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BALLAST RESISTOR!!! Change that and she'll fire up. They go without you knowing and all it does is crank till she dies. Mike

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START YOUR CAR AGAIN (IF YOU CAN) AND WHILE IT IS RUNNING, PULL OFF THE POSITIVE BATTERY CABLE.. SEE IF IT STAYS RUNNING.. IF IT DIES RIGHT AWAY, SOMETHING IS WRONG IN YOUR CHARGING CIRCUIT.. BAD ALTENATOR, REGULATOR, SHORTED WIRE, BAD GROUND, ECT.. THAT IS DRAINING YOUR BATTERY..

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I'm sorry but we've been disconecting batteries for a lot of years and never blew a curcuit or gauge doing it. The new cars suffer from those syptoms but the older cars don't. You just pull the cable and the car will imediately die if the charging system is bad. You are disconeced for less than a 1/2 a second.

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I know this used to be a widely used method, but that doesnt make it right. Any component with an IC (integrated circuit) is at risk. This means electronic voltage regulators, etc...

Just because its disconnected for a less than a second doesnt make it ok. If you stick your tongue in a 220V welder plug for less than half a second, does that mean you wont get a shock?

Also, the damage is often not immediately apparent, often times the component is damaged but still operates, and may do so for 6 months or 6 seconds, but its still damaged and will fail sooner than it would have originally. This goes for welding on a vehicle as well.



no disrespect intended.

Its an outdated method that should not be used on anything, especially when a voltmeter can be had for under 10 bucks. All you will know by disconnecting the battery with the car running, will be told to you by hooking up a voltmeter to the battery. Its probably even easier.

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START YOUR CAR AGAIN (IF YOU CAN) AND WHILE IT IS RUNNING, PULL OFF THE POSITIVE BATTERY CABLE.. SEE IF IT STAYS RUNNING.. IF IT DIES RIGHT AWAY, SOMETHING IS WRONG IN YOUR CHARGING CIRCUIT.. BAD ALTENATOR, REGULATOR, SHORTED WIRE, BAD GROUND, ECT.. THAT IS DRAINING YOUR BATTERY..

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not to any more but when my pos. cable came off on my bike it cooked every light bulb on it (about 17) the fuel pump and the horn. so i cost my self over 150 in parts cuz i didnt get the bolt out for the cable . after that ill never take the pos. cable off

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START YOUR CAR AGAIN (IF YOU CAN) AND WHILE IT IS RUNNING, PULL OFF THE POSITIVE BATTERY CABLE.. SEE IF IT STAYS RUNNING.. IF IT DIES RIGHT AWAY, SOMETHING IS WRONG IN YOUR CHARGING CIRCUIT.. BAD ALTENATOR, REGULATOR, SHORTED WIRE, BAD GROUND, ECT.. THAT IS DRAINING YOUR BATTERY..

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not to any more but when my pos. cable came off on my bike it cooked every light bulb on it (about 17) the fuel pump and the horn. so i cost my self over 150 in parts cuz i didnt get the bolt out for the cable . after that ill never take the pos. cable off

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Im still having a hard time seeing it.

an ignition coil works because there is a winding around a metal core which induces a higher voltage on a secondary winding.

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**** Here's your answer. Whats inside your alternator?? a winding (stator) around a metal core (rotor) which induces a higher voltage into the secondary winding (stator)

Some alternators have an "S" terminal on them. The S is the stator take off, they use this high voltage AC take off to heat windshields and various other applications in high end vehicles.

Disconnecting the battery opens the DC circuit and you now have a high voltage condition in the vehicle, not in the battery.

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but like I said, I'm gonna try it and see what the voltage goes to. unfortunatly, I need to wait until i replace my battery cause it died a while ago and I havent replaced it yet.

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Dont do it. You risk some pricey components!

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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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I've enjoyed the debate, but can't help but wonder if thedriver ever got his problem solved?

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I've enjoyed the debate, but can't help but wonder if thedriver ever got his problem solved?

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I've enjoyed the debate, but can't help but wonder if thedriver ever got his problem solved?

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Not yet.

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oh well, first disconnect the battery while its running and ......

Sorry, I couldnt resist.

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I found this on a website ( http://www.marine-electronics.net/techarticle/battery_faq/b_faq.htm )

it says under MYTHS about batteries:
"10.6. Test the alternator by disconnecting the battery with the engine running.

