What is the correct way to use jumper cables to start one's car?

Dear Car Talk

Dear Car Talk | Jun 01, 1997

Dear Tom and Ray:

Although it's too late for me, since I've already done irreparable damage,
just what is the correct way to use jumper cables to start one's car? I've
heard every possible variation on how to connect and start one's dead car
with the tangled mass of black and red cables that live in every trunk.
Thanks. -- Shelley

TOM: Is THAT what that tangled mass of cables in the trunk is for? I
thought that was the speaker wire for my eight-track player.

RAY: You have three objectives when you're jump-starting a car, Shelley.
The primary objective is to not cause an explosion. Objective No.2 is to
not damage either car's electrical system. And objective No.3 is to get the
"dead" car started.

TOM: The way to avoid damaging the electrical systems is to clearly
identify the terminals of both batteries. Make sure you know which terminal
is positive and which is negative before you start. Unfortunately, the
markings aren't always as clear as they should be, so you may have to
scrape off some dirt or use a flashlight to get a clear look at the marks.

RAY: Once you've identified all of the terminals, you can proceed to the
next step: avoiding an explosion. When batteries explode, it's because a
spark has been created in the vicinity of the battery. So the key is to
avoid creating a spark anywhere near the battery.

TOM: Sparks can only fly when you make the very last connection of the
jumper cables and complete the circuit; in other words, when you have three
cables' ends hooked up and you're attaching the fourth. That's why you want
to always make that last attachment away from the battery, and here's how
you do it.

RAY: So here's the procedure. First, make sure the "live" car's engine is
turned off. We find this cuts down on necktie-in-the-fan-belt syndrome.
Although you could start with either car, we're going to suggest you start
by taking one end of the red (positive) cable and clamping it securely onto
the positive terminal of the "live" car's battery.

TOM: Next, take the other end of that red cable and clamp it to the
positive terminal of the "dead" battery.

RAY: While you're over at the "dead" car, take an end of the black
(negative) cable and attach it to the negative terminal of the "dead"

TOM: And then, to avoid an explosion, you attach the other end of the black
cable to some large, metal part of the engine block on the "live" car --
not to the negative terminal of the "live" battery. The negative terminal
of the battery is simply a ground terminal. And by using another, more
remote piece of metal to ground the circuit, you can make sure that any
sparks that do fly are kept far away from the battery.

RAY: OK, now you've avoided both damage to the electrical systems and a
major explosion. Good! You're two-thirds of the way there. Now you're ready
to start the car.

TOM: Make sure all accessories (lights, air conditioner, etc.) are turned
off in both cars. Then start the car with the "live" battery and keep it
revved at medium speed (about 2,500 rpm) for a few minutes. The alternator
of the good car will then be effectively "charging up" the "dead" battery.
Then try starting the "dead" car. If it doesn't start, rev the "live" car
for five or 10 minutes more, and try again.

RAY: If it doesn't start after that, give up and call for help, because
something else is probably wrong. Continuing to mess around with it at this
point is only going to increase the chances of failing to achieve Objective
No.1(see above).

TOM: Once the car with the dead battery has started, you remove the cables
in the exact opposite order that you put them on, again taking care not to
create sparks or get your ponytail stuck in the fan.

RAY: Then you throw them back into the trunk so they can resume their
historic role as a mass of tangled black and red cables. But clip this
article and tape it to the cables so you'll know what to do next time,

* * *

What's one secret of financial success? Driving a used car! Read How to Buy
a Used Car: Things Detroit and Tokyo Don't Want You to Know. You can order
it by sending $3 and a stamped (55 cents), self-addressed, No.10 envelope
to Used Car, PO Box 6420, Riverton, NJ 08077-6420.

Get the Car Talk Newsletter

Got a question about your car?

Ask Someone Who Owns One