Weak alternator?

2005 Mercury Grand Marquis, 102K miles

I noticed the other day that the voltmeter on my instrument cluster was indicating a bit low from normal (dead center). Knowing better than to blindly trust a cluster gauge I checked voltages with a multimeter, and it read 13.3V at the battery, and 13.4V at the alternator. This was after a 35 mile drive home from work with the A/C running virtually the whole time. Later on that evening I ran an errand, and noticed that despite the car sitting for a couple hours, the voltmeter was in the same low-ish spot. Checked voltages again with the multimeter, and they had not changed either, still 13.3/13.4V.

This morning I drove to work, and saw that the gauge (and voltages) did NOT drop off. Meter was dead center like it should be, and testing showed 13.9V all around. Today I drove home and kept the A/C off, and once again everything stayed right where they started, 13.9V. As an experiment I then turned on the A/C once I got home and rechecked voltages: within seconds it was back down to 13.3V, and it did NOT recover back to 13.9V after shutting the A/C off. It looks like once the voltage drops, it stays there for a while regardless of A/C being on or not.

Anyone seen these particular symptoms before and have a thought? Seems to me like it’s possibly a slightly weak alternator (I don’t like that it never quite reaches 14V under any circumstances). Another thought I had was that perhaps the belt tensioner is weak, as I did notice that when the A/C compressor clutch is engaged, the tensioner wiggles back-and-forth a bit, then returns to steady and solid when the clutch disengages. I should also add that the battery voltage when off is always good, 12.5V to 12.7V.

Are all these measurements taken with the engine running? battery voltage not charging should be between 12.2 and 12.8 (low state of charge and high state). Charging, it would be 1-2 volts higher.

When charging is the voltage at the battery and at the alternator about the same? Are you moving the ground connection as well as the hot lead?

Considering the likelihood that the underhood temperature was considerably higher when the voltage seemed low compared to when it was normal it is likely that everything is good. Voltage output drops somewhat as temperature rises. I still work on the basis of 13.8v being ideal for an alternator but I understand that today 14.8v is very common. I would be concerned if output voltage exceeded 15v though. It is good to pay attention to any changes in the gauges. You may have a problem developing and it’s good to get ahead of the curve and save yourself from Mr Murphy, @budd2049.

@BillRussell - yes, those are voltages with the engine idling. All voltages were checked both at the battery and the alternator (using the positive output stud, and the alternator case for ground); at most there was a 0.1V difference, if any.

I don’t see any problem with those numbers. The battery seems to be fully charged, showing 12.5 to 12.7 volts with the engine off. You have no problem cranking the engine, right?

What the battery voltage and alternator output voltage read depend on the circumstances. If the battery is partially discharged – like the car has been sitting for a week without every being run, or you’ve cranked the engine for 5 minutes unsuccessfully and drained the battery – the alternator voltage (once the engine starts) may be 14.5 volts or even 15 volts. That voltage will reduce as the