A battery as like a voltage stabilizer or filter to the pulsating DC produced by the charging system. Disconnecting a battery while the engine is running can destroy sensitive electronic components, for example, emission computer, audio system, cell phone, alarm system, etc., or even the charging system itself. These damages can occur because the voltage can rise to 40 volts or more. In the 1970s, removing a battery terminal was an accepted practice to test charging systems of that era. That is not the case today. Just say NO if anyone suggests this."

So like I said, old vs new. The alternator puts out a rectified DC signal. That means it was AC but the diodes rectified into DC. there is still a small amount of ripple on the output. with the battery in place, the battery acts like a shock absorber which mellows the ripple out. This ripple can ruin electronics in NEW vehicles. Old stuff like ours aren't affected because the technology is more resistant to the ripple.

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Read it again it says 70s cars, and "or more" doesn't mean 360v more. I do use a digital meter. The Canadian school system must be fantastic when a trade school grad knows more than an electrical engineer who works systems for the most advanced tactical strike fighter on earth.

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You have not convinced me or too many other people that your right just stubborn but you're young you'll learn you can't always win. I believe the only shunt here is the one between your ears you don't even recognize sarcasm when you read it.

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No, actually correct facts are what is accepted and so far you have spouted all kinds of FACTS but you still haven't proven your point, cause and affect. There is a very large difference between the new ICs and the original electronics. The early stuff was a lot sturdier and less affected by surges or spikes, or that ever you want to call it,than the stuff they use today mainly because it was simple. If it wasn't for dinosaur methods of testing your parents would have been walking every where they went because us dinosaurs did quite well with or testing methods long before you learned your way and long after. I can completely dismantle my car into it's smallest parts and rebuild it without opening a book or resorting to overpriced diagnostics machines can you?

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are these the facts youre bringing to the table??

I think we need an icon of a big guy beating a dead dinosaur!

find somewhere in the last 100 years where it says to disconnect the battery with the engine running, and show it to us. then laugh, til then, we laugh at you.

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My vote is for the Starter.
There are many failure modes for starters, and I've experienced the one you are describing. I don't think you need a more powerful replacement, just one that cranks without seizing itself!

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Fascinating Thread! FULL of great info regarding batteries and charging systems.

I still think it's the starter.

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Check the grounds for corrosion including the starter to block. I've had a 13 1/2 to 1 engine and never had starter/cranking problems. Check the grounds and the starter.

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Read me again;

starter, starter, starter.

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it says right in the article that I posted that it was an accepted method back in the 70's. is that what you wanted to see?

if so, here it is again for you:

"In the 1970s, removing a battery terminal was an accepted practice to test charging systems of that era. That is not the case today. "

and why isn't it the case today?? well becuase today we have "sensitive electronic components, for example, emission computer, audio system, cell phone, alarm system, etc., or even the charging system itself."

You see, back in the day, the "pulsating DC produced by the charging system" was not bad enough to damage the heavy duty electronics they used back then.

if you want to think it does, I will let you believe that. I don't.

Actually, I think you're enjoy trying to rile someone up. I'm not riled though. we were trying to edumacate but I see you can't be swayed. You have actually made me smile quite a bit with your argument. I dont buy it one bit. Keep it up. Thats what we live for!!!

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Stumpy, I see you brought your facts to the table again. Youre in over your head now, the dead horse is not a life preserver.

all due respect.

Have a great day fellas!

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I see no need in repeating what has already been posted for you but you just don't seem to understand.
THEDRIVER I am sorry for helping to steal your post. I hope you get the car fixed.

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Well if he uses proper practices, he may be able to diagnose it. If I was there, with a snap on AVR Id diagnose it properly in under 60 seconds, but I'm not there. In order to diagnose it properly he needs a minimum of a carbon pile and a voltmeter.

and stumpy, repeating whats been said wont save your arguement, i've shown it dont hold water.

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can I have some too?........

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Im very familiar with EMI, thats part of being a real auto technician.

So the best you can come up with is that these are robust and are resilliant to the voltage??

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Phone that alternator rebuilder I mentioned earlier, and let him tell you how robust they are! A simple sulphated battery can cause the the rectifier to fail. Rectifiers fail because of heat, so alot depends on the heat dissipation capabilities of the alternator, where its mounted in the car, the load placed on it etc....

