What are the tell tale signs of connecting a jumper cables the wrong way. Will other sensors blow first?

If there is a voltage difference between cars could the alternator blow.

I helped jump someone and their alternator is broken. If I am told I am responsible, how would I know? Thanks

  • Welcome to Motor Vehicle Maintenance & Repair! Dec 4 '19 at 1:23
  • Happened to a workmate of mine. Fuse blew, all the lights in the car were blinking oddly on and off, would not start, would not crank. Question: Why did you have to help them to jump their car? Was their alternator already bad?
    – Chris
    Dec 4 '19 at 2:01

If you connect a set of jump leads the wrong way, you're likely to see a pretty big spark as you connect the last one - even if the battery you're jumping to is completely dead, the alternator's diode pack can carry a couple of hundred amps. If there's a fusible link for the alternator that will fail pretty quickly.

Most likely, the car won't even crank, let alone run with the jump leads connected backwards, so if it did so it would indicate that it was connected correctly. With older direct drive starter motors that usually had a wound field, these will run the correct direction even if connected backwards, and a points based ignition system will still generate a spark, so that it would maybe start, this would be true of many cars from the '60s or earlier. Newer cars with permanent magnet field gear-reduced starter motors will not turn the engine if reverse connected. Anything with electronic ignition will not start. By design, the electronics are supposed to be able to tolerate both reverse connection and load dump (more on that later).

The alternator has by design a limited maximum current, it won't run at that point for more than probably 10 minutes without getting excessively hot, but it'll tolerate a couple of minutes charging two batteries in parallel.

On disconnecting the jumper cables, if the alternator on the running car can produce a spike if the jumped battery is still very dead and has a high internal resistance. This is known as a load dump, and many alternators now have protection built in in the form of Zener diodes in the bridge, or an additional suppressor on the positive terminal.


If you did it the wrong way I would expect the main fusible link to have blown on one or both cars.

If the alternator failing is not the reason they needed the jumpstart in the first place, alternator failures are very common if someone drives the car trying to use the alternator to charge the battery all the way up from dead. Alternators are designed to maintain battery level, not do a full up charge.

  • Not all cars have fusible links...
    – Solar Mike
    Dec 3 '19 at 17:39

It is possible to damage an alternator during a jump start and even after a jump start. The voltage regulator is solid state so any voltage spike from the finall connection spark can fry it. That doesn't necessarily mean you perform the jump improperly. Usually crossed polarity fries the fuses. Also, if you jump start the dead car and let it run, the alt in the dead car immediately goes to full output the instant you remove the jumper cables. That sudden voltage spike can damage the alternator. Again, not your fault. These are all side effects of jump starting a modern car with computer while using jumper cables. That's why the professionals always use a jumper pack. It avoids voltage spikes.

  • How does a jumper pack avoid a voltage spike? Wouldn't a spike still occur when the jumper pack is disconnected? Dec 3 '19 at 22:33
  • Itl lessens the possibility because it only outputs 12+ volts. When you disconnect jumper cables, the donor vehicle is usually outputting 14+ volts.
    – user9181
    Dec 4 '19 at 1:06
  • Makes sense, thanks. Dec 4 '19 at 3:17

There was no big spark, my car is a 2.6 VW Microbus 2006 and his car a 1.4 Reno clio or something. His negative cable melted, and was smoking after I had: 1) Checked to see if I could roll start it - didnt roll start - or have any lights turned on - completely dead. 2) Started my car, 3) Then connected jupmpers 4) Then went to his divers 5) Tried to turn it on 6) Saw smoke coming from the battery 7) was the negative cable - from previous experience small thickness cables have melted

Appreciate your answers guys.

  • You should edit your question to add that information rather than putting it in an answer. It makes it easier for someone coming along and reading the question if all the information is in the one place. Dec 4 '19 at 12:05

The correct way to jump start: put red jumper cable to positive post or remote positive terminal on discharged vehicle. Attach other red clamp to working vehicle. Attach black cable end to negative post on dead battery. Attach other black cable to engine ground on working vehicle. As mentioned, it is risky to jumpstart newer cars.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.