenty. If the engine ground strap goes bad, the engine will seek another ground through these cables. Often, the car will run and start just fine. Over time, however, these cables will melt themselves to their housings. Replace the affected cables and clean or replace the engine ground strap.

Dim Headlight

Sometimes, one or both headlights will be dim. One of the headlights has a bad ground and is grounding itself through the other headlight. In doing so, the headlights change the wiring configuration from parallel to series. When wired in series, they each share half the voltage and glow dimly. Clean or replace the ground(s).

Brake Lights Turn off Taillights

This is a variation on the dim headlight problem. A bad ground is causing the brake lights to ground themselves through the taillight circuit and vice-versa. Clean up the grounds, and everything will work fine.

Turn Signal Problems

When a bulb burns out, most turn signals will either flash quickly or not at all. Sometimes, they do so even though all bulbs appear to be working. Other times, they may flash, but very slowly. If both left and right circuits act the same, suspect the flasher unit or the switch. If only one side has a problem, corrosion is at work. The solution is to first check and clean all the grounds, which often requires removing lamp assemblies to clean the bolts and attachment points with a wire brush. Sometimes, the base of a bulb will corrode, and simply replacing the bulb will solve the problem. Other times, the bulb socket is corroded and should be cleaned.

Blown Fuse

Finding the cause of a blown fuse can be difficult. A component in the circuit is either dead-shorted to ground, or is causing too much load on the circuit. If something is dead-shorted, fuses will blow the instant they are replaced and the circuit is turned on.

Physically search the wiring in the circuit, then disconnect components attached to the circuit one by one until you find the short. If something is generating too much load, the diagnosis is similar, but more difficult. Try to isolate any device on the circuit and see if its use blows the fuse. If you still don’t find the problem, check a manual or consult an expert for testing each device and ensure each device is in spec.

Intermittent Problems

Intermittent problems are the hardest to solve. If you can’t get the problem to happen while you’re looking for it, shake the car or the wiring harness and see if that causes it. Loose or corroded connections are common causes for intermittent problems and such shaking will often bring them about. If you still can’t solve it, call in an expert.

Wiring Additional Circuits

First and foremost, follow the accessory manufacturer’s instructions. However, many instructions suggest wiring directly to the battery to ensure a good power supply. Avoid this if possible. First see if there is an available accessory circuit in the factory wiring that can handle the necessary current. Many factory systems have extra accessory circuits and fuses built in for owners to expand. If you still want to wire directly to the battery, make sure you have a fusible link, fuse, or circuit breaker as close to the battery as possible. Also, avoid the self-resetting circuit breakers since they may reset before you know there is a problem.

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friedgreencorrado UltimaDork
9/27/09 9:55 p.m.

Articles like this are why this is the best magazine for people like us on Earth. I'm still a little afraid of going through my Alfa (the PO replaced a lot of the GTV6 switch gear & gauges w/Alfetta parts), but at least now I know how to start looking.

boxedfox (Forum Supporter)
boxedfox (Forum Supporter) Reader
12/8/20 8:32 p.m.

In reply to friedgreencorrado :

If it's any consolation, you can't make it any worse than it already is. From my experience, the electrical systems on those things randomly release electrons into thin air even when everything is wired properly.

David S. Wallens
David S. Wallens Editorial Director
12/8/20 10:45 p.m.

Hey, guess what article (and article author) helped diagnose an electrical problem tonight? laugh

Thanks, Carl!

Vigo (Forum Supporter)
Vigo (Forum Supporter) MegaDork
12/8/20 11:54 p.m.

I would just like to say that we are lucky to live in a world where a decent multimeter now starts at around $12. 

Martt New Reader
12/9/20 9:02 a.m.

This is a marvelously well-written article and Carl has provided a resource that should help many puzzled and electrically-inexperienced mechanics and owners.   I know for sure that if I had carefully read, understood and absorbed Carl's tips 60 years ago when I got started working on cars, they would have saved me lots of time and having to learn from hard experience.   And tips like the one about the starting system 'finding' an alternative ground path via control-cable sheaths are marvelous.    My only caution for the inexperienced is, (1) read what Carl says carefully, probably several times; then (2) ensure you understand what the test is actually measuring.   Once the underlying logic becomes clear, e.g. putting the test light across the power and ground terminals of a device to determine whether its ground is OK or faulty, then Carl's advice will suddenly make great sense and you, the novice electrical trouble-shooter, will have learned a lesson that will stand you in good stead over the years to come.


