Rick's Free Auto Repair Advice

Car Won’t Start With a Jump: The Biggest Mistake DIYers Make

Learn why a car won’t start with a jump

If your car won’t start with a jump, it’s almost always because the battery is too discharged, or your jumper cables are garbage. Read on to learn more about jump-starting a vehicle.

1) Cheap jumper cables will let you down.

Let’s use a water pipe analogy. Think about how much water can flow through a 1″ diameter water pipe compared to a 1/8″ diameter water pipe. You’ll agree that the small water pipe can’t flow as much water, right? Well, the same thing applies to electricity. Small gauge wires, the kind found in cheap jumper tangle free jumper cablescables, simply can’t transmit as much power as heavier gauge cables. It takes almost 60-150 amps at 12 volts to run a starter motor with no load; in other words, not connected to your engine.

With a cold engine, it takes even more power to rotate the engine. If you’re using a cheap set of jump cables, you’re trying to transmit power through a straw.

If you’re using a cheap set of jumper cables, don’t be surprised if your car won’t start with a jump.

I’ve helped people jump-start their cars using my heavy-duty set of jumper cables when their jumper cables couldn’t get any response at all. When it comes to jump-starting, thicker cables are always better. See this post on jumper cable basics and how wire gauge and the length of the cables determine how much power they can carry.

2) It won’t start because you have a seriously dead or shorted battery

Most DIYers think that the power flows from the good vehicle right to the starter motor of the dead vehicle. They forget there’s a battery in the circuit and that battery is d-e-a-d. The instant you complete the jumping circuit you’re starting to charge the battery. In fact, that dead battery is like a black hole, and that is where all the power is going; to recharge the dead battery. The biggest DIYer mistake is not allowing enough time to recharge the battery using a good vehicle as the charger.

Leave the cables connected for at least 10 minutes before trying to start the dead vehicle. If it still won’t even crank, chances are the dead battery has an internal short. If that’s going on, the car won’t start with a jump. You’ll have to replace the bad battery.

3) You left accessories on while trying to jump-start

Turn off the headlights, blower motor, heated seats, rear defogger in BOTH vehicles while you’re jumping the dead battery. A vehicle alternator can only output about 1/3 of its maximum capacity when idling, so there’s no way it can recharge the dead battery and still power all those electrical accessories at the same time.

If you’ve followed those instructions and the dead vehicle still won’t crank, try cleaning the battery posts and terminals. High resistance in the posts/terminals will prevent power from recharging the dead battery and the starter and probably caused the dead battery condition in the first place.

4) Bad connections

All the instructions tell you to connect the negative jumper cable to a good ground location. But with so many plastic components in cars, that can be hard to find. I’ve seen people connect the negative cable to the plastic engine cover and then wonder why the car won’t start with a jump.

Use the flashlight on your phone and find real metal. Wiggle the clamp to make sure you’re getting a good bite. You can’t transmit 150 amps if you don’t have a good connection.

How to clean battery terminals

If you don’t know if the problem is a dead battery or bad alternator, read these articles:

Test your alternator

Find what’s draining your battery

Charge your battery

©, 2014 Rick Muscoplat

Posted on by Rick Muscoplat



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