1998 S90 alternator output

[jimmy57]

Oh, and I'm getting 13.4v on mine 1998 S90

13.4V initially with engine speed at least 1500 or 13.4V after it has been running and engine is idling? What accessories on?

The alternator amperage output at idle is 55-60% of its rating. As result of the load on the alternator the voltage when idling is reduced proportional to what is turned on in car (the loads). You can't get rid of fuel pump and injection system load but the total of that and the few other things you can't turn off is 6-8 amps. Headlights, a/c with blower motor, radio amplifier when equipped, heated rear glass, seat heaters, and the few other possibilities can give load up to 50 amps. If all those were on and the alternator had been charging 15 minutes the voltage would likely be 12.6-12.8 The battery might even be kicking in a few amps at that point, definitely if cooling fan is running.
The voltage regulator has a temp based feature and the voltage will be higher initially to assure starting drain on battery is replenished and then the voltage drops to mid 13V range to keep from heating battery and losing electrolyte to evaporation.

[beav700]

I get it when idling with cold engine

[thefisch]

So I swapped out the battery this weekend. Old battery was only showing 10.6v with 15CCA left. Took it to NAPA for a warranty swap and they didn't even bother with charging to test it - figured it was a bad cell for the CCA to get so low. At any rate, got a decent prorated credit towards a new NAPA Legend Premium (model 8448).

Thanks again for the tip on the front hold down - mine was actually intact and was able to loosen and tighten just fine. Probably one of the easiest battery swaps given all the room you have.

With a fresh battery showing 12.67v, the alternator at startup with climate off, radio off and DRL on was putting out 13.7-13.8v (using the AC V setting, the reading was 28.9v). With climate on full blast, radio on, and DRL on the voltage dipped to 13.3-13.4v volts but held steady. Drove the car on a 2 hour roundrip with AC on and radio on and used an in-car voltage reader (via socket) that showed 13.3-13.4v the whole trip.

Only when I stopped at home did voltage dip to 12.9-13.0v while idling in driveway. Not sure why, but I will check the car again in a few days. After letting the battery settle for a few hours, I got a reading of 12.88v. I plan on keeping an eye on the state of charge after letting the car sit as I am a little concerned there could be a draw that got the last battery down so low. Although it could have just been its time.

Thanks again jimmy57 and beav700 for all the help.

[lummert]

Could the belt be slipping?

[jimmy57]

In FL with a/c on the cooling fan was likely running and with Daytime running lamps on, the idle charge rate is low enough for it to be that low. If the alternator was bad the voltage would likely have been 12.4-12.5 or even lower.

[thefisch]

The belt was replaced a few months ago and seem to be fine. No issues with PS pump, and even when battery went south and alternator was only putting out ~12.7v (due to load from very low battery), there was never an issue with electronics while driving the car. So I don't think the belt was the issue, just a bad battery.

jimmy57, thanks for confirming that alternator output at idle after the drive wasn't too low - you are right that the AC was running too.