David S. Wallens
David S. Wallens Editorial Director
12/9/20 12:54 p.m.

In reply to Martt :

Thank you for the kind words. Carl is the best. 

The M3 has a battery drain. It has had one since we bought the car. Figure we should figure out why. Carl's easy tests confirmed that, yes, it has a drain and, no, it’s not a few likely culprits. So now we're digging deeper--and more to come as an update once we know more. But, yeah, this is a methodical approach vs. simply throwing parts/money at the car. Tools use so far are shown above: test light, volt meter, 10mm wrench, flashlight. (Not shown: cell phone to call Carl.) 

Thanks again. 

californiamilleghia SuperDork
12/9/20 1:14 p.m.

Anything on changing tail light and turn signal bulbs to LED bulbs and  how the turn signals flash ?

Also that antitheft systems are always on , and always draining the battery a little , at least the older ones.....

fearlesfil New Reader
2/14/21 7:58 p.m.
  • When troubleshooting, don't jump around looking at all manner of things. Work logically from one end to the other (either direction works). E.g. with a bad start circuit (the small wire that operates the solenoid) first check for 12V at the start wire at the starter. If that's zero volts (and assuming the starter is probably good) move to the neutral safety switch, then ignition switch, and finally the fuse block. Between where the readings go from bad to good is where the problem lies. Things can sometimes (rarely) get weird, like when I went through 4 new starters and traced the above circuits and all appeared good. Checking the voltage at the starter told the story: it dropped from 12V (disconnected with key to start) to 5V (connected) when trying to crank. The starter manufacturer had the wrong solenoid resistance and it just drew too much current! I could have switched starter brands (the best idea), or do what I did: let the starter circuit (low current) operate a relay with a bigger fuse and wire (higher current) coming directly from the battery. Been starting well for many years.
  • Fuses: in most cases the fuse is there to protect the wire it feeds. You may sometimes find a wire diameter good for 10 amps with a device that only draws 2. The fuse isn't sized wrong, it's protecting the larger wire, not the end device. On some newer cars with 286 fuses in 4 panels, there may be some desire to also protect the end device. But those low amperage fuses usually tree up with other circuits to a larger fuse up stream. When you look at the schematic diagram you may see two or more fuses between the battery and the end device. Don't let that confuse you, as the logical one-end-to-the-other method still works.
  • Fuses 2: When adding a fused circuit, the fuse goes as close to the power source as possible, and again is sized to protect the wire. That way, if the wire is shorted anywhere along it's length the fuse will blow, saving the wire. When you find and fix the short, the wire can return to service.
  • With more modern cars, use a test light designed for "computerized" cars. These draw very little current and are less likely to damage a computer or other sensitive circuitry.
  • Once in my 45 years of troubleshooting, a wire DID break. Recently a knock sensor showed an open circuit per the DTCs. Doing a resistance (ohm) test from sensor end of the wire to the computer end (note the battery should be disconnected for most resistance testing) showed an open too. There was no visible damage to the harness, pins, tape or connectors. Using the sharp point on the tester to check along the wire's length, the open was located about 2" from one end. Spliced around it and it was back in business. I think there was just a lot of vibration there behind the engine, and after 13 years of very small but repeated bending it just gave up.
engiekev HalfDork
4/21/21 7:57 a.m.

I need to get a powerprobe! They help a lot with being able to test circuits and power them up remotely to diagnose issues at the sensor or part.

 That, in addition to a good multimeter should be in everyone's electrical toolkit.

For really advanced stuff, an oscilloscope is also useful. PicoScope makes an affordable USB 2 channel scope that runs on any PC, well worth the cost of $115.  WAY more portable than a bench o-scope.


Scott Skillman
Scott Skillman GRM+ Memberand New Reader
4/21/21 4:19 p.m.

One exception to wires do not break is the NC1 Miata built from 2006 - 2008. The engine wiring harness can create lots of codes that are not fixed by replacing components due to bad wires in a loom. For me is the MAF sensor and O2 sensor heating circuits, and then a dead car. 

All was good once we replaced the wiring harness.

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