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Place a volt meter across your Alt, disconnect the battery and see for yourself what the voltage jumps to. Often times it will exceed 400 volts. In automotive electrical school (MOPAR) This was the first thing taught to us dealer weanies. DO NOT UNHOOK A BATTERY WHILE THE ENGINE IS RUNNING! I'm glad to hear you have never had a problem. A better way to see if the Alt is working would be to place a screw driver against the rear center, if it sticks the unit is at least somewhat working.

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There ya go, its the FIRST THING a technician should know!!

the screwdriver on the rear bearing is a good way to tell if your regulator is working, because it picks up the magnetic field from the rotor. If the field is energized it will be magnet. The alternator may still not work though, especially if some back yarder was previously diagnosing it by disconnecting the battery. Bye bye rectifier! This can be confirmed by performing a ripple pattern test on the diodes.

sincerly, THE PROFESSOR

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ARRRGGG! What you keep ignoring is that you are NOT DISCONNECTING the alternator field by disconnecting the battery!!! all youre doing is disconnecting the BATTERY. The field wires are still hooked up to the regulator.

the alternator PRODUCES voltage. The coil is a transformer that takes voltage and converts it. Disconnecting the battery should have absolutely no effect on the field wires on the alternator.

Im still gonna try it. heck, ive done it plenty of times before (just like stumpy says he has) and I never saw anything fry. So, now I'll put a voltmeter on my alternator and disconnect the battery. Im not afraid. I dont drive my truck until it snows anyways.

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Im not ignoring anything. The whole part of it IS TO LEAVE THE FIELD ENERGIZED while the battery is disconnected! Then you have a transformer. By disconnecting the battery with the car running, the alternator still has the field energized!! Thus generates voltage that has nowhere to go!!!!! What do we know about electricity?? It takes the easiest way out!! Usually across a semi conductor of some type. The voltage will rise and rise, until it finds an escape.

If you disconnect the battery, the rest of electricaal system is subjected to this high voltage. If you just disconnect the alternator output terminal, you just subject the alternator to this high voltage.


Where do you think the power generated by the alternator goes when the field is still energized and the battery is disconnected??????????????

I'll give you a hint, you can use it to weld with!

I cant believe you guys think this is ok. A little knowledge is totally dangerous!!

just cause you got away with it before doesnt make it ok. Like I said earlier, the damage may not be immediately apparent!!

You guys really need to be educated properly in auto repair. Phone any local alternator rebuild shop and tell them the alternator you bought from them doesnt work because you disconnected the battery and the car stalled. Dont phone auto zone or whomever , phone a real rebuilder with electrical knowledge!

This is what separates real technicians from back yarders. A real tech knows the facts. I suggest you folks educate yourselves on this. This is why auto makers made it more difficult for back yarders that know eveything to work on thier products.Too many people with too little knowledge!

If you go try this on your Ramcharger, I hope you have a nice expensive stereo in there, and make sure you rev it up good and hard!

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There is no way in Hades a car alternator will put out 400v it's physically impossible. As for spikes every time you turn on the headlights, the heater, the wipers, or the radio you cause a momentary spike and nothing blows. That is one of the reasons the main feed is a fusible link because it will outlast these spikes where a fuse won't. The welders are putting out high amperage and that is what welds not voltage. Put the books down and grab a wrench and get real world experience.

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My friend I have been a dealer mechanic since 1961 so I think I do have a small bit of on the job experience. What do you do for a living?


I'm a professional auto mechanic since 1988 I graduated with a 4.0 gpa and achieved 93% on the Ontario trade exam after completing the required time in school and a 9000 hour apprenticeship program. I have taken all the ASE tests and received master tech status, but since ASE isnt worth anything here, its just a way to waste money on credentials that arent recognized here

The voltage regulator maintains the steady output. It will only allow what is needed to go to the system.

The voltage regulator turns the field off and on to reach a happy medium in the electrical system. It detects this by monitoring the system voltage via the sense terminal.

Removing the battery does not disable the regulator.
Nobody said it disables the regulator. The regulator sees the battery voltage as low and keeps the field energized. Just like the battery was low, it keeps the field energized because its trying to reach its set point of approximately 13.5-14.2 volts


The reason you can weld with the AMPERAGE produced is that the draw causes the regulator the allow current to flow as drawn on up to the max amperage of the alternator.