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very good, but is it still advisable to disconnect the battery with the engine running???

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what about your aftermarket stereo, your electronic autometer tach, or your replacement mechanical voltage regulator with the electronic internals?? anyone have an MSD? or some type of spark amplifier?

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just how robust are the Chryco electronic regulators?? or ignition boxes?? Maybe you can test one???? And issue us safe limits with a mil spec on them??

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how many of peoples cars have no up to date electronics in them??? Or an alternator??!!! and what makes you so sure the old chryco ones can take it??? and if so, why would you subject them to that abuse???

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get a voltmeter for $9.99!!

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is it still ok to disconnect it???

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yes

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Im very familiar with EMI, thats part of being a real auto technician.

So the best you can come up with is that these are robust and are resilliant to the voltage??

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Yup! what do you want, SEM photos of the die? I dont have it. you have to trust me on this

Sure, if you have them! and I never trust anyone because they say so.

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Phone that alternator rebuilder I mentioned earlier, and let him tell you how robust they are! A simple sulphated battery can cause the the rectifier to fail. Rectifiers fail because of heat, so alot depends on the heat dissipation capabilities of the alternator, where its mounted in the car, the load placed on it etc....

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Yes, sulphanted battery causes the alt to put out higher voltage and current and the diode overheats and dies. Thats what I said kills diodes- HEAT


yes heat kills them. Ive said that a while ago, step into today! also exceeding their capabilities will kill them.

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very good, but is it still advisable to disconnect the battery with the engine running???

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on a 70's car with 70's electronics (which is what I have been saying all along), it can be done. yes

In the beginning of this discussion did anybody ask the owner to ID all of his electrical components?? Did anyone go do an inventory on his vehicle? And maybe he has an aftermarket regulator thats most lilkely been replaced since 3 decades have passed?

Is this something we need to do before we heedlessly determine if the car will survive the voltage increase that you fail to realize exists??

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what about your aftermarket stereo, your electronic autometer tach, or your replacement mechanical voltage regulator with the electronic internals?? anyone have an MSD? or some type of spark amplifier?

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Change the situation and I'll change the answer. These are modern electronics. more susceptible but I doubt they would be harmed because I dont believe you'll see an increase in current or voltage by disconnecting the battery

If you trust me, I might trust you! But I assure you, youre wrong, and if you were exposed to the autmotive world you would have conquered this inability to see how this really works.

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just how robust are the Chryco electronic regulators?? or ignition boxes?? Maybe you can test one???? And issue us safe limits with a mil spec on them??

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Want specs? I'll get them. theyre at my house

Specs are good! maybe we can ascertain how well they will survive given the circumstances, then again, we could just use a proper diagnostic method which would eliminate all this typing and arguing. WHY CONTINUE ?? How can any logical human being state that its logical to disconnect the battery knowing what we know??

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how many of peoples cars have no up to date electronics in them??? Or an alternator??!!! and what makes you so sure the old chryco ones can take it??? and if so, why would you subject them to that abuse???

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Youre saying you get 400v when I disconnect the alternator. I say your nuts. You say I'll blow all my circuits. I say Ive done it many time before and Ive never had that happen.

Pay attention , I said 200 was more likely. I never said 400. That was stated by a gentleman that went to chyco electrical school (im sure they teach all the wrong stuff) I dont say you WILL blow all your circuits, you said that (lawyer?) I said youll be likely or at least succeptible to it, and did not mention all. I said anything with an IC is succeptible and also the rectifier in the alternator. Ive got enough words in my mouth without you adding your own. You may never have anything bad happen by having unprotected "relations" but that does not make you immune. Ive said that before enough times, go read them.

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get a voltmeter for $9.99!!

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My voltmeters (I have several)cost several hundred dollars. I dont use cheap tools

good for you, I too own a labscope and a graphing multimeter and other expensive tools. It still wont save you from disconnecting the battery with the car running like a moron. A simple $9.99 voltmeter is enough to diagnose if the vehicle's alternator is at least trying to charge the battery. This is much safer, reliable, and practical. Surely anyone can see that.

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is it still ok to disconnect it???

