Electrical issue: alternator ok, battery ok, but low

So here are the symptoms: noticed that the voltage gauge was lower than normal sitting about 12.5. Then as I would sit at idle, it would go to 12 volts. I turned off the car and let it sit. Started it up 5 minutes later and the voltage started at 14 (or just under), then no more than a minute or two later the voltage went back to about 12.5. I took the car to Advanced Auto, they hooked up the battery/alternator checker and said that the batter was good, but once the car started and was at 2000 rpm, it only produced 13.36 no load and 12.38 loaded with wipers, fan on high, lights, stereo. Then I pulled the alternator off and had them check the alternator on the alternator machine. It tested good! AC Ripple 0.6, Engine running volts 13.6, voltage set point 13.9, DC output 13.2. I also checked voltages I was getting at the cables that run from the alternator to the right side of the engine bay and to the starter, no voltage drop. Same as at the battery if that means anything. Any ideas?

Quotes source post: 11732189

Most likely a bad alternator regulator, which shows symptoms only when hot, especially if you have low voltage even without load. You may want to try replace the regulator first and see if that helps. If you have not replaced the starter "y" cable to the updated version, it would be good to replace that as well. More work but the stock cable is prone to corrosion and could fail anytime. When you saidno v drop on the starter cable, where did you take the measurement?

I took it at the alternator lug to ground and at the positive terminal in the little black box on the passenger side to ground. Both got 12.47 volts.

Quotes source post: 11732236

Is the "alternator lug" the terminal at the back of the alternator where the starter cable connects to? See post #9 here http://www.renntech.org/forums/topic...tage/?p=240055

Yep. The best I could describe it. It goes from that lug to the starter and around to the terminal on the passenger side from what I can tell.

No corrosion on any of the cables that I can tell. They look brand new, but don't look like they would be the updated larger diameter cables.

If you can access that lug at the back of the alternator and you see 12.47v, that means your alternator is generating 12.47v, which is way below spec. It's the alternator for sure, maybe it's just the regulator. There are many cases where a bad alternator tested out OK when cold. One other less likely possibility is something is drawing too much current that overloads the alternator. You can put an amp clamp on the cable that comes out from that little black box to check the current. That should be about 50A with low beam and a/c on. Btw, you don't happen to have an underdrive pulley, right?

No underdrive pulley. Also, I was checking to see if I could see the voltage at the battery at the lug at the back of the alternator. The alternator was not in the vehicle at the time. So 12.47 was what I was seeing at the battery also. Just checking to see if there was voltage drop over the cable. Not sure if I can tell anything by that.

Quotes source post: 11732333

To measure the voltage generated by the alternator, you need to put the two probes on the alternator lug and on the alternator casing, or the engine carrier instead.

Note to measure v drop on a charging system, a high current flow (10s of amps) through the cable is needed. That's why you saw no v drop. To measure v drop, you would need to put evevrything back and run the engine with low beam and a/c on to generate current flow.

Thanks. Ended up getting a remanufactured alternator. Installed and fixed the issue completely. Even though the alternator tested good, the voltage regulator must have been bad and didn't show up on the Advance Auto testing machine.

Quotes source post: 11733826

Glad you got it fixed. Yeah, most of the bad regulators only show their symptoms when hot, which a bench test will not simulate.

Similar problem as other were experiencing in this post but mine persists despite my best efforts. The voltage output on my '99 C2 drops to about 13 V as the air temperature rises. In the morning with air temp around 75-80 it will stay just slightly below 14 V mark for the entire 35 miles drive to work. On way home with air temps in the mid 90s, the voltage indicator slowly drops down to about 13V. Starting is no problem hot or cold. Things I've done so far - replace voltage regulator, replace the "Y" cable with the later "03" part number, cleaned the ground strap between engine and body, cleaned negative cable at the battery, replaced the battery. Nothing has changed the behavior when hot. About the only thing I can see that is left is replacing the entire alternator but don't want to do that unless I can verify that is the problem. Is the electrical gauge reasonably accurate as that is all I looked at to based my conclusions so far. Any ideas on how to test or what other possible issues could be causing this?

Quotes source post: 12373186

It's not difficult to daignose but you need to be systematic. When the symptom shows up, run the tests described here and you can diagnose it with 100% accuracy, as opposed to throwng parts at it. http://www.renntech.org/forums/topic...s/?hl=charging Note you need a good digital voltmeter, some long wires, and a home made insulated hook to probe the back of the alternator for output. To eliminate the alternator, do test 5 first. If it fails, you should replace at least the regulator if not the whole alternator. If you're not comfortable, just bring it to a shop and they should be able to pinpoint the problem. Your problem is likely the alternator/regulator or the y-cable as it seems to be temperature dependent. However, only the tests above can confirm that.

Btw, get this tester if you don't trust the voltmeter in the cluster. You can leave it plug in the cig adaptor 24/7 w/o draining the battery. You can see the morning voltage before you unlock the car and open the doors and you can judge if there's excessive current drain overnight. Amazon.com: INNOVA 3721 Battery and Charging System Monitor: Automotive Amazon.com: INNOVA 3721 Battery and Charging System Monitor: Automotive

Thanks Ahsai, I'll go through those tests and report back. Kevin

Quotes source post: 12373186

Just for reference, there are two different alternator/starter cables: one ends in 01 and the other in 03. The one you need is driven by your VIN number. My car requires an 01, but it had an 03 installed, which is not factory and it FAILED. The 03 cable is no better than the 01 cable. Purchase the one specified by your VIN. Btw, Ahsai knows his ****. :-)

This subject seems to come up frequently and causes many wasted $ and hours. So here is a a link to some other helpful work by Ahsai on the Y connector and charging faults: Ahsai has another useful thread here: https://rennlist.com/forums/996-foru...ter-cable.html "Y" connector is 996 607 019 03 .List price is $72. Don't make the mistake of thinking the Boxster has the same part but just a 986 part number.number .The Boxster part # is 986 607 019 02.It looks different on PET