Should car still run on alternator with ground terminal off battery?

[Johnny Mac]

So I had always heard a simple alternator test is to pull the ground cable off the battery post, and see if it still runs. Had also heard you should be able to feel a magnetic pull on the back of the alternator when its working. My alternator does neither. engine Dies with the ground off the battery, and I can feel no magnetic pull on the back. Drove it to autozone and they tested the alternator and claim its good. I have my doubts....this car has killed 2 batteries since I got it. I believe it has a slow draw on the battery when off...but also not sure the original 1978 alternator or voltage regulator is working. I looked at the voltage gage the autozone guy had, and it showed over 12 even with a load.
If it matters amp meter in car shows positive even at idle.

My race car kill switch will kill the car through the positive battery cable, so the positive cable killing the car makes sense. but again I thought the negative cable could be used to test the alternator? Anyways...let me know what you guys think. Thanks.

 

[cudascott]

Car will not run with the ground cable disconnected from the battery never. A method known to check the alt. is pull the positive cable. The best method is to Use a multi meter
to check for charging voltage from the alt. by first checking the volts at the battery with the key off and no lights or accessories on then star up and check again if the alt. and it's components are doing their job you should see around 14 volts on a charged battery while the engine is running

 

[Rustedwrench]

What I can gather from your post is this. Nothing. Don't know what kind of car you are working on. Don't know if it is a street car or race car. Don't know if any modifications have been done to the wiring. See the problem?

It sounds like you may have two different problems. Search this forum for "alternator test" or something similar. Member 67Dart273 has posted at least 4,752 times about how to test this stuff. Solve one problem at a time.

And stop pulling the battery cable off the "test" the alternator.

 

[Johnny Mac]

Car will not run with the ground cable disconnected from the battery never. A method known to check the alt. is pull the positive cable. The best method is to Use a multi meter
to check for charging voltage from the alt. by first checking the volts at the battery with the key off and no lights or accessories on then star up and check again if the alt. and it's components are doing their job you should see around 14 volts on a charged battery while the engine is running

Thanks for the clarification. I always heard it was the ground cable. Confuses me as to how the positive cable kill switch works on my race car then.

And rusted wrench, I did search for alternator test. Got nothing. Referenced a 1978 above. Stock dodge street car. If that is even relevant to the cable or magnetic field test I asked about. Thanks so much for the valuable input.

 

[skykeith]

NEVER DISCONNECT ANY BATTERY CABLES WITH THE ENGINE RUNNING!!! Google "alternator load dump" and read about how much component damage this can cause. If the rear bearing housing isn't magnetized, it ain't working. Take it somewhere else and have it tested. A qualified technician should be able to tell you if it's an alternator or regulator problem.

 

[67Dart273]

Car will not run with the ground cable disconnected from the battery never. .

Sorry not true. removing EITHER cable on a running and charging vehicle will result in the engine running on, but as others said "don't do so"

A method known to check the alt. is pull the positive cable.

A person should NEVER do this.

 

[67Dart273]

And stop pulling the battery cable off the "test" the alternator.

NEVER DISCONNECT ANY BATTERY CABLES WITH THE ENGINE RUNNING!!!

Correct. Do NOT pull battery cables on a running car

It has the potential to damage electronics, but WORSE it is NOT a good test. WHY?

Two possibilities..........

1.....Let's say you have a car with a "bunch" of loads, a great big fuel pump, electric water pump, big electric fans, and an 80-100A alternator.

Let's further say that this alternator has some bad diodes and that it's only putting out.........?? 20-40A or so.

So it's BARELY working. You sit there idling fast enough that it will "run the engine" pump and ignition, and you disconnect the cable and it keeps right on running. You WRONGLY conclude that the alternator is "OK"

========================================

2......Situation there is actually NOTHING wrong with your alternator, but you have the engine idling a bit too slowly, and you pull the cable.......and the engine quits. You again WRONGLY conclude the alternator is bad, when if fact the engine is simply idling too slowly.

