Can you jump a car using a car with a dying alternator? (vehicle, auto)

[Kluch]

School me on this. Is it possible to jump start a car with cables using a different vehicle who's alternator (possibly) isn't working as well as it should. Situation. My daily driver (a 2006 jetta) wouldn't start this morning, the starter was clicking but no engine turnover, not even a little. It started fine yesterday and ran for over 30 minutes no problem. It was real cold last night though (in the 20s) and I wonder if this somehow caused the battery to drain...?
Anyways, had to drive our van to work. Shortly after it started the battery light on the van then came on (and then off and then on again, and then off again) but currently remains off. I stopped by advanced auto to have them check the battery. The vans battery is good based on their reading but is low on charge (though I have had no issues starting the van). So my guess is the alternator might be going which stinks since it's probably toast on my Jetta daily driver too.
I want to jump the jetta tonight when I get home but I fear I'll drain the Vans battery in the process. Is that possible?

[Quietude]

If the van's alternator is not charging the battery, jumping the Jetta will drain it noticeably, yes.

A battery charger might be a good investment. A good "jump box" even more so. One or the other will get you through this situation and be a nice reserve to have on hand for next time.

Batteries often fail "suddenly" due to temp changes or a collapsed cell. My rule, hard-learned, is that once a battery over a few years old fails once, replace it. Trying to ignore it or pretend it was a one-time thing will just lead to more strandings, in places less convenient than your driveway.

[kwong7]

I'm with Quietude on this. You might be able to get away with a weak battery for a few days in warm climate, but you need those CCA for cold weather. If you're alternator is on its way out, you won't be able to conventionally recharge your battery with the motor running and you'll be electrical system won't have the proper voltage/amperage while running. I strongly recommend testing your batteries and alternators and replace them accordingly.

[easy62]

Yes you can but don’t shut off the van with the bad alternator when you’re done until you get to a garage to have van charging system checked. And a battery that’s 12 yes old like your VW to just die is pretty good. Batteries only last maybe 7 or 8 years depending we’re yo are located. Hot climates and cold climates are very hard on a battery.

[M3 Mitch]

Originally Posted by Kluch

School me on this. Is it possible to jump start a car with cables using a different vehicle who's alternator (possibly) isn't working as well as it should. Situation. My daily driver (a 2006 jetta) wouldn't start this morning, the starter was clicking but no engine turnover, not even a little. It started fine yesterday and ran for over 30 minutes no problem. It was real cold last night though (in the 20s) and I wonder if this somehow caused the battery to drain...?
Anyways, had to drive our van to work. Shortly after it started the battery light on the van then came on (and then off and then on again, and then off again) but currently remains off. I stopped by advanced auto to have them check the battery. The vans battery is good based on their reading but is low on charge (though I have had no issues starting the van). So my guess is the alternator might be going which stinks since it's probably toast on my Jetta daily driver too.
I want to jump the jetta tonight when I get home but I fear I'll drain the Vans battery in the process. Is that possible?

Actually, you first need to at least get a VOM and check the charging voltage on the VW. I have a 92 Subie Legacy and the starter was doing this - cold mornings it would not engage, unless I put a battery charger/booster on it. In the afternoons it was warm enough to start on it's own. But the battery and alternator were fine, it was the starter (no doubt the brushes are about gone in the old starter) (I kept the core as I have a good local rebuilder who will replace the brushes if that's all it needs for a very good price).


So, check your battery in the VW, and check the alternator, at least look at the battery voltage with the engine running, then turn on some accessories and see how much the voltage sags.


This is not a definitive check of the alternator like a first-class shop would do, but it will give you a general idea.


If the battery and alternator check out, you may want to think about replacing the starter, if it's not hard to access.



If you don't have a good automatic "smart" battery charger, you ought to get one, assuming you have mains power available at least near your car.

[easy62]

Jump the VW then get a new battery, take van and get charging system checked stop wasting time on check this check that take it to were they have the right testing equipment to tell you exactly what’s wrong and I’m not taking auto zone. I’m talking bust open the wallet and let a Pro look at it not a parts counter person.

[duke944]

You can buy a digital multimeter from places like Walmart, home depot, auto parts stores etc for $10-$20 and it will tell you how many volts the alternator is putting out by putting the meter's leads on the battery terminals while running. Should be around 14v running. If you don't want to buy one can't help you with any diagnosis.

[Kluch]

I'm taking the van to a shop in about an hour and they're going to diagnose and see if something is up with the alternator (or elsewhere). Once that mystery is solved I'll jump the VW and see how that goes.

[tickyul]

Accurately diagnose exactly what is wrong, then go from there.

[Homi]

14.2V alternator output or new alt needed...

(Simplified: 14.2V reading at battery terminals with car running, using a digital volunteer, can be bad for like $25 from Amazon and are really useful,.)