koolaid_kid

I'm just sayin'. You really only have two things it could be: an intermittent ground or a faulty rectifier bridge. The function of the rectifier is to convert AC to DC. The ripple of AC leaking into the DC would be an excellent explanation for the effect you are seeing. If you wish to cheap out you can find just the rectifier for half the price. But for me, you would be penny wise and pound foolish. You would be better served to just rebuild the entire thing at 1/5 the price of a rebuilt alternator, which installs the same kit and then charges you 5 times as much.

rv93

it does sound like its the rectifier. but why does it work normally at idle (<800rpm)? could it be that i just dont notice it that low?

koolaid_kid

Yes. At idle the AC output voltage may only be 20 volts (working from a motorcycle output, but probably similar). As RPM increases, AC voltage increases. On a bike, it may increase to 70-80 volts AC. AC is a sine wave. The higher the voltage, the taller the peaks and valleys in the sine wave, so the ripple effect increases in the DC output.

rv93

i thought it would all cancel out the same at all RPMs, but if you say so, alright then. thanks for the help!

TGGodfrey

A side note which probably wont matter to the Original poster.

RPM of Alternator does not affect voltage (in a way it does but not the way it appears to be mentioned here). Alternator voltage output is designed at a specific value based on the windings of the rotor and stator. Speeding up the rotor in the AC half of the alternator increases the frequency. In the DC side (after the rectifier) the 14V nominal output is actually a pulsed output not a perfect linear 14V. As you increase the frequency on the AC side you are also increasing the Freq on the DC side. The DC actually looks like a series of speed bumps on an occiligraph or many positive cycles of a sinewave. More peaks in 1 second = higher frequency (hZ).

Amperage is drawn, never "pushed" as many people think of it. If you increase the frequency of the 12V pulses your increasing the amount of amperage the alternator CAN output.

Batteries are not force fed to charge. When charging a battery you always charge at a rate higher than the nominal voltage. Current in AC and DC always flows from higher voltage to lower voltage. That is why the Alternator is rated at 14v and many high amp chargers are 16V. Ok with that said, Batteries at lower charge have a lower impedance than a battery that is fully charged. Low impedance means less resistance as a bottom line. So when you go to charge a dead battery, the ammeter of the charger will show say 30amps and by the time its fully charged it may show 5 amps. The most accurate battery charging should always be done with an ammeter. This is also why a battery becomes hot when fully or over charged. The Cells reach a high impedance state and with resistance comes heat.

Why does the alternator show 12.5 volts at idle and 14.5 volts at 2000 RPM? Surely that means Voltages increases with RPM right? Not exactly. Think back to Ohms law. Voltage and amperage are inversely related; As amperage increases voltage will decrease if the source remains constant. At idle your alternator has a given load on it of say 40 amps. With no load the alternator will output 14.5V but once you have a load of 40amps on it, the voltage sags at 12.5. Increase the speed of the rotor will increase the frequency of the 14.5 pulses and enables the alternator to support a higher amperage, remember I said the output was pulsed? Less frequency means the valleys are larger and there are gaps between the true 14.5 peaks. Increase the frequency and the valleys are smaller and there are more 14.5V Peaks.

Now go one step farther to the rotor. The rotor is energized by the system voltage and that is where the alternator starts creating power. If you have low voltage to start with then your using an undervoltage rotor to create power. So at idle the alternator is getting kicked in the nuts from both sides. Lower voltage to create with and sizeable demand for base electronics. Once you spin the alternator faster the frequency of the output is increased allowing the alternator to supply more amperage. This reduces voltage drop and also allows the rotor voltage to increase 1.5 volts which gives the alternator even more muscle.

Why cant we just supply 14.5v to the rotor (field voltage) at all times? We dont have a 14v battery thats why. Design a secondary excitation circuit to power up the rotor windings and naturally you can up the voltage supply to an extent. Your actually increasing vars into the AC portion of the alternator which will result in higher DC voltage. This is how AC in created in Nuclear Steam and Fossil generation.

Why does a bad alternator sometime output 18V. An alternator output is based off the regulator for the most part. The stator windings are relevant but affect the amerage more than anything. The regulator acts as a transformer for discussion purposes. If the AC portion of an alternator puts out 28v AC, a 2:1 ratio regulator would give you a nominal of 14v. If the windings on the high side of the regulator were to short; you could end up with a regulator of 1:1. 28v AC : 28v DC.

Now some newer style alternators with PCM controlled regulators are able to fully regulate the alternator output at idle and RPM ranges so that the alternator outputs a constant voltage no matter what RPM. The 01 Camaro and older is NOT one of them.



Kinda sounds like circular discussion doesnt it.....thats becauce it is!

TGGodfrey

I wish more people had that attitude with diagnostics. I know it takes more time but that is where you find more knowedge and skill. The dealers and shops ruined the "true mechanic" by pushing the concept of parts changers. Now you have people who just know symptoms and possible solutions. People just dont take the time to understand and diagnose based on how everythig must operate. Kinda unfortuneate for those who have to pay to fix thier car.

rv93

youre right, not very relevant to the original question, but interesting info indeed. i misspoke. i wasnt trying to imply that voltage goes up with RPM. just still not sure why the regulator seems to function fine at low RPMs, but fails at higher revs. however, im becoming more and more sure that the problem is somewhere in the alternator, if not the regulator. i disconnected all connections to the alternator, and the problem went away. then i connected it back, and with all electric accessories off, checked voltage as close to it as i could (battery terminals), and problem was there (used a 12V bulb to test; voltmeter doesnt show any oscillation)

TGGodfrey

Voltmeter might not be sensitive enough, Bulb gives you a reassuring indication though.

As for why it seems ok at low RPMs and bad at High RPMs could be explained by understanding how different styles of Diodes work. Every Diode has a voltage rating. As you already understand a diode allows current to flow in one direction only correct? That is true but only as long as the voltage remains in the diode's operating limits. Lets say a Diode has a limit of 5V. That would tell you that 5v and less the diode will only flow in one direction but when the voltage spikes over 5v the diode will allow it to bleed off the excess voltage. Kind of like a water dam it begins leaking. As components get old their ratings are affected. This is a huge problem with small electronics like ECU/PCM's. Alternator regulator circuitry is just as likely to have issues related to age as well.

Why would a diode have such ratings? why not make it just a one way valve sort to speak? I hate the water comparisons but think of a huge water pipe like the mains that feed fire hydrants. If you open the hydrant real fast all this water comes rushing out and the water in the pipe builds velocity. If you slam the valve shut, the velocity cant just come to a halt and the result is a buildup of pressure at the valve. The pipes will burst. Same concept applies to a Turbo and Blow off valve if you are into turbo system design. Same deal for DC current only tracking and arcing, and component failure are the result. Fuel injector circuits, relay circuits, and motor (iac ect) have this pull up circuit for this very reason.

So now that we have covered Diodes and how/why they work; think of the AC coming out of the alternator and entering the regulator circuit. You have the portion of the AC wave that is positive and the half that is negative. You want the diode bridge (typically 6 diodes 3 and 3) to only allow the positive portion of the wave to pass though but becuase the 3 diodes that limit reverse 12v (-12v) are failing, your getting -12v exposed to the system. With the flickering lights your seeing the alternator actually pull on the 12v+ of the cars electrical system. Your seeing depressions in the voltage supply.

rv93

that explains it. i knew the basics of more advanced electronic components like diodes, but i never knew specifics like that.

TGGodfrey

Years of building, modifying and diagnosing Megasquirt ECU's.

rv93

haha, that would do it, i suppose. im still in college...much to learn.