I thought this was a no-no, but my 82 year-old hot-rodder uncle says one way to test for a bad alternator is to disconnect the negative battery cable with the engine running. If it stops, he say the alternator is bad. Just curious (I know there are other ways to check.)
Disconnecting negative cable to test alternator
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He is saying that if you disconnect the negative battery cable on a car that’s running it will stop if the alternator is bad?
And what will happen if you disconnect the negative battery cable on a car that is running and the alternator is OK does it keep running?
And what will happen if you disconnect the negative battery cable on a car that is running and the alternator is OK does it keep running?
Google…
In the pre-computer days, you could pull it off without damaging anything. Today, you risk frying every electrical device in your vehicle. The second you disconnect the battery, the voltage regulator pegs the alternator to put out maximum power. With no battery in the circuit to act as a buffer, the alternator can put out up to 150 volts, depending on engine rpm. When the smoke clears, that “simple test” could end up costing you several thousand dollars for new electronics.
Introduction
Don't test an alternator by disconnecting the negative battery cable. Use a voltmeter instead to conduct a simple, safe test.Tools Required
- Voltmeter
In the pre-computer days, you could pull it off without damaging anything. Today, you risk frying every electrical device in your vehicle. The second you disconnect the battery, the voltage regulator pegs the alternator to put out maximum power. With no battery in the circuit to act as a buffer, the alternator can put out up to 150 volts, depending on engine rpm. When the smoke clears, that “simple test” could end up costing you several thousand dollars for new electronics.
Never open a live circuit. period. Even at 12V. Use a voltmeter. they're cheap.
I told my uncle I thought he was crazy...
Crazy uncle is one of the predefined and required roles in every family. Sounds like he's got you covered.
Maybe it worked on a 48 Ford with 6 volts and a generator.
My uncle told me never leave a car battery on a concrete floor because concrete will drain the battery. I wouldn’t call him crazy, just misguided.
Voltmeter is great.
I love my Solar battery tester:
My uncle told me never leave a car battery on a concrete floor because concrete will drain the battery. I wouldn’t call him crazy, just misguided.
Voltmeter is great.
I love my Solar battery tester:
Last edited:
jmackro
Well-Known Member
At the risk of sounding like someone's crazy uncle, yes, that's how it works. A car with a working alternator will continue running if you disconnect the battery. You can disconnect either the "+" or "-" side - or remove the battery entirely - the alternator will keep things going.
Now, this may be a bad thing to do with a solid-state regulator, or on a 21st century car with 35 microprocessors. But with an old-time coil & points regulator, it doesn't seem to cause any damage.
skk said:
Unless you are starting the car, there really isn't much current going into or coming out of the battery. With just the ignition drawing power, even if the alternator is dead, the battery won't be putting out much current. In other words, there isn't much of a spark when you pull the cable to perform the "crazy uncle test".
No argument. But you'll never be remembered as the crazy uncle if you do things the prudent way.