Fact or Fiction, Electrical System Test?!
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Posting Guru
Joined: Jun 1999
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From: Northern California
Fact or Fiction, Electrical System Test?!
This is not a joke. Repeat, not a joke... 
Today, while visiting with in-laws, for Dad's day, my brother-in-law swears this works. If you suspect an alternator that is not working (or charging), all you need to do is this: With the engine running, remove the POS (+) battery cable. If the engine stalls, immediately, then the alternator is not working, or barely working. If the engine stumbles, yet continues to run, then the alternator is fine. Says he's done this "test" for 3 decades, and it has not failed him yet. How about it? I cringed, and have issues with his "test". 1) I'll never let him drive, or come anywhere near my Aerostar! 2) I told him, on a modern, computer controlled car or truck, removing the cable, while the engine is running, would possibly cause current "spikes" in the electrical system? To me, it just seems not a good thing to do. On the other hand, maybe this "test" is okay? Have you done it? Heard about it? Any opinions on this would be appreciated... 
Today, while visiting with in-laws, for Dad's day, my brother-in-law swears this works. If you suspect an alternator that is not working (or charging), all you need to do is this: With the engine running, remove the POS (+) battery cable. If the engine stalls, immediately, then the alternator is not working, or barely working. If the engine stumbles, yet continues to run, then the alternator is fine. Says he's done this "test" for 3 decades, and it has not failed him yet. How about it? I cringed, and have issues with his "test". 1) I'll never let him drive, or come anywhere near my Aerostar! 2) I told him, on a modern, computer controlled car or truck, removing the cable, while the engine is running, would possibly cause current "spikes" in the electrical system? To me, it just seems not a good thing to do. On the other hand, maybe this "test" is okay? Have you done it? Heard about it? Any opinions on this would be appreciated...
Senior User
Joined: Aug 2003
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From: Corpus Christi, Tx. 78418
I have heard of this before. My Dad used to do the same test as a preliminary, but he would also check the voltage at the battery before and after starting the engine to determine if the alternator was putting out proper voltage. Of course, this does not verify the amperage out of the alternator.
Whether or not disconnecting the battery with the engine running could cause a spike in volts/amps,...well, I don't know. You'd think in the newer cars/vans they would be designed to avoid just such an occurance. But, who knows.
Ron
Whether or not disconnecting the battery with the engine running could cause a spike in volts/amps,...well, I don't know. You'd think in the newer cars/vans they would be designed to avoid just such an occurance. But, who knows.
Ron
Elder User
Joined: Dec 2002
Posts: 624
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From: Georgia
I've not tried this either. The battery acts like a very large capacitor and will smooth out spikes and the like. If removing the battery causes voltage spikes.... that's not good for the computer. On older cars that did have any of the modern electronics, that wouldn't be a problem.
Besides, when you can take your car into Autozone (or somewhere else) and they can test the battery and alt for free, why take a chance on blowing your computer?
Besides, when you can take your car into Autozone (or somewhere else) and they can test the battery and alt for free, why take a chance on blowing your computer?
Senior User
Joined: Oct 2004
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I would NOT try this on a newer veihicle. Back in the day, you could check an alternator by unplugging the connection on the back of it. If the engine RPMs increased, it indicated that the alternator was working and putting a load on the engine. If you do this now it will indicate that the alternator WAS working, and you just burned it up with your test.
Hotshot
Joined: Aug 2002
Posts: 12,679
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From: BC, Canada
A safer way to tell if the alternator is charging is, with the engine idling, touch a screwdriver to the bearing cap on the rear of the alternator. If the bearing cap is magnetized then the alternator is charging. You will still have no idea how much it's charging but you'll know whether or not it's charging at all.
Posting Guru
Joined: Jul 2004
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From: Mitchell, SD
DO NOT REMOVE THE POSITVE CABLE WHILE THE ENGINE IS RUNNING!
This will blow your PCM along with tons of sensors, fuses, and modules. There is a slight chance that nothing will happen, but it is like playing russian rullette with 5 chambers loaded.
The battery is very important in controlling spikes and keeping the current flow even.
His theory worked great, on any vehicle pre 1980 that did not have much for computers, even then you could do some damage to some of the solid state controls.
Take it to autozone/ect and have it tested for free. Or if you have a multi-meter, you can measure the output of the alternator (should be putting out a steady # of amps to the battery not exceding the alt designated amperage) and battery (running should be around 14.4v) yourself on the vehicle.
This will blow your PCM along with tons of sensors, fuses, and modules. There is a slight chance that nothing will happen, but it is like playing russian rullette with 5 chambers loaded.
The battery is very important in controlling spikes and keeping the current flow even.
His theory worked great, on any vehicle pre 1980 that did not have much for computers, even then you could do some damage to some of the solid state controls.
Take it to autozone/ect and have it tested for free. Or if you have a multi-meter, you can measure the output of the alternator (should be putting out a steady # of amps to the battery not exceding the alt designated amperage) and battery (running should be around 14.4v) yourself on the vehicle.
Last edited by 93nighthawk; Jun 20, 2005 at 12:42 PM.
Postmaster
Joined: Sep 2003
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From: Southern California
Or just hook up a voltmeter. If it reads 14.5V, you are OK. If it reads say 12.5V, you know it's shot! Or drive it for a week, if the battery dies, it's dead 
What you brother-in-law did is the equivalent of punching a guy in the stomach while he's taking a long pull from the beer bottle.
What you brother-in-law did is the equivalent of punching a guy in the stomach while he's taking a long pull from the beer bottle.
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Elder User
Joined: Dec 2004
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From: North Jersey, New Jersey
I agree with copper maybe on the older vehicles before the days of distributorless ignition systems and PCMs this was safer but with all the computers today it seems risky.
Its not really necessary anyway with the dashboard idiot lights on most cars the Aero even goes as far as a gauge both the idiot light and the charging guage told me when I had to replace mine
. Plus the Alt started squealing probably due to failing bearings.
That must have been the easiest thing I ever replaced on an Aero.
Its not really necessary anyway with the dashboard idiot lights on most cars the Aero even goes as far as a gauge both the idiot light and the charging guage told me when I had to replace mine
. Plus the Alt started squealing probably due to failing bearings.That must have been the easiest thing I ever replaced on an Aero.
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FTE Community Team
Joined: Nov 2001
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From: Oak Harbor, Washington
I'm coming late to the party, but what they all said. Old cars, no transistors or ICs, generators that make fairly smooth DC, yes, that works. New cars, alternator makes pretty ripply DC and lots and lots of IC modules, you are asking for very large repair expenses. A $10 voltmeter on the battery, turn on headlights, rev engine, look for >13.5v . If it won't do that, there is a problem, though it may not be the alternator.
FTE Community Team
Joined: Nov 2001
Posts: 1,843
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From: Oak Harbor, Washington
Oh, and on the "charging system check": I use a $1300 Snap-On product daily for this, and like most tools, it's only as good as the person using it. I've had it pass batteries with a dead cell! It will fail an alternator because (like a Tercel or a OEM 35a '66 Chevelle) the alternator can't keep up with the internal test load the tester puts on the system for the high-RPM phase of the test. I've had the clamp-on ammeter read over 300A on charging. I've had it pass alternators with 500mv ripple!
IOW, those things are only a starting point -- an experienced and trained tech can do a lot more than just say, "the tester says your system's OK."
IOW, those things are only a starting point -- an experienced and trained tech can do a lot more than just say, "the tester says your system's OK."
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