Here's a strange one.
Does anyone know what the consequences are of spinning an alternator at high speed (6500rpm crankshaft speed) without it being turned on?
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If the regulator is not turned on... so it's not making power... I don't see an issue. It would be like spinning the A/C compressor without the clutch engaging.
What would it matter if it's putting out power or not?
If there's an infinite load then it makes no difference on the rpms of the alternator or the output.
Sure, it would get hot over time from frictional loads but with no electrical load, it'd just spin it's little heart out.
Misnblu.com
Newbie member since 1999
Thank you Dave and JBO! 
Haven't looked at this post for a bit. Honestly, I didn't expect any replies to it.
I was a bit worried about damaging and destroying a perfectly good alternator if it's switched off, while cranking the engine at over 6000rpm. I'll be rewiring my car, and I figure it might be good for a couple of HP if I just wired in a switch to interrupt the signal to the alt so that it doesn't turn on. Though I don't have much electrical load typically anyways, so I probably am not putting too much load on it anyways. Would only be used for a short time period, like dyno runs and 1/4 passes and such. Other than that, pretty much useless mod.
Guess I'll just try it, and find out what kind of damage gets done.
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As long as the bearings can handle it... there shouldnt be a problem..... an alternator is actually much easier to spin with no power going to it... so it might even be better for it.
You still have to cooling action from the fan behind the pully and it isnt putting out as much heat because its off.....
If it can do it with power going to it...... it should definitely be OK without...
Haha, Jason has a good idea.
You've got my old brain thinking now. lol
I would guess if you're going to use a switch of some sort, you'll have to have something heavy duty enough to handle the output.
Give us some ideas man.
If not, I'll figure it out. lol
Misnblu.com
Newbie member since 1999
Thank you Dave and JBO!
Misnblu wrote:Haha, Jason has a good idea.
You've got my old brain thinking now. lol
I would guess if you're going to use a switch of some sort, you'll have to have something heavy duty enough to handle the output.
Give us some ideas man.
If not, I'll figure it out. lol
Personally.... I would use an older mechanical Ford starter relay inline with the bat wire ( one wire that way.... theres 3 I think for gounds)...
personally all i do is take the belt off..i mean damn, it takes what?2 minutes to get our belt off ? lol i dynoed 8whp more accross the midrange with my belt off on my dyno runs...only 1-2 peak hp but a gain in the mid is still a gain. not really sure what youll get off simply turning the alt off
scott (section8cav) wrote:personally all i do is take the belt off..i mean damn, it takes what?2 minutes to get our belt off ? lol i dynoed 8whp more accross the midrange with my belt off on my dyno runs...only 1-2 peak hp but a gain in the mid is still a gain. not really sure what youll get off simply turning the alt off
Not on my car. The way the upper motor mount is on my car you actually have to jack up the motor, unbolt the mount then slip the belt out that way. I wouldn't really feel comfortable just letting the thing hang in there. Also, mine seems to get up to temp REALLY fast, and it doesn't take too long to just sit still without the fan running before it'll start to overheat. I can't imagine how quickly it'll do that without the water pump running. By turning the alternator off you lose the field and won't have the resistance from the alternator charging the battery. Might be a small gain, but a gain nonetheless.
Misnblu wrote:Haha, Jason has a good idea.
You've got my old brain thinking now. lol
I would guess if you're going to use a switch of some sort, you'll have to have something heavy duty enough to handle the output.
Give us some ideas man.
If not, I'll figure it out. lol
It shouldn't take a whole lot of a switch to do it. I don't believe there's much current running into the alt, just 12V+ reference. I'm not talking about disconnecting the B+ wire. Where the little plug is, you have 2 wires (should be red and grey). The red one is a signal wire going to the alternator, if that doesn't get 12V to it, the alternator will not turn itself on, and will never charge the battery. Simply wiring in a switch and opening the circuit would cuz that voltage reference and the alt would just stop trying to charge.
I learned some time ago that I can run for a long time (though my battery was relatively new at the time, and now it's not) with that plug disconnected. Apparently I didn't have it pushed all the way in and it came unhooked on me. But I know I can drive with it unhooked, but that's driving and not ripping it up to redline in every gear.
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^^^^^ How hard is it for you to get to that plug with the alternator on the car?
IF you have small enough hands you might just be able to reach in there and pull the plug out enough to disconnect it quick before you race....
Either way.... if you do this.... post the difference in times.... if any...... kinda curious
I have the alternator on a switch in my circle track car. At first I did a few laps with it off and didnt really notice a diffrence so I switched it back on and noticed a diffrence right away. Its nothing huge, but if its enough for me to notice then its definatly worth it.
