Your car’s Electrical System – How to spot a fault

What’s going on with your car’s electrical system – In this article our aim is to highlight what’s happening with your car electrics detailing the major components of your cars electrical system.  We will also highlight some hints and tips along the way.

Let’s start your car’s battery.

Your car battery’s main function is to start the engine and operate the electrical elements in your car.  Things such as your windows, lights, radio, sat nav all need the battery to work.  The battery itself typically consists of six cells of positive and negative lead plates, immersed in electrolytes made up of water and sulfuric acid mixture.  Care should always be taken if you are charging or jump starting your car because a chemical dangerous gases vent through the batter cover vents as a result of the chemical reaction occurring with the lead plates and the electrolyte.

For a safer battery you may want to look at batteries that use a gel instead of an electrolyte.  You can now use maintenance-free batteries so that you minimise concerns for the electrolyte element.  Eventually the batteries will, by their very nature, deteriorate over time so it’s not a complete solution rather an alternative more efficient solution

How to recognise when you are having battery issues

If your car is having problems starting the use can start with a very basic test.  If you know the exact condition of your battery you will be able to identify if it is actually the battery at fault or possibly another electrical component causing the issue.  If could be that you need a new starter motor or a replacement alternator.

All you need to test the battery is a battery, battery charger and a decent DVOM (Digital volt Ohm Meter). Your car handbook will normally tell you what colour to look out for.  Simply look at the DVOM for the colour to tell you if your battery is in good condition, needs charging, or has low fluid and needs replacing.

If you are guessing that the battery needs replacing, then beforehand, it might be an idea to just to take your car to an auto mechanic just to ensure it is the root cause of the electrical fault with your vehicle.  After all the last thing you want is to spend money and time replacing the battery only for you to find out that it was actually a parasitic drain on the system actually causing the battery failure.  It’s not just potentially expensive, it could actually be dangerous too!

It’s good to know that the average life of a battery is typically between 3 and 5 years.  Your local car electrics specialist is usually the best person to help you with this.


Let’s talk about your Alternator

The alternators main function is to help run the electrical accessories in your vehicle.  This includes the ignition and the engine controls system.   So how does it work.  In short It produces electricity to maintain battery storage charge.  It is driven by the engine producing an alternating current (AC).  We always recommend that you should check your car’s repair manual or with your local mechanic to obtain the correct information before working on your alternator.

Typically, the alternator will last around 3-4 years.  This is due to the demands placed on all the modern electrical devices (stereos, lights, windows, heated seats etc., you use in your vehicle. What’s always useful to know is that if your alternator is coming close to its end of life, this can put additional stress on the car battery.  By keeping your car servicing up to date, and keeping an eye on your engine management lights, you can often prevent these subsequently related issues.


In the table below we have highlighted some typical power usage on a car’s charging system:

Equipment Electrical Draw
Rear Window Defogger 25 amps
High Blower 20 amps
Headlamps (low) 15 amps
Windshield Wipers 6 amps
Ignition 6 amps
Brake Lights 5 amps
Total 77 Amps

When you consider if you charge your phone or laptop whilst in your car or have customised your vehicle this will create additional stresses.

It’s really important to pay attention to your car’s warning light for the alternator.  This will definitely help you catch any problems before they become a major issue.  Most modern cars have alternators that have the electrical current passing through the filament of the warning light is what energizes a circuit in the alternator to start charging.

To check the warning light circuit, turn the ignition switch to the “on” position without cranking or starting the engine; if the light does not come on, remove the plug from the alternator and ground the wire that terminates to the #1, I, L or D+ terminal (depending on manufacturer). If the light comes on, the wiring is okay but the alternator is defective; if the light still does not come on, the wiring to the light circuit and the bulb should be checked. (our mechanics at Widnes auto electrics would always recommend that you consult a trained auto electrician or your local garage if you are unsure about what this all means)

You should always check the fuse that controls the light circuit, too. It’s good to know that different cars use different labels such as “charging”, “regulator”, “meters”, “gauges” or “engine”. In some cars, if the fuse is out, the warning light will come on but may not go off. In others, a burned out fuse may make the warning light work in reverse order; that is, when the key is on, the light is off but as soon as the engine starts and the alternator starts charging, the light will come on.

Checking out these simple circuits first can greatly reduce the time spent trying to find out the cause of your car troubles and could help to prevent the unnecessary replacement of your alternator.

When it comes to charge light indicators, we find that in some cases it is normal for the charge indicator light to come on when nothing is wrong with the alternator.

According to information published by some of the leading car manufacturers, a car may have a low voltage reading or lights that dim when electrical loads are heavy at idle. Furthermore, this condition is normal and no repairs should be attempted unless a fault has been found.

