Rick's Free Auto Repair Advice

Car jump start tips

How to perform a car jump start safely

Car jump starting has always carried some car jump start cablesinherent risks. Jump starting a deeply discharged battery, for example, can sometime cause it to explode due to the escaping hydrogen gas. That’s why all car jump starting instructions tell you to make the final connection to an engine or body component far away from the battery. That way, any spark created during the final connection won’t be near the hydrogen gas. But there are several other safety tips.

Turn off ALL accessories before connecting car jump start cables

The surge spark that’s generated when you make the final connection is an arc caused by the flow of power from the donor car to the dead battery. But if you leave any accessory on in the dead vehicle, you create even more power demand when connecting cables. Worse yet, that arc produces very high voltages that can damage sensitive computers. So turn them off.

Turn off headlights, blower motor, wipers, rear defroster, seat heaters and even your radio in the dead vehicle! Seriously, why risk damaging a $500 factory radio during a jump start.

Car jump start refresher course

IN the old day most car makers used red insulation on the positive battery cable and black on the negative. That’s not always the case anymore. So you must look for the + and – marking on the case near the terminals.

Connect positive from the dead vehicle to positive on the donor vehicle

Turn off the headlights, seat warmer, blower motor, rear window defroster in the donor vehicle before connecting the cables. The alternator in the donor vehicle cannot generate enough power at idle to power all those accessory AND still pump out enough current to charge the battery in the dead vehicle.

With the engine on the donor vehicle running, connect negative on the donor vehicle to a metal connection on the engine of the dead vehicle as far away from the battery as possible. Let the donor vehicle charge the battery in the dead vehicle for at least 5-minutes before trying to start the dead vehicle.

Once the dead vehicle starts, disconnect the negative cable first.

Cranking tip during the jump start

The owner of the dead vehicle most likely tried multiple times to start the vehicle and may have pumped the gas pedal. In other words, they may have flooded the engine. If the engine on the dead vehicle cranks strongly but won’t fire up and you suspect a flooded engine, depress the gas pedal all the way to the floor and hold it there. That puts the computer into “clear flood” mode and the ECM shuts off the fuel injectors. The full throttle with no added fuel can help evaporate the excess fuel in the cylinders. After cranking for a few seconds with full throttle, release it and see if you get any signs of life from the engine.

After jump starting the engine may run rough

When battery voltage drops to below 9.6-volts, the ECM loses its adaptive memory; those changes to factory programming that compensate for engine wear. Don’t panic. Your ECM will relearn that data over a period of time, usually the next 10 cold starts.

Engine won’t idle or accelerate after jump start

As mentioned above, the computer loses its memory when battery voltage dips below 9.6-volts. On a late model vehicle, that may mean the electronic throttle body no longer recognizes it’s “home” position. In that case you may have to perform a throttle body relearn procedure. Unfortunately, the procedure is different for every year, make, model and engine. I’ve posted some of the relearn procedures on this site. Just search for the term “relearn” to see if yours is included.

©, 2019 Rick Muscoplat

Posted on by Rick Muscoplat



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