You’re stranded with a dead battery, and you suspect the alternator is the real culprit. Can you jumpstart a car with a bad alternator? The short answer is yes, you usually can, but it’s a temporary fix with some very important caveats.
This article will guide you through the process, explain what happens next, and help you understand the risks. Knowing these details can prevent further damage to your vehicle and get you to safety.
Can You Jumpstart A Car With A Bad Alternator?
Technically, a jumpstart provides the initial power to crank the engine from an external source, like another car’s battery. If the alternator is failing, a successful jump can get the engine running. However, the alternator’s job is to recharge the battery and power the electrical system while the engine runs. A bad alternator won’t do this, meaning your jumpstarted car will only run until the borrowed charge in the battery is depleted.
How an Alternator Works With Your Battery
Think of them as a team. The battery provides a burst of power to start the car. Once the engine is running, the alternator takes over. It generates electricity, replenishes the battery, and powers everything from the headlights to the radio. A healthy alternator keeps the battery fully charged for the next start.
When the alternator fails, it stops supplying power. The car is forced to drain the battery to run all its systems. This quickly depletes the battery, leading to dimming lights, slow electronics, and eventually, a complete stall. That’s why a car with a bad alternator often dies while driving, not just at startup.
Signs Your Alternator Might Be Bad
Before you even try to jumpstart, look for these clues that point to an alternator problem rather than just a dead battery:
- Dim or Flickering Lights: Headlights and dashboard lights that dim at idle and brighten when you rev the engine.
- Warning Lights: The battery or ALT warning light on your dashboard is illuminated while driving.
- Electrical Failures: Power windows moving slowly, a weak stereo, or malfunctioning accessories.
- Strange Noises: A grinding or whining sound from the engine bay can indicate a failing alternator bearing.
- The Car Dies While Driving: This is a classic sign. A pure battery issue typically only prevents starting.
- Burning Smell: A smell like hot wires or rubber could mean the alternator is overheating.
Step-by-Step: How to Jumpstart with a Suspected Bad Alternator
If you need to get the car moving to reach a repair shop, follow these steps carefully. Remember, this is a get-out-of-trouble tactic, not a solution.
What You’ll Need
- A set of jumper cables (heavy-duty recommended).
- A donor vehicle with a good battery (turned off).
- Safety glasses and gloves (optional but smart).
The Jumpstart Procedure
- Position the Cars: Park the donor car close to yours, but ensure they do not touch. Turn off both engines and set the parking brakes.
- Identify Terminals: Locate the positive (+) and negative (-) terminals on both batteries. They are usually marked clearly and may have red (positive) and black (negative) covers.
- Connect Positive to Positive: Attach one red clamp to the dead battery’s positive terminal. Connect the other red clamp to the donor battery’s positive terminal.
- Connect Negative to Ground: Attach one black clamp to the donor battery’s negative terminal. Connect the final black clamp to an unpainted metal part of your car’s engine block or chassis. This is a ground connection and is safer than connecting directly to your bad battery’s negative terminal, which can minimize sparking.
- Start the Donor Car: Start the engine of the donor car and let it run for a few minutes. This allows its alternator to begin putting charge into your battery.
- Attempt to Start Your Car: Try to start your vehicle. It may crank slowly at first. If it starts, let both cars run connected for another minute or two.
- Disconnect in Reverse Order: Carefully remove the cables in the exact opposite order: your car’s ground clamp, the donor’s negative clamp, the donor’s positive clamp, and finally, your car’s positive clamp.
What Happens After the Jumpstart
If your alternator is truly bad, your car will now be running solely on the limited charge that was transfered during the jump. You have a very limited window of operation. Here’s what to expect and do immediately:
- Minimize Electrical Load: Turn off EVERY non-essential electrical item. This means the radio, air conditioning, fan blower, heated seats, and even the defroster. Use only your turn signals and headlights if absolutely necessary for safety.
- Do Not Turn the Engine Off: If you shut the car off, you will almost certainly need another jumpstart, as the battery has not been recharged.
