I have a brand new alternator, new battery. And yet when I I drive the car and it is in neutral with the AC on the volts drop down to the low 12's. When I accelerate with the AC on the volts will be steady around 13.4. When the AC is off it always stays around 13.4. Which is good.

I wondering why it drops down like that when the AC is on? I took a mulitimeter and read the plug going into the alternator with the car in the "off" postion it read 12.4. I then put it in the start postion and there was no reading at all. I have a feeling that the car is just running on the battery alone. this has been going on now for about 200 miles. It is a very strong batt. but I think it will not last very long if I keep this up.

Any help will be greeatly appreciated.

 

Oh and another thing, before I also had a Stinger SP800 and that batt. died on me and I thought that the Alt. went out so thats why I got the new Alt. So now its happening again.

 

check/replace/upgrade all your grounds.

 

If you have a feeling that the car is running on battery only and the alternator is not giving you sufficient electrical power while the engine is running, then try this simple test. While the engine is running, disconnect your battery cables, one by one and make make sure the engine keeps running. If the engine dies when you disconnect either one of the battery cables, something is wrong with your alternator or relative wiring.

 

I have the same problem and if i figure it out, I'll post here You car is running off the battery at idle which needs to be fixed but it sounds like it is charging at all other times so your battery shouldn't tank it soon. I don't think it is possible to see in the 13 volt range if the car was running off of the battery alone. You could use a trickle charger each night until you get it figured out.

Grounds could be the issue but weren't for me. The Hyper ground kit solved some amp draw issues but did nothing for my idle voltage drop. Nor did upgrading the alt to a 150 amp unit.

A coupe of things to try:

- With the car running, test voltage at the alternator and then also at the battery. Does it match? If not, then you have a voltage drop across the system. Could be grounds or poor wiring to the battery. Is your battery relocated? If so, is the cabling thick enough for the relocated battery? 2-4 gauge would be a minimum for me.
- What size alt pulley do you have? If it is stock, then you could put on a smaller pulley to increase the spin rate of the alternator to get the alt to charge at idle. This could be a simple fix for you if your pulley is a stock (read large) size. You might need to go to a smaller alt belt if you do this but they are cheap. http://www.excessiveamperage.com/ sells crazy small billet alt pulleys all the way down to 0.5 inches which is insanely small but something like this might spin the alt enough to solve the issue.


Hope this helps and good luck. PM me if you want to discuss all the things I've tried to no avail.

 

Cool thanks for the info. guys I will try them and post up some results.
BTW the batt. is relocated to the rear drivers side bin.

 

Just like mine. Did they relocate the voltage wire sensor also? It's the top wire of the two-wire plug into the alternator. It needs to be there to provide voltage feedback to the alternator. You can test this easily by using your volt meter with the ignition off and by testing the voltage at the wire in the clip. If it is at 12 volts with the ignition off you know the wire is getting battery feedback/power. If not, it needs to be hooked up and this might solve the problem.

 

Like Goodfellas stated, upgrade and add grounding point.

You car is actually running fine. Yes voltage will drop when you are running ELECTRICAL items. AC draws the most voltage. As long as you are not going below 11.8 volts and getting consistently lower, you are fine.

BTW if your car was running on battery power only, you will not even get a block down the street before your battery was dead since there isn't an alternator working to changer it back.

 

Oh and disconnecting the battery while the engine is running is not a good thing. You can send a voltage spike to your ECU or something similar and fry it.

 

No not true, the car is not running fine and the low 12s isn't correct. 11.8 is a bare minimum before most of the car's systems tank it and you are stranded. There is a very high likely hood the car is not charging at the levels he's discussed, the low 12s, and it will be a matter of time before the car dies on him.

He can take the car in to any shop to verify this. My car was right at the same levels he is describing and the car was not charging at this level. Yes, it charged at above 1,500 RPMs but not at idle and the voltage he is seeing I am confident is coming directly from the battery not the alt. Consequently, the battery will degrade over time and the alt will overwork itself trying to charge the battery and eventually fry itself.

And FYI, you can go about 30 minutes on a charged battery and no alt, depending on what you are running. I know as I went 20 minutes before I could get to a shop to diagnose the above. Had a Battery Bug installed on the battery to monitor volts and it started to chirp, warning me the alt wasn't charging.

You are correct that AC draws a bunch of amps, thus volts, but my fuel system draws more than anything else, 20 amps. Dual fuel pumps will do that.

 

OK I found out that the pully that I have on the aftermarket alt. is not the same. Its from the stock Alt. So the pully I have on there now is larger then the one I have bought, I have a smaller pully with a brand new Alt. but I need to get a smaller belt to compensatie. Like you said David, I will try the samller the smaller pully. Its just a big mix up with Alt. and pullys.

 

That would be an easy and cheap fix. No more room to adjust the belt you have? No big deals a belts are $20.

Not so lucky for me. I'm staring at buying a custom alt that will product more voltage at idle. Cost - $450