Wiring problem, erratic voltage

[Doc Holiday]

My alternator spits out erratic voltage when I throw the voltage tester on it. I had the alternator tested at the local parts store, and they are telling me that it checks out just fine, so I must have something wrong with my car. The alternator uses an internal regulator, and has a post where youhook an excitor wire to it. I checked my exciter wire, and even without the alternator in the car at all, the exciter wire spits out a fluctuating voltage. I'm thinking this isnt right. Can someone let me know? The car has been wired with a Painless wiring kit (thewhole car, the factory harness is no longer in the car). The painless kit I used is called the 50002 Race car kit/12 circuit. The battery is in the back of the car. I've also found that the voltage is lower by almost 1 volt at the battery than it is in the front of the car. Anotherweird issue is that the voltage when tested at the MSD box is 1 volt higher (15.5V). This is all tested with the car running. With the car off, the voltage is a consistant 11.91 volts EVERYWHERE, even the battery (which telles me the battery is dying). Any advice, because I cant figure out howto get the alternator to stop fluctuating.

[Rick_R]

You may need a diode in the powerline of the MSD box. some of them put out AC back into the system and it makes the regulator go funky. I have seen it before on an older box, not sure about newer ones. I'm not sure how good a parts store alternator test is, they probably don't have a scope to see if there is ripple in the voltage, could be a diode issue, this one has Ray written all over it....

[RRRAAAYYY2]

First thing it sounds like you need to do is junk your battery. If its at rest voltage is under 12.5 volts, yours being 11.91, it is toast. This could be the cause of your alternator going crazy. A battery at 12.5 is almost at a 50% state of discharge for a cranking battery. A good healthy battery will be over 12.72 volts.

Next you have a 1.0 volt drop from front to back, this is too high as well. The cable you have going from front to back is too small. Either that or you have a ground issue. Either way you should not have more than a 0.5 volt drop in your system, and that is just to get it to everyday vehicle standards. That should be considered 0.3 allowable voltage drop on the positive side, and 0.2 allowable voltage drop on the negative side.

The first thing I would do is charge your battery with a charger, not the alternator. Once it is fully charged, have it both load tested, and electronically tested. I really suspect it is going to fail. If it is good put it back in the car, if not replace it. If it tests good I would also consider trying a different, known good, battery before putting it back in the car.

It the battery has a cracked plate it can pass both the load and electronic test, but still be a problem. The only way to check for that is to see what it requires to float charge it. i.e. once it is fully charged, how many amps does it require to keep it fully charged. In other words if the battery is fully charged (automatic charger for at least 24hours) but it stays on a 5+amp draw when connected to the charger, it likely has a cracked plate and needs to be replaced.

The battery may very well might cure your problem with the alternator. If it doesn't call me at 1-888-214-7774, and we can go over some tests from there. Hard to explain on the net, and would take tons of typing back and forth.

Once we get the alternator solved, I would then voltage drop test your car. Here is an explanation of how to do that:
http://www.svapowerproducts.com/html/voltage_drop.html

Doing the test this way, will not only tell you what the voltage drop is, but WHY and WHERE it is occuring. (Just in case Six Shooter is reading, :) ) . When doing the test front to back, you can use a set of booster cables to extend the one lead of the multimeter. This will help you figure out why you have different voltages in the car when it is charging.

[Doc Holiday]

I am going to buy a new battery tomorrow and throw it in the car. I will let you know what happens. Thank you for the quick response.
BTW, while I have your attention, why is it that when I check all my wires with the car off, the voltages are all the same, but with it running, they are different all throughout the car? Is this because I have poor wiring, or is it from the erratic alternator output? Does testing with the car off tell me if I did a good job? Or do I have to have it running with a steady voltage output and then test it to know if I did a good job? Thanks again.

[RRRAAAYYY2]

When you check the car with everything off, you are only trying to pass enough amperage through the system to make the multimeter read. In other words one strand of one wire is capable of carrying the amperage without a voltage drop.

This is why you test it with the system you are checking turned on. That way you get real world data. Don't be surprised to find several places where you have big drops. Your starter's main battery feed will likely be one, especially with the battery in the trunk. To get an allowable drop you would need 0000 or larger wire, so you have to make a comprimise there. We like using 1/0 as it keeps the drop to within the tolerances of the starter.

I will try and use the starter as an example. We will say the battery is in the trunk, and you have a 10 foot lead to and from it. So we have installed 1/0 lead to it. Testing the lead with:
1amp load = 0 volts drop
10amp load = 0 volts drop
100amp load = .2 volt drop (guessing)
140amp load = .36 volt drop
300amp load = 1 volt drop (gear reduction starter load)
600amp load = 2+ volt drop (older direct drive starter load)

The above 100, 300, 600 amp loads are approx's . Continuing we see that trying to use this system with the older style starter is not going to work. The starter itself pulls so much amperage that it will drop the voltage 3 volts, add the 2 volt drop from the wire, and all of a sudden the motor is trying to crank at 8volts. Which means the maperage will go, so will the heat, which increases the resistance, etc.

So we then turn to the PMGR starter. Its load is less, and will only take 2 volts from the battery, the line adds another 1 volt drop, and we end up still with over 10 volts. The starter will work fine at this voltage.

Now say you use that same wire to be the main power feed for your fuse panel. You have a 100amp load in the system, so the wire only drops .2 of a volt, which is no big deal and very acceptable.

So by doing the voltage drop test with the devices you are using, under their normal working conditions, gives you the most accurate answers. In the above example you would have failed the wire if it was being used with a direct drive starter, but it is fine with a PMGR starter, and great for a power feed to the fuse panel. Same wire, three different answers.

It sounds like you never clicked the link to the explanation of how to do the test. When you do, read through it as if it is the easiest test in the world to perform. Everyone tries to over think, add stuff to it, and make it complicated. I am serious, my 8 year old can do the testing no problem, it is that easy. Once you test two wires you will laugh about it. It is as simple as it sounds in the link.

[Doc Holiday]

I am ashamed. You are right, I didnt click the link, but I have now, and I will use this method to test everything on the car I can think of. I hope I'm not scared at what I find. Thanks for your help. I really appreciate it. I'll be back here crying if it doesnt look good. Ha ha.

[RRRAAAYYY2]

:), and you are welcome. If you need it, and from your original description I think you will, I can email you a wire sizing chart. sva-sales@worldchat.com . There is a downloadable version on my site, but it sucks.
I don't think you will find your race car being as scary as much as if you checked your daily driver, especially if it is a 90's Chevy pick-up.