I cant begin with whats wrong with this statement. First of all the regulator is just a switch with a zener diode in it. It senses the voltage and either turns the alternator field on or off. Its not magic. The alternators ability to supply enough amperage to weld with comes from the size of its windings and ability to dissipate heat. These traits made the Chryco 100amp a good choice


Every thing you have brought up requires the regulator the be stuck wide open and that doesn't happen when you pull the battery cable.


What voltage does the regulator see when the battery is not hooked up?? Disconnect your battery and check to see if the field stays energized. It will


What happens is exactly the oppisite the regulator senses the lower draw and lowers the output to match.
Regulators dont sense draw, they sense voltage. and they tell the alternator what to do, turn on or off. When the voltage reaches the setpoint determined by the zener diode, it shuts off the alternator field.




BTW if voltage welds why aren't your battery cables being welded now.

Because like I said earlier, voltage creates the spark, not amperage, and batteries are only 12V, but they are capable of over 800 amps!! thats why they arent welding together right now. How is it your ignition coil can jump the spark plug gap and your battery cant??




The reason is that there isn't a high amp draw at the battery.
Put your greasy cresent wrench across the battery terminals and see how low the amp draw is!!










Also spike and surge are interchangeable terms.

A spike refers to a voltage spike ( i.e sudden rise) in voltage! a surge is referred to as a sudden rise in current flow(i.e. the stuff that causes blackouts when everyone has their air conditioners on. )



Spend some more time reading you'll get it right eventually.
The scary part is I've got this right, and I havent been doing it 45 years. This is basic auto repair knowlege. I dont know what credentials you have, but getting credit with snap on doesnt make you a technician.




You are going off the deep end on this with absolutely no proof.

The proof is out there. Ever change an ECM ?? Why does it come in an anti-static bag?? Why should you ground yourself before touching one? Why do the electronics workers wear anti static suits??? Its because of static shocks!! static shocks dont carry amperage, but they are capable of high voltage! just like your alternator. These people know the failures may or may not be immediate, but are aware of the dangers if they are.

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There you go again. We are talking about old cars not new computer cars. Make up your mind. And my credenals consist of a Chrysler master mecahnics certificate that I earned before you were old enough to go to school. And it is recognized here. The end!

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Here I am again, tell me whats different about a new car and an old car??? tell me! Id like to know! Is there a magical mystery date somewhere?

youre talking electrical, and Im talking electronic! they merged around the time of "transistorized ignition, and electronic voltaage regulators, and the transistor radio" These are all after you got your master chryco tech status!

an IC (integrated circuit) is an IC !!! which if you read back to my first post on this, youll see that I mentioned any IC was at risk by using these dinosaur methods of diagnostics.

maybe you should renew your credentials, they seem expired to me. afterall, thats before calculators had been invented, and man landed on the moon. So much more has since been learned! I encourage you to explore it.

and the facts are whats recognized here!

have a great day!

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I welded a screwdriver between the + side of a battery and the fender when I shorted it out once by accident. but thats neither here nor there.

lik stumpy said and as I have been saying, the voltage regulator (and hence the alternator field) is not disconnected when the battery is removed- so no jump in voltage.

The regulator still being connected and operational is what causes the field to remain energized, this causes the alternator to keep working, building up a charge with nowhere to go since the battery is no longer in the system

the most I'll give you is that there is an overshoot of voltage when the battery is unplugged and replugged. voltage of maybe a few volts. But no way do I see damage occurring.

thats just a surge, thats not the issue

what about using jumper cables??? here you attach a battery to a car with a dead battery (dead battery with no voltage is close to no battery at all) and then DISCONNECT it after it starts.

thats a serious mistake but only determental to the car with the dead battery. Its not relevant to this arguement. It causes the alternator to work full tilt til the battery is charged. An alternator is designed to keep a battery charged, not charge up a dead one. The result will often be a burnt rectifier. Sometimes they whistle. Most people get away with it though.



so people should be blowing all there electronics using jumper cables. thanks but no thanks.

there is a possibility but minimal. Best not to do so.

same with using a big battery charger to start a dead car. Did it lots of times, never any damage.

no damage that youve seen immediatley. Seen it many times myself, and know of other knowledgeable mechanics that have. Becoming a bigger issue these days with body shops running down batteries then boosting them


never say 400v either- ever- unless the car hit a power line.


Try it and tell me never. 400 is a tad high, but its possible. 200+ is more common! try it, keep your field energized, then disconnect the output wire on your alternator. Check the alternator output with a voltmeter. rev it up. Tell me how your readings wont cause troubles!! Just like I said earlier, try it with your battery charger. It works the same way!

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