For clarification, as a car idles for extended periods of time during high heat conditions, a number of things happen that contribute to “lowered” alternator output that coincides with physics and the design of the alternator:

  1. As heat within the alternator increases, the electrical resistance in the alternator also increases, which reduces the alternator’s charging capacity.
  2. As temperature rises, the voltage set point of the regulator is lowered to reduce the chance of overcharging or “boiling” the battery.
  3. More modern alternators have a “delay/soft start” built into the regulator circuit. This delays the load being placed upon the engine when starting up from a stop, so that the smaller engines in use today are not loaded down upon acceleration due to charging demands. This can delay the charging by up to 15 seconds.

With the alternator’s capacity for charging reduced by heat and other factors, an alternator may only be able to produce up to 70% of its rated output under these conditions. So an alternator rated for 100 amps may only be able to produce 70 amps when hot at idle when there is 77 or more amps of demand on it.

So the question to consider here is…If it can be considered normal for warning lights to glow while a healthy alternator is running, how do you know if the alternator is in good working order or if there are other problems waiting to happen?

A detailed diagnosis is always the best route to determining whether or not the alternator is at fault, but there are times when diagnosis time is short and you still need a positive identification of the problem. Cases like this require specialist tools which will enable you to isolate the alternator from the car’s wiring harness and lets you see if the alternator is at fault or if there is a wiring problem elsewhere within the car’s wiring harness. These tools will be able to you to see quite accurately if it is indeed the alternator causing you your car electrical problems if there is another mechanical failure at play!


Let’s consider faults with your car’s Starter Motor

What is a Starter Motor?  Put simply, the starter motor is simply a DC motor that turns the engine crankshaft through the flywheel, starting the combustion process by creating compression within the cylinders. Voltage to the starter is supplied directly from the battery and is controlled by a relay and/or solenoid operated from the key switch inside your car.  Starters can be of varying types and designs depending on the requirements of your car.  That said whatever the type, they all offer the same basis service, to get your car started

On older vehicles a slow cranking engine can typically be a sign of a bad starter motor.  On most modern cars however, it’s due to low battery voltage, poor electrical connections at the battery or a failed relay or fusible link.  It’s good to know that If you are choosing to buy a new car, most starters will easily outlast a new vehicle warranty if it’s not overused, if good connections are maintained and if it’s not overheated.

You can help reduce the workload of your starter motor (and increase its life) by simply starting your car with electrical components switched off (eg radio, air-con etc., window wipers.). To help you with this a lot of manufactures now have it so that the AC compressor and alternator will not turn on until after the vehicle has been started. (find out more on our dedicated starter motors page

Maintenance Tips For Your Car’s Electrical System

Its highly recommended that you get your car’s electrical system checked and tested at least every two years or whenever whenever you spot a problem with your vehicle. Many problems are often caused by voltage variations and must be one of the first steps in identifying any problem. Because there is a computer on board in almost every car built now, even quite minor voltage changes can alter the controls.

Your car’s electrical system must be load tested to certain standards, which can be simulated by turning on all the accessories and lights for simple voltage drain but that is not an all-inclusive test. Measuring circuit loads with an ammeter, circuit voltage drops with a DVOM, variable circuit load testing, etc. is usually the only way to fully check function. With electrical systems operating at 80%-100% of capacity nowadays (see Power Demand Chart), it is crucial that it be up to standards.

Whilst you may consider yourself capable of doing this testing process yourself, you might find that the test equipment to buy is too expensive when you think about how often you will actually use it during your car’s lifetime.  This is why we would always recommend an auto electrical expect who will have the right tools you’re your car manufacturers system.  This could save you a lot of time and money in the long run


Our top 5 helpful tips to help extend the life expectancy of your car’s electrical system.

Tip #1: Always keep your battery and its connections clean to avoid clogged battery cover vents and overtaxing your starter. This will also allow for proper ventilation of dangerous, explosive gases from your battery.

Tip #2: When replacing your battery, always buy one of the same or higher CCA rating (cold cranking amps) as the original battery and make sure it’s the same or compatible “group size” to fit your battery tray and cable connections.

Tip #3: Due to the varying nature of car electrical systems, never jump start your car using another car that is running. Use the other vehicle’s battery power alone to start it because a 14.5 volt running system can seriously damage a 12.6 volt system due to the overvoltage.

Tip #4: Start your car with the major electrical components turned off – A/C, stereo, etc. – to ease the load on your battery and starter and extend their lives.

Tip #5: Have your car’s electrical system completely checked and tested at least every two years or whenever you have it serviced for any type of driveability issue.

Widnes Auto Electrics  – Remember to come back  to check for new maintenance topics.

Note : these repair tips are designed only as a starting point.
Please seek the assistance of a professional mechanic
for all repair problems beyond your capabilities.


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