Jump starting someone’s car can kill my alternator?

Hi! Yesterday morning I jump started my neighbors car. Her car is a significantly newer and bigger svu my car is a 2004 Toyota Camry. Apparently some stuff in her car is already fried. Now that same night my battery light came on and made a low humming v whining sound when I started it and when it ran. We took the battery which is only a few months old and we took it to get checked out and they said it’s was perfect but the alternator was running severely low. I just took it to the dealership a week ago and they said everything was fine in my car. So can jumping someone else’s car kill my alternator and if so what’s the best and cheapest way of fixing this issue?

Yes it can and a new alternator is in your future . From now on don’t jump from vvehicle to vehicle . That is why they sell rechargeable battery packs or do as I do and call road side service.


Remove it and take it to an auto electric shop. They can diagnose and fix it. Reinstall it.

(Be sure to disconnect the battery negative cable first, and know how to wake up your radio when it’s all reinstalled: there may be a security code to enter on the radio.)

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I don’t know if you have any damage, but I can recommend one of those jump starter / phone / table power supplies. Keep it as part of your travel kit and recharge it in the car.

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I have seen too many horror stories. AAA jumped my 2017 with a portable thing, daughters battery died after warmed up at a gas station. $70 I think for the portable, No more jumping cars in my future.

Hi, did you notice an idle speed above the normal (around 1400 or 1500 rpm) ?
Sometimes, when the ECU detects an abnormal charge to the battery, it revs up the engine to compensate the poor alternator functioning.

Also, it might be useful to measure the voltage with a multimeter to check if the value is normal.

This is what you need. That thing never failed me

The cables were connected wrong, the polarity was reversed. This is also what damaged the alternator in your car, the diodes are burned.


That is a leap to a conclusion, things happen even when connected correctly.

I have repaired a number of vehicles that were towed in after a jump start fail. The main fuses blow, some fuse assemblies can cost over $200.
If the cables are reversed the diodes in the alternator will overheat in a short time.

Occasionally, Woot! has the NOCO boost packs and battery chargers on sale. I got the 400Amp version for $50 last fall.

Why are you telling me ? I don’t have this problem .

I agree with Nevada. All wiring and components are protected by fuses and fusible links. The only thing not protected is the battery positive and negative cables and since they appear to be fried…

Old saying about no good deed goes unpunished.

Absolutely! Jump starting can kill your alternator, and lot of other things on a modern computerized car. Damage is possible even if the cables are not reversed. A simple fuse does little to protect a computer from the instantaneous voltage spike caused by the arc that occurs when jumper cables are connected or disconnected.

  1. Ideally, you should use a dedicated jumper battery as other have suggested.
  2. Use safety glasses if possible.
  3. If you must use another car, connect the positive to the battery terminal and the negative to the a good body ground on both cars. Positive first. Under no circumstances should either car have its key in the On position while cables are being connected.
  4. If you can, start the helper car, speed it to fast idle, and let it charge the dead car for 5-10 minutes or so. Then turn off the helper car. Then try to start the dead car.
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Excuse me. The reply function on this furom software is different from I’m used to. I intended to respond to the OP.