The links between a bad alternator and an engine misfire!

(Last Updated On: September 12, 2023)

A bad alternator can cause a misfire which will be explained in more detail, but it is rare. An alternator is a component that supplies the electrical load (voltage) required to power a cars electrics with a motor running. Without the engine running, voltage is taken from the battery, the energy store for the electricity supplied by the alternator.

A bad or failing alternator fails to supply the voltage at a consistent rate which puts pressure on electrical components to work with a reduced power source, causing a misfire.

Why can a bad alternator cause a misfire?

Modern engines use many supporting electrical components to keep them operating correctly; there are a couple of ways an alternator causes a misfire. An alternator supplies a stable specified voltage consistently with the motor running.

Because a bad alternator fails to supply the correct voltage levels required, these electrical components fail to operate correctly; this can cause limp home mode, the engine management light to illuminate, or just a simple misfire.

For example, suppose the coil pack fails to operate correctly because of a reduced voltage problem caused by the alternator. In that case, the spark supplied by the coil pack to the plugs is insufficient to ignite fuel in all cylinders, which is what a misfire is.

A coil pack/ spark plugs not receiving an excellent electrical supply is the most common cause of an alternator-induced misfire. This is the case for the coil pack and many other electrical components.

What are the symptoms of a bad alternator?

The symptoms of a bad alternator start as electrical components in the car struggle to operate. If you are trying to use anything electrical, the alternator must supply the voltage to power them. An increased load on the alternator that no longer provides the correct voltage consistently will result in the car’s electronics struggling to operate. Most vehicles use a 12volt supply, but some older classic cars use 6volt, and then large trucks tend to use 24volt. The most common found symptoms are:

Dim/dimming headlights

The headlights require a constant 12-volt supply to stay at their brightest. When you have a bad alternator and are driving along, the voltage supplied by the alternator is irregular. The lights will only receive a fraction of the voltage then all of a sudden, the voltage spikes, and the headlights become bright for a second.

To explain, only supplying a 12-volt headlight with 6 volts will result in dimmer lights. Providing the lights with over 12 volts and the bulbs will become bright, which will blow the bulb if it’s weak.

Radio not working

For the same reason as headlights, the radio will become temperamental without an entire 12-volt supply from the alternator. The sound will from the speakers be quieter, if any sound at all, and the lights on the radio display will also be dimmer. Again this is because more than the voltage supplied by the alternator is needed to keep it fully operating.

Windshield wipers struggling 

The wipers are another item that relies on a 12-volt supply when operating. The additional load on the alternator when using the wipers will reduce the speed and chances they will manage to clear the screen. They may work as expected for one or two cycles, but you will notice they start to struggle, and their movement will slow until they fail to operate.

Difficulty starting the engine

Although the battery stores the electrical load to start the battery, with a bad alternator, there is no voltage going into the battery to store. A typical 12-volt car requires the battery to have between 11.4 and 12.4 volts to turn over, some vehicles will start with a lot less than 11.4 volts, so these are just typical numbers. 

A failed battery is one of the distinctive signs the alternator may have failed; it doesn’t always mean an alternator is bad. Of course, batteries fail on their own. It doesn’t even tell you the battery needs replacing if the alternator fails, and you should get the car going with a jump or bump start. But you will ultimately struggle to start the vehicle without any intervention.

Smoking alternator

An alternator smoking is usually overheating; this isn’t always because it is overworking; it’s generally because a bearing inside the alternator has failed. An alternator overheating stops supplying the correct voltages required and then overworks till it dies.

The bearings dry out because they are faulty or with age; the contact of dry bearings on the outer casing creates friction, where the heat and smoke come from. The other thing that happens with dry bearings is they are noisier, so you will hear them.

Battery light

The illuminated battery light indicates that the alternator is not putting out enough volts back into the battery People get confused and assume it means the battery is failing, but it is the alternator. It doesn’t always come on in many vehicles, but if it does come on, you can be sure of the issue.

Can you drive with a bad alternator?

You can drive with a bad alternator if the vehicle starts, especially during the daylight without the headlights on; the issue comes if you stall or turn the motor off. As soon as you turn the car off, the alternator may not have supplied enough voltage to the battery to restart it again.

The trick to driving with a bad alternator is to reduce electrical consumption as much as possible, So no heater, no lights, no radio, no aircon, no charger cables, and no operating electric windows. That way, you have a chance that the alternator may just about put enough voltage back into the battery to restart it. If the alternator is too far gone, you will have no hope. Once jump started, the car may stall, wasting your time trying to drive it.

How do you check if a bad alternator is causing a misfire?

Checking to see if the alternator is working is the first thing to do. Connect a voltmeter to the battery and have the motor running at idle The voltage displayed should be between 13.4 and 14.4 volts if the alternator is good. If it is below or above that or an inconsistent reading, the voltage spikes and drops rapidly, and the alternator is no good. From there, you have a good indication the alternator may be causing an issue.

You can check the voltage supplied at each component is sufficient enough to operate correctly, but this is a lot of time-consuming work to identify that the alternator is at fault. It would be entirely academic when you already know the alternator isn’t operating correctly.

Final thoughts on a bad alternator causing a misfire

As briefly mentioned in this article, a bad alternator can cause a misfire, although it is rare. A misfire is usually created by other failed components rather than the alternator. Because the alternator supplies the electricity required to power said components, it is the reason you occasionally get misfires caused by the alternator. Solving this problem involves replacing the alternator; if you have a bad one, replacing the alternator first will indicate if you have another issue or if the alternator was the problem.

My name is Tom although my friends call me Tommy. Messing around with cars and bikes has always been a hobby of mine even from a young age. So I made it my day job 17 years ago. I am a fully qualified mechanic as you would expect. I've worked in all different areas of the motor trade, valeting, panel beating, engine repairs, I'm sure you get the idea. I enjoy sharing my wealth of knowledge and experience with others, which is the reason I spend a lot of time here writing for this website.

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