=========================================

As above check battery with voltmeter. If there is any doubt charge the battery and retest, and or test the battery. A battery going bad........dead cell, etc, can give you skewed results

 

[Johnny Mac]

Thanks all. Thought the cable pulling damage was in reference to newer efi cars. Anyways. Will get a voltmeter on it. Alternator isnt super expensive. Just didnt want to have to replace it if it was working. Figured one of tje above tests was a quick and dirty way out as my time is limited, and I am usually working, racing, or working on the race car, opposed to this old cruiser

 

[cawley]

So I had always heard a simple alternator test is to pull the ground cable off the battery post, and see if it still runs. Had also heard you should be able to feel a magnetic pull on the back of the alternator when its working. My alternator does neither. engine Dies with the ground off the battery, and I can feel no magnetic pull on the back. Drove it to autozone and they tested the alternator and claim its good. I have my doubts....this car has killed 2 batteries since I got it. I believe it has a slow draw on the battery when off...but also not sure the original 1978 alternator or voltage regulator is working. I looked at the voltage gage the autozone guy had, and it showed over 12 even with a load.
If it matters amp meter in car shows positive even at idle.

My race car kill switch will kill the car through the positive battery cable, so the positive cable killing the car makes sense. but again I thought the negative cable could be used to test the alternator? Anyways...let me know what you guys think. Thanks.

With the car running, u can pull the negative off the battery & it will stay running, unless your alternator is bad. Then it will shut off

 

[67Dart273]

With the car running, u can pull the negative off the battery & it will stay running, unless your alternator is bad. Then it will shut off

Stop posting this ****, and **** is what it "is" I guess you did not bother reading the rest of the thread.

 

[bcschief]

Pull the alternator off and take it to your nearest autozone they can test it and see if it works they can also test your battery. Actually drive your car there and they can test both in the car.

Brian

 

[Lustle]

The only way this "test" would work is if your alternator was completely pooched. And you would long suspect something was wrong before this test would prove it in my mind. You'd have numerous electrical issues before the alternator wouldn't be capable of running the engine on its own.

 

[RustyRatRod]

Correct. Do NOT pull battery cables on a running car

It has the potential to damage electronics, but WORSE it is NOT a good test. WHY?

Two possibilities..........

1.....Let's say you have a car with a "bunch" of loads, a great big fuel pump, electric water pump, big electric fans, and an 80-100A alternator.

Let's further say that this alternator has some bad diodes and that it's only putting out.........?? 20-40A or so.

So it's BARELY working. You sit there idling fast enough that it will "run the engine" pump and ignition, and you disconnect the cable and it keeps right on running. You WRONGLY conclude that the alternator is "OK"

========================================

2......Situation there is actually NOTHING wrong with your alternator, but you have the engine idling a bit too slowly, and you pull the cable.......and the engine quits. You again WRONGLY conclude the alternator is bad, when if fact the engine is simply idling too slowly.

=========================================

As above check battery with voltmeter. If there is any doubt charge the battery and retest, and or test the battery. A battery going bad........dead cell, etc, can give you skewed results

3......Situation You remove the positive battery cable when there is nothing wrong with your extremely high amperage alternator. You accidentally ground your metal wristwatch band to the radiator support, because you didn't notice the positive cable end was touching the watch band, instantly frying your watch band to your arm. 30 minutes later in the ER, you are having your watch surgically removed from your arm because it is cooked on.

Seen it happen, first hand. Once with a ring and once with a watch. It was the SAME GUY within a month of the first accident.

 

[Johnny Mac]

3......Situation You remove the positive battery cable when there is nothing wrong with your extremely high amperage alternator. You accidentally ground your metal wristwatch band to the radiator support, because you didn't notice the positive cable end was touching the watch band, instantly frying your watch band to your arm. 30 minutes later in the ER, you are having your watch surgically removed from your arm because it is cooked on.

Seen it happen, first hand. Once with a ring and once with a watch. It was the SAME GUY within a month of the first accident.

Lol same bone stock monaco barge you have assisted me with on the A.C. thread. But duely noted! Thanks. As you can all tell. Im used to building drag cars, and just buying new performance parts. This old cruiser with bone stock components is giving me a new education.

 

[Magiver]

Anyone who says it's ok to pull any battery cable off while the motor is running is full of ****.
That's a very stupid thing to do. Along with the previous mentioned bad stuff, the battery, when being charged even a small amount will produce hydrogen gas. The spark produced when pulling off a cable can ignite the hydrogen and the battery will explode like a bomb. Seen that happen first hand. It's not pretty.

 

[73 Swinger 8]

Just go to any AZ they will test the battery and alt. for free, in the car. It's usually safer...lol

 

[Johnny Mac]

Just go to any AZ they will test the battery and alt. for free, in the car. It's usually safer...lol

One of the first things I said was I took it to AZ and it tested good according to them. However i feel nothing from the mag field test, and AZ Isnt exactly known for their genius "tech". I believe it was them that sold me a dead battery years ago (that I had asked them to check for charge in store before I got it home). Got it home, car still didn't start, ended up then changing a starter motor.... for nothing.

