Jump Starting Classic Cars



My first car with an alternator was a ’64 Chevy. To jump-start, we had to unplug a wire. Is that still a must-do?


Having grown up in Minnesota, I have spent a fair amount of time jump-starting cars. At -40°, it simply becomes a way of life. My first car was a ’63 Chevy Impala SS, which spent a fair amount of time connected to jumper cables, either because of the weather or my personal lack of funds to purchase a new battery. I can even recall improvising jumper cables by touching the bumpers of the two cars together to establish a ground then using a tire iron (or two) to complete the connection of the two positive terminals. I don’t recommend doing it this way but sometimes you just need to improvise. In my experience, I have never needed to unplug anything to complete a jump start.

I have owned countless cars in the years since, with both alternators and generators and have never needed to disconnect anything prior to jump-starting. I have asked others as well after receiving your question, and can’t find anyone who recalls ever needing to unplug anything.

Although, if the car was older and had a generator as opposed to an alternator and was without ANY power it may be necessary to polarize the generator before starting.

HOW TO POLARIZE A GENERATOR – (Always check your cars service manual for specific polarization procedures for your year and make.)

For most Ford products: Disconnect the FIELD terminal wire at the voltage regulator. Momentarily touch this wire to the BATTERY terminal a couple of times. Note: Failure to disconnect the field wire at the regulator (using a jumper wire) could overload the regulator and ruin it.

For most other makes: Momentarily connect a jumper lead between the regulator BATTERY and ARMATURE terminals after all leads have been connected, but before the engine is started.

If you do need to jump-start your car with an alternator, leave the cars connected together for several minutes before starting to restore some charge to the dead battery. It is important to recognize, the alternator was NEVER intended to be a battery charger. If you forget and leave your lights on, never jump-start the vehicle and “drive it around” to build the battery back up. I know grandpa said you could do that, and in grandpa’s day of 35 amp generators, you could easily get away with it. However, most alternators are capable of making over 100 amps of current, but only for short periods of time. If you drive on a discharged battery, chances are, you will overheat your alternator. If it is not destroyed right then and there, it will surely be damaged and its life greatly reduced. Your first option should be to place the battery on a charger for a while before starting.

Wrench Safe,
– Mark

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13 Responses to “Jump Starting Classic Cars”

  1. ferd

    One time I did have to unplug a wire in order to jumpstart a car. I had to disconnect one battery cable on the car that I was trying to jumpstart (I disconnected the positive cable because it was more convenient). The dead battery in the car I was jumpstarting was overloading the alternator on the car that was providing power. After I accomplished the jumpstart I reconnected the cable and allowed that car's alternator to do its job. That car then drove away, and a few days later when I saw its owner again he said he hadn't had more problems but he did replace his battery as a precaution. I've jumpstarted a lot of cars over the years, and that was the only time I had to disconnect anything in order to get the job done. For some reason that one dead battery presented too much of a load. From what I remember, the car being jumpstarted was a typical midsize American model made in the late 1980s or early 1990s, with an original battery that was about six or seven years old. My jumper car was an early 1980s full-sized Pontiac with a 63 amp alternator and young (no more than two year old) battery. I jumped many cars using that Pontiac, and only had to disconnect the other car's battery that one time.

  2. Dan

    Back in the '80s, driving older '70s cars, my dad had told me to disconnect the alternator wire (on GM) of the car doing the jump so as to not damage the alternator on that car, bit on the dead battery car, you didn't need to. Not sure if it ever reallyade a difference, except maybe not overworking the good alternator.

  3. Steve

    I appreciate these articles and comments. Having jumped cars and pickups from the mid-60's to now, I never heard the "un-plug" precaution, or saw others using it. The generator equipped vehicle precaution is a good one to know about polarizing the generator, as, although people I saw jumping such vehicles in those days, did not do that (but that is a very old memory...). Bob's comments, above, about batteries blowing up is a good one to watch out for, as well as simply checking (for lead acid batteries) that fluid level in the cells is correct can help out as well. Starter relay condition can also make a car look as though it has a low or discharged battery. If the relay contacts get burned or overheated through extended low voltage/high amp cranking conditions, the car may crank like it has a weak/undercharged battery.

  4. Tj Donahoe

    Been jumping cars and pickups since the fifty’s And never disconnected any wires …. Never had a problem

  5. joe laplaca

    you talk about a63 chevy with a 64 chevy in the backround?

  6. Jimmie

    What about 6 volt battery jump start?

  7. Bob Marty

    Having worked part time and full time at my dads gas station for many years, living in Wisconsin l jump started and towed many vehicles over the years. Never, ever unplugged any wires before jump starting. The biggest lesson I had to learn (twice) was to never hit the switch on the Goodall starting unit sending 300 amps to a FROZEN battery on a cold winter day. Yep, twice I experienced battery’s blowing up. Didn’t need a 3rd lesson.

  8. Bobby

    Just the opposite maybe? Although if the car was older and had a generator as opposed to an alternator and was without ANY power it may be necessary to polarize the generator before starting

  9. Paul

    I have a 1950 gmc 1.5 ton truc It has a 6 with 4sp in it. I want to put a 6.0 with a 5sp or my 350'383 and 6 deuce set up but I have a borgwarner 4 sp for a old vette I had

  10. Paul

    I have a 6 deuce set up but the problem is I can not find someone to tune them after being rebuild 383