Sherwood, what do you mean by the "charge voltage" should be 14.5 if the A/C is on? By charge voltage do you mean the combined voltages as measured at the battery should still read 14.5 Volts with the A/C on, at idle? So is this what your car reads at the battery with the a/c on?

As far as the voltage being high 12's or 13 with the car off for it to be in spec, I just checked mine, (new optima battery) and it does read 12.99 volts with the car off.

EDIT - Dan, if what sherwood is saying is correct, it appears you still may have a problem since you stated you measured: "Car battery at Idle with A/C on 12.9". According to sherwood, the reading should be 14.5 volts w/ A/C on, not 12.9??

Also, If I am getting around 14.5 Volts at idle, but when I turn on my A/C it drops to 12V could this indicate a faulty regulator? I've replaced my alternator before, but I don't remeber where the regulator is. Usually arent' the regulators on the alternator unit itself? So in order to check this (and/or replace) I'd have to pull the fan, etc...??

A few months back we got a 911 in the shop for poor charging . Took reading at the battery and found 12.6 or battery voltage . Checked voltage at the starter connections and found 13.9 .

Hmmmm......... checked voltage at connection from alt and 13.9 .

Checked voltage at starter battery cable going forward to battery and found 12.7 .

Cut off old lead and soldered on a heavy duty copper lead to the battery cable end at the starter and rechecked battery voltage at battery , car running at idle and found 13.8-13.9 volts . After charging the battery 14.2 was found at battery .

Not saying that's your problem but Tyson Schmidt reminded me of something I've seen .

Please listen to your friends advice .

Never remove the battery cable with the car running .

Best to you ,

SxS

rnin: has really nothing to do with the Honda shop..I was simply playing off of foamy911's earlier comments at the beginning of the thread,..during the more frustrating interpretive early phases of this thread......a suggestion to take it to someone who knows what they're doing, before things get damaged...LIKE DISCONNECTING THE BATTERY WHILE THE FRIKIN CAR IS RUNNING,..(can I get a "hello")

All in all,..nothing bad intended.........in any-case sounds like the thread has progressed and Pelican University has great teachers (and students).

My best,

"Sherwood, what do you mean by the "charge voltage" should be 14.5 if the A/C is on? By charge voltage do you mean the combined voltages as measured at the battery should still read 14.5 Volts with the A/C on, at idle? So is this what your car reads at the battery with the a/c on?

Once the engine starts, all electrical energy should be created by the charging system - at the tune of around 14.5 volts. If the charging voltage goes below battery voltage, the system uses whichever is higher. If your electrical system drops to 12.5 volts with the A/C ON, your battery is probably discharging (i.e. the charging system isn't providing enough voltage).

If the alternator is new and good, if the battery is new and good, if the electrical connections are good, if there are minimal parasitic current drains, I would suspect the voltage regulator. Big ifs though.

Sorry I can't help more in detail.

Sherwood

Sherwood & Dan,

Well, I measured voltage at Idle (at the batery) and this is what I'm getting.

13.86V or so at Idle.
13.6V at fan position 1 w/ a/c on
13.5 at fan position 3 w/ a/c on.

So my voltage is dropping slightly by .3V. I also do notice my idle drops too, although I've checked my ICV and it is working properly.

Is 13.5V too low with the a/c on?? Also, if 13.86V is out of spec and not the proper "charge voltage" of 14.5V, is this cause for concern?

I was apparently getting faulty readings where I chose to tap the voltmeter gauge. I am going to re-wire the voltmeter gauge directly to the battery and the fuseblock....

As long as the charging voltage is above the open circuit battery voltage, you should be okay.

However, what happens when you also switch the headlights ON and/or the rear defroster and/or any or all other accessories? What if you have high wattage high beams or a large audio amp? If the voltage and current used to operate these accessories exceeds the charging output, the extra voltage/current is then provided by the battery, and it will eventually discharge to a point where the engine ceases to operate (min. electrical power for ign. and fuel system).

Until you locate the malfunction source, keep the electrical loads to a minimum.

Sherwood

Thanks! Well, I was curious, could it be that there is some minor variation among cars/alternators?? Or must it "always" be 14.5V??

Dralph12 said he has a "new" rebuilt alternator and he's reading "Car battery at Idle with A/C on 12.9". I too changed my alternator years back, but I'm at 13.5V w/ A/C on at idle.

So you're saying if I still may have a problem at 13.5V, seems like Draplph12 still needs to check his system too then, and should probably be even more concerned, if he's getting 12.9V at idle w/ a/c on...?

If the red light goes ON, the electrical energy being used is coming from the battery (i.e. the alternator isn't providing it). As long as the alternator can supply the electrical energy for the system AND recharge the battery (after cranking), you're okay. 14.5 volts is sort of the spec for most charging systems. Could be a few tenths less. Check the spec for your MY 911.

What's the off-idle charging voltage with A/C ON? The closer it is to "14.5", the more margin you have in case you have to operate A/C, high beams, rear window defrost, max. fan speed plus other accessories concurrently. You don't want to operate on battery deficit for any length of time. That is a one-way ticket to being stranded.

Sherwood

You have a voltage drop somewhere. One quick way to find is remove one fuse at a time,check voltage intill you find the circuit.

I found that one of my problems with low alternator voltage at idle was due to the car Idling @ 650 rpm's not 850 where it needed to be. This made a big difference with the alternator voltage @ Idle.