There you are — running late, coffee in one hand, keys in the other — when you turn the ignition and… click. Silence. You grab the jumper cables and call a buddy over, thinking it’s just another dead battery. But here’s the kicker: even after the jump, your car dies again mere minutes later. What gives?
The real culprit might not be your battery at all — you might be dealing with a bad alternator, instead. So, can you jump-start a car with a bad alternator? Here's what you need to know.
Will a Car Jump-Start With a Bad Alternator?
Providing that your battery holds a charge, yes — but don’t expect it to last long. When you jump-start a car, you give the car battery enough juice to crank the engine and get the vehicle running. Typically, this is where the alternator would come in.
The alternator charges the battery and keeps the electrical system alive after the engine starts, which is why jumping a car with a bad alternator is a bit of an exercise in futility. With a failing or dead alternator, charging may be unreliable or nonexistent, leaving your battery to do all the heavy lifting — something they’re not built to do. Eventually, your car will likely die and need to be jumped again.
Jumping a Car with a Bad Alternator — How Long Will it Run?
When the alternator is bad, your car runs on borrowed time — and that time is usually measured in minutes. We can’t provide concrete estimates of how long your vehicle will continue running with a bad alternator, since it depends entirely on your battery's size, charge, and health.
However, if your battery was fully charged, in good health, and accessory usage (AC, radio, headlights, etc.) was limited, you might make it quite a way without the car dying. But with a freshly jump-started battery, or one with low charge, the engine would shut off within a few minutes. Ultimately, driving without a functioning alternator isn’t worth the risk. In the best-case scenario, you get stranded in a parking lot or on the side of the road. In the worst-case scenario, your car dies somewhere you can’t pull off from, and you put yourself and others at risk.
Signs of a Failing Alternator
So, how do you spot the red flags before you’re stuck on the side of the road Googling “tow services near me”? Here are some of the most common signs of a failing alternator:
- Warning Lights: The battery warning or check engine light will typically illuminate when alternators fail.
- Inconsistent Headlight Brightness: If your headlights are dim or flickering, or counterintuitively, brighter than usual, it can be a sign that your alternator is providing inconsistent voltage, or none at all.
- Repeated Dead Batteries: If you have to jump your car frequently or keep changing batteries, but your battery keeps dying, your alternator probably isn’t pulling its weight.
- Strange Electrical Issues: Random glitches in electrical components, like slow window movement or issues with your radio, can also hint at an alternator that isn’t producing enough voltage.
- Strange Noises: If you hear growling, whining, or grinding noises from your engine bay, coupled with other issues on this list, you may have a mechanical failure in your alternator.
For a more in-depth exploration of alternators, and to rule out your battery and starter as the issue, read our post, “Battery vs Alternator vs Starter: Who’s the Culprit?”
What to Do If Your Car Won’t Stay Running After a Jump
So, you’ve jumped your car, driven a few blocks, and boom — your vehicle is dead again. After getting to a safe place or securing a tow home, here’s what you can do to diagnose the issue:
- Look for Warning Lights: When the car was running, did you see any illuminated warning lights on your dashboard? Most modern vehicles have a battery warning light or message that will appear on your infotainment center if the alternator isn’t charging your battery. If one of these indicators lit up, it’s time to bring your car in for service.
- Inspect Cables and Connections: Loose or corroded wiring can prevent the alternator from charging the battery. Make sure to disconnect your battery before touching any of these connections during troubleshooting.
- Check the Battery: If your battery is old, swollen, or the terminals are corroded, it could be part of the problem. For one older than three years, recharge the battery and check its output with a multimeter; a healthy battery typically provides 12.6 volts. If your battery is swollen, replace it ASAP — it’s a safety hazard. For corroded terminals, simply unhook the battery wires, clean the terminals and wires off, and reinstall.
- Check Your Alternator: You can use a multimeter to see if your alternator is generating power. To do so, start your car. While the engine is running, carefully connect the black probe of your multimeter to the ground on the back of your alternator, and the red probe to the output terminal. (This is VERY dangerous because of rotating parts.) If the voltage is around 14 volts, your alternator is generating sufficient power. (Some vehicles may output more voltage.) If the voltage is lower than 14 volts, the multimeter is likely just reading the voltage of your battery, signaling that your alternator isn’t functioning.
Ready to Stop Jumping and Start Driving?
A bad alternator can leave you stuck, and while a jump-start might get your car running briefly, it probably won’t keep it going for long. If you find yourself constantly reaching for the jumper cables or troubleshooting your electrical system, visit your local Tires Plus for a battery and alternator check to ensure your vehicle stays powered up and ready to roll.