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yes

ya, go right ahead. A little knowledge is dangerous. Maybe you get lucky 999 times out of 1000, good for you, but that still does not make it right. and again, only a moron would do it and subject himself to future problems. Totally unprofessional and if anyone did it, anyone of the automotive technicians I know would laught themselves silly at them

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TO THE DRIVER, STUMPY & AAR CUDA & 5537SG,

I AM THE GUILTY MOPARTS MEMBER THAT STARTED THIS 5 PAGE AND COUNTING THESIS ON THE EFFECTS OF REMOVING A POSITIVE BATTERY CABLE FROM A RUNNING CAR TO CHECK IF YOUR ALTENATOR WAS WORKING OR NOT... I KNOW IT WAS THE BACKYARD (NOT DEALER TRAINED) MECHANIC WAY TO TEST AN OLDER CARS ALTENATOR, BUT THE FACT IS PEOPLE WHO WORKED ON 60'S AND 70'S CARS HAVE DONE THIS PROBABLY MORE THAN ONCE WITHOUT A NUCLEAR REACTION HAPPENING.. I KNOW I HAVE DONE IT ON A COUPLE OF MY CARS WHICH WERE A 1969 AND 1971 YEARS (WITH POINT IGNITIONS) AND HAVE NOT HAD ANY OF THE SHORT OR LONG TERM ELECTRONIC FAILURES DESCRIBED..

TO 5537SG'S POINT(S) I REALIZE NEW CARS ARE ALOT MORE COMPLICATED THAN OUR OLDER MUSCLE CARS, HAVE MORE SENSITIVE ELECTRONICS AND REQUIRE ALOT MORE EDUCATION AND TOOLS TO FIX AND REMOVING THE POSITIVE CABLE IS NOT TO BE ATTEMPTED...AND I WILL USE A VOLTMETER NEXT TIME TO CHECK IF MY ALTENATOR IS WORKING OR NOT..JUST TO BE SAFE

BUT RIGHT OR WRONG, I AGREE WITH AARCUDA AND STUMPY WHEN THEY SAY IT WAS THE COMMON METHOD PERFORMED LOTS OF TIMES ON 60'S AND 70'S CARS WITHOUT CHERNOBAL TYPE MELTDOWNS.. I'VE SEEN IT DONE TOO MANY TIMES..

THE FACTS IN THIS THREAD CAN BE DEBATED, ARGUED ABOUT, CUSSED AT... BUT THAT WAS NOT THE INTENTION OF MY POST.. I WAS TRYING TO HELP SOMEONE GET HIS CAR GOING, NOT TO DAMAGE IT FURTHER.. SORRY IF MY BACKYARD WAYS CAUSED ANY ILL FEELINGS, BLOWN-UP RADIO'S, TACH'S OR THE CHINA SYNDROME..

GUESS I SHOULD OF KEPT MY 2 CENTS IN MY POCKET ON THIS SUBJECT..

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Hey guys the problem is the newer car electrical system if I understand correctly are what they call a CLOSED LOOP system where earlier cars had a openloop system. so when you unhook a new car battery while it running you taking a comoponet out of the circuit!! am I correct on this??

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Open and closed loop refer to a state the vehicles electronic engine controls are operating in. In a nutshell, open loop means the car is running off pre-programmed parameters that would be close to ideal. They typically operate in open loop during warmup , which is basically until the oxygen sensor comes online. It might not allow the trans to shiift into overdrive etc.. Each manufacturer has different ideas.

Closed loop is when the computer will make appropriate changes to the operating parameters when the vehicle is warmed up and the oxygen sensor is sending logical information to the ecu.

When the O2 sensor reports logical information to the computer, it will adjust fuel injector pulse widths (amount of fuel the engine sees) to keep the vehicle operating close to ideal fuel/air ratios. That's what is called closed loop.

additionally, most cars are using heaters in the oxygen sensors to help get them online sooner. Also to cope with 02 sensors mounted downstream in the exhaust system (post catalytic converter) They may run too cold there.

keep in mind during all of this, an oxygen sensor may be reporting a signal thats "skewed" enough to change the ideal fuel mixture for the situation. The computer sees a signal thats relatively normal, and continues on like everything is fine. This is when things get harder to diagnose. Keep in mind, if the computer sees an illogical signal, that just cant make sense, it realizes this and turns on your check engine soon light. The computer saves this data, and in post 96 cars, saves a whole freeze frame of everything the computer is monitoring. Your speed, what gear youre in, how far the throttle was depressed etc..

Open and closed loop are not actually what we are discussing in this situation, but I hope that helps your understanding of it! I can get more in detail if you desire.