Some people say its no real advantage cause everyone else is also running an alternator so its no big deal. I think what they don't realize is that the alt is not always charging. If you are pulling off a corner next to a guy and your alt is charging and his isn't then he has an advantage. The switch makes sure its always off, except for caution laps on longer races of course.
And what scott said about just removing the belt all together. That only works on the 2.4 cause the water pump runs off the timing chain. The 2.2 alternator belt also runs the water pump, so us 2.2 guys cant do that, that belt is important.
^^^^^^ if your car is running..... your alternator is charging....
The alternator constantly charges.... the amperage just changes when needed for load..... and the more its putting out... the harder it is for the engine to turn. (think of how magnets push themselves away from eachother)
Your car technically has more power going to the ground with your lights off but it's prob not even enough to measure.
Your alternator doesnt "shut itself off" like a air compressor motor does.
Actually, that's not quite right. Aside from the PCM telling the alternator to turn on or off.... There's also an internal voltage regulator that controls the field current on the rotor, which limits the voltage output. It's something like 400 or 500 cycles per second... the amount of time the alternator is "on" or "off" depends on the rpm of the shaft turning... it's off most of the time at high speeds, and on most of the time at lower speeds (ie. at idle).
While it very rarely shuts off 100%... most of the time while driving, it's off 90% of the time.
Either way regardless of its on and off function, we all agree an alt not charging what so ever is better than an alt charging, so this is worth it for oval track, drag or whatever it is you're doing.
Umm..... if that where true my volt meter would jump around because the second the alt stopped charging it would drop below 12 volts....
Yes at times the output is minimal..... but the thing is always on..... theres always some juice coming out of the alternator when the car is running.... the only time the alt doesnt put out anything is if the engine isnt turning or the ignition is off.. You would be surprised how much electricity is needed form the battery for an average car to run correctly.... the alt needs to be on all the time to keep the battery level the same. IF charging stopped.... you would almost immediately drop to a little under 12 volts because of current draw... (output is closer to 14... all according to a volt meter btw... I know its really amperage changes your seeing) sure a car can run on that for awhile.... but going back and forth would kill the battery... and since the alternator is designed to put out what the car is drawing from the battery plus a little more.. it's always charging.
And are you sure the ECU has anything to do with the alternator? Im pretty sure it doesn't... You have a battery wire, a ground, and a wire that I think is hooked to the ignition to tell the alt to turn on (kinda like a remote turn on in an amplifier)
Weebel wrote:Umm..... if that where true my volt meter would jump around because the second the alt stopped charging it would drop below 12 volts....
Yes at times the output is minimal..... but the thing is always on..... theres always some juice coming out of the alternator when the car is running.... the only time the alt doesnt put out anything is if the engine isnt turning or the ignition is off.. You would be surprised how much electricity is needed form the battery for an average car to run correctly.... the alt needs to be on all the time to keep the battery level the same. IF charging stopped.... you would almost immediately drop to a little under 12 volts because of current draw... (output is closer to 14... all according to a volt meter btw... I know its really amperage changes your seeing) sure a car can run on that for awhile.... but going back and forth would kill the battery... and since the alternator is designed to put out what the car is drawing from the battery plus a little more.. it's always charging.
And are you sure the ECU has anything to do with the alternator? Im pretty sure it doesn't... You have a battery wire, a ground, and a wire that I think is hooked to the ignition to tell the alt to turn on (kinda like a remote turn on in an amplifier)
For your first question, the plug is right on top. I'd just like to have the switch because I can switch it off at the beginning of a run (or once the car is staged up) and flick it back on at the end of a run.
Most newer cars the alternator is PCM controlled. My 05 TrailBlazer will actually, when it's really cold out, not even turn the alternator on for about 30 seconds of the engine running just to keep that load off of a cold engine. Kind of a strange thing, but it is computer controlled. And like Lenko said, the internal regulator is PWM to help reduce load on the engine (probably why the PCM controls it too) thus giving you slightly better MPG. NO the alternator does not need to be hooked up to the PCM to function properly, or my 3400 alternator would not work. At either rate, the alternator will adjust for load, if it doesn't need to make power so much when the battery is fully charged, it won't try to.
You would be suprised how little current draw it takes to run my cav. I had an alternator go bad and managed to drive the car for more than an hour with the alternator generating no power. You won't instantly drop below 12 volts unless you have a bad battery. An SLI battery is made for short times of large current draw anyways. And a 12 volt battery is dead when it reaches 10.5 volts under load, so being under 12 volts is not instantly a bad thing.
www.gmscf.com
Hey, I'm only an electrician by trade and an electronics technician by training... so it's not like I know anything about this stuff
Now, without trying to deliberately take this thread away from the original topic... I suppose it's related...?