- Drive Directly to a Repair Shop: Your goal is to get the vehicle to a mechanic before the battery drains completely. Avoid idling for long periods; the alternator charges better at higher RPMs, so driving is actually better than sitting still.
- Be Prepared to Stall: As the battery drains, your dash lights will dim, and the car may begin to run roughly before finally stalling. Try to pull over safely if this happens.
Risks and Why It’s a Temporary Fix
Jumpstarting with a faulty alternator is a band-aid. The core risks include:
- Stranding You Again: The car will stall once the battery charge is gone, potentially in a dangerous location.
- Damaging the Battery: Completely draining a car battery repeatedly can ruin it, turning one repair (alternator) into two (alternator and battery).
- Overloading the Donor Car: In rare cases, a severely bad alternator can have internal shorts that might put a strain on the donor vehicle’s charging system.
- Electrical System Issues: As voltage drops, your car’s computer modules may behave erratically or get damaged.
How to Test if the Alternator is Really the Problem
You can do a simple test after a jumpstart to confirm your suspicions. Once the car is running, carefully disconnect the negative battery terminal. Warning: This is an old-school test and can be risky on modern computer-controlled cars; it may cause voltage spikes. A safer, modern method is to use a multimeter.
- With the engine off, a healthy battery should read about 12.6 volts.
- After a jumpstart and with the engine running, have a helper measure the voltage at the battery terminals. A functioning alternator should produce between 13.8 and 14.8 volts.
- If the reading is below 13 volts or fluctuates wildly with engine RPM, your alternator is likely not charging properly.
Many auto parts stores offer free charging system tests, which is the safest and most accurate option.
Permanent Solutions and Repairs
A jumpstart is not a fix. You need to address the root cause. Here are your options:
- Alternator Replacement: This is the most common fix. A mechanic will install a new or remanufactured alternator.
- Belt and Connection Check: Sometimes, the issue is a loose serpentine belt that drives the alternator or a corroded connection. A mechanic will check these first.
- Battery Replacement: If the battery was deeply drained multiple times, it may need to be replaced along with the alternator. A good shop will test both.
Towing the vehicle directly to a repair shop, while an upfront cost, is often safer and cheaper than risking damage from driving with a failing charging system.
FAQ Section
How long will a car run with a bad alternator after a jump?
It depends on the battery’s state of charge and your electrical load. With all accessories off, you might get 20-30 minutes of drive time, sometimes less. It’s very unpredictable.
Can a bad alternator ruin a new battery?
Absolutely yes. A bad alternator will fail to charge the new battery and will eventually drain it completely, which can significantly shorten its lifespan or kill it outright.
Will jumpstarting a car with a bad alternator damage the donor car?
It’s unlikely but possible if the bad alternator has a severe internal short. For most standard jumpstarts, the risk to the donor car is minimal if the connections are made correctly.
What’s the difference between a dead battery and a bad alternator?
A dead battery means the car won’t start, but if jumpstarted, it will continue to run normally because the alternator is working. A bad alternator means the car may start (if the battery has charge) but will die because the battery isn’t being recharged. Often, a bad alternator leads to a dead battery.
Can I just keep jumpstarting my car instead of fixing the alternator?
This is not a sustainable solution. Each jumpstart drains the battery further, and you will eventually ruin the battery. The car will also become unreliable and strand you. The only proper fix is to repair or replace the alternator.
Is it safe to drive with a bad alternator if I just got a jump?
It is only safe to drive directly to a nearby repair shop with minimal electrical load. It is not safe for a long trip or daily driving, as the car can stall without warning, leading to a loss of power steering and brakes.
In conclusion, while you can physically jumpstart a car with a bad alternator to get it moving, it’s a short-term emergency measure. The success depends on the battery retaining enough charge, and the car will not run for long. The moment you get that alternator warning light or experience the signs of charging failure, your next stop should be a mechanic’s shop. Ignoring a faulty alternator will inevitably leave you stranded and likely turn a single repair into a more expensive one involving the battery as well. Always prioritize a proper diagnosis and fix for your vehicle’s charging system.