 

[67Dart273]

One of the first things I said was I took it to AZ and it tested good according to them..

I tend to agree. To do a proper test "at home" you really need at minimum some method of loading the system, and this is traditionally done what is known as a "carbon pile." You CAN find these occasionally at garage sales or on CL. There was one advertised locally the other day. I already have "a couple" and damn near bought this one!!!

http://spokane.craigslist.org/tls/5223987868.html

FIFTEEN BUCKS for a battery load tester!!!

You can, however, "improvise." One way is to leave the headlights on for a period of time, depending on how good the battery is, 1/2 hour or so. Then see if the alternator will maintain at least 13.5 to the battery.

Please read the shop manual. This is the BEST traditional source of info. You can download them free at MyMopar

You can do a "full field" test and get some idea of whether the alternator will put out enough. Depending on whether you have a 69/ earlier, or a 70/ later system, will dictate how you get battery rigged into the field.

Of course if you have an aftermarket alternator and or a "one wire" this is more difficult.

 

[krazykuda]

Just go to any AZ they will test the battery and alt. for free, in the car. It's usually safer...lol

They should be able to test it in states other than Arizona...

 

[Johnny Mac]

I may have better luck driving to arizona than to autozone lol

 

[bobg450]

OK, here's my 2C- Connect a voltmeter to your battery with the engine running. Should read about 14 VDC. If it's not and dropping, go to the field lead on the back of the alternator (small wire) and you should read something. If not, then the voltage regulator or it's wiring are not doing their job. If you do read something, you either have a bad alternator or bad ground somewhere. Mine was a bad voltage regulator. It was that simple, for me.

 

[73 Swinger 8]

One of the first things I said was I took it to AZ and it tested good according to them. However i feel nothing from the mag field test, and AZ Isnt exactly known for their genius "tech". I believe it was them that sold me a dead battery years ago (that I had asked them to check for charge in store before I got it home). Got it home, car still didn't start, ended up then changing a starter motor.... for nothing.

lol It was late I missed that....but I did say it's usually safer

 

[67Dart273]

OK, here's my 2C- Connect a voltmeter to your battery with the engine running. Should read about 14 VDC. If it's not and dropping, go to the field lead on the back of the alternator (small wire) and you should read something. If not, then the voltage regulator or it's wiring are not doing their job. If you do read something, you either have a bad alternator or bad ground somewhere. Mine was a bad voltage regulator. It was that simple, for me.

Sorry but it's NOT (quite, always) that simple. It "can be" but maybe "isn't"

I tend to divide the charging system into "rough blocks"

"Not in order"

"The parts".......alternator, regulator, and sometimes, the battery

"The output" circuit, IE the path all the way from the alternator output stud to the battery. On unmodified cars, there is at least 5 or 6 major terminals in the path that can fail

"The field" wiring. This includes the path from the ignition switch, through the bulkhead to the voltage regulator, and onto the alternator field. On a 70 / later, this also must include the second blue field wire.

 

[bcschief]

One of the first things I said was I took it to AZ and it tested good according to them. However i feel nothing from the mag field test, and AZ Isnt exactly known for their genius "tech". I believe it was them that sold me a dead battery years ago (that I had asked them to check for charge in store before I got it home). Got it home, car still didn't start, ended up then changing a starter motor.... for nothing.

Then most likely it is good the charging problem is elsewhere IE battery, wiring or voltage regulator. I also agree the rear shaft bearing should attract a screwdriver when it is running. You should of had them test the starter before you replaced it.

Brian

 

[bobg450]

Sorry but it's NOT (quite, always) that simple. It "can be" but maybe "isn't"

I tend to divide the charging system into "rough blocks"

"Not in order"

"The parts".......alternator, regulator, and sometimes, the battery

"The output" circuit, IE the path all the way from the alternator output stud to the battery. On unmodified cars, there is at least 5 or 6 major terminals in the path that can fail

"The field" wiring. This includes the path from the ignition switch, through the bulkhead to the voltage regulator, and onto the alternator field. On a 70 / later, this also must include the second blue field wire.

Love the quotation marks and the caps. Just sharing my experience and not trying to sound like a "know it all". Guess I should have kept my 2C to myself..............I've only been a licensed electrician for 38 years and worked as a technician in a Nuclear Power Plant for 33 years, with a 50 year passion for cars.