Batteries have a two ratings, reserve capacity (the amount of time, in minutes, to discharge to 10.5V at a constant draw of 25 amps) and cold cranking amperage (aka CCA; the minimum amperage the battery must deliver for 30 seconds while at least 7.2 volts). Batteries are between 2.0 and 2.2 volts per cell, and have 6 cells... so they can be anywhere between 12.0 and 13.2 volts fully charged - depends on the battery. Most I've seen on cars are around 12.6 fully charged. (Locomotives are what I work on usually... they have 8 batteries at 8 volts each, makes 64 volts.. for the starter motors... charging voltage is around 72-74 volts depending on the type of unit) You want to charge it around 14.0-14.5 volts, most GM alternators have the voltage regulator internally set to that voltage. It you go over 18.5 volts, it'll actually throw a code for high power circuit voltage...
The alternator plug should have at least two wires in it... those go to the PCM and nothing else. Here's a quote from Corvette forums that describes it perfectly:
Quote:
The PCM monitors the alternator through the red and grey wires
The L-terminal circuit from the generator is a discrete circuit (a discrete circuit has no splices and only one source and destination) into the PCM. The PCM applies ignition voltage to the generator L-terminal circuit. A small amount of current flows from this circuit through the generator windings to ground to create a magnetic field which starts the generator process. When the generator is at operating speed and producing voltage, a solid state switch for the L-terminal circuit in the generator opens and the PCM detects that the initial startup current flow has stopped.
The PCM expects to detect low voltage on the L-terminal circuit prior to the generator rotating at operating speed and conversely expects the circuit to be at ignition voltage potential when the generator is operational. When the PCM detects a fault (circuit shorted to ground, or circuit shorted to voltage), the Driver Information Center will display Charging System Fault.
The generator has an input to the PCM called the F Terminal to indicate the percentage of total capacity that the generator is producing. This signal is detected by the PCM as a duty cycle from the generator and displayed on the scan tool as a percentage. The PCM can monitor the generators output under all conditions to determine if it is functioning normally.
When there is low demand from the electrical system on the generator, a low duty cycle percentage will be displayed. As more accessory load is placed on the generator, the duty cycle output detected by the PCM will approach 100 percent. A normally functioning generating system will never reach 100 percent as indicated on the scan tool.
The L and F terminals are the red and grey
OK.... I know exactly how the charging system in a car works.....
JOHN.... I think for the most part we are saying the same things but in different ways... and confusing each other... so I'll leave it at that...
As for the computer controlling the alt.... thats new.... I didnt know that..... seems a little uneeded to me though.... pretty sure my car doesn't do that..... they just gotta keep adding features dont they :/
SHOoff.... I might be thinking of times when I was working on cars and saw this happen IDK..... and I quess your right.... I had an alt quit on a Sunbird I used to have once and I mad it about 30 miles before it wouldnt run anymore... although theres always enough draw to justify it charging on a normal street drivin car even if it really doesnt need to be.
My main point was about the racing thing where your alt might be charging and the other guys not charging thing that was posted by "mr efficiency".... IDK what chunk of air he pulled that out of and was just trying to explain to him why he's crazy... are you really blaming the fact some one pulled on you out of a corner was because of his alternator LOL
Sorry to throw things off like that.....
I am curious what difference this makes...
Weebel wrote:SHOoff.... I might be thinking of times when I was working on cars and saw this happen IDK..... and I quess your right.... I had an alt quit on a Sunbird I used to have once and I mad it about 30 miles before it wouldnt run anymore... although theres always enough draw to justify it charging on a normal street drivin car even if it really doesnt need to be.
My main point was about the racing thing where your alt might be charging and the other guys not charging thing that was posted by "mr efficiency".... IDK what chunk of air he pulled that out of and was just trying to explain to him why he's crazy... are you really blaming the fact some one pulled on you out of a corner was because of his alternator LOL
Sorry to throw things off like that.....
I am curious what difference this makes...
For power the figure I came up with, is at maximum load the stock J-body alternator should be able to make about 1.5kW of power. Which is about 2hp. So it's probably taking 3-5hp to turn the thing if it's trying to charge a completely dead battery. Not a lot of whp would be gained by doing this for sure.
Other things that would be a concern is the voltage drop from the alternator not running. The coils will still charge fine so ignition will work without any noticeable difference. But the flow rate of the fuel will slow down some. I was pulling nearly 10.5:1 on the dyno last year, so I know that I have some room to work with. And the MS unit can be set to compensate injector duty cycle for voltage loss. I'd naturally assume that a stock PCM would do the same trick.
Not looking for a huge gain here, its certainly not something that's going to be overly impressive in the power department. But it'll give me something else to play with.
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So at the drag strip it might shave off a thousandths of a second then LOL..... still something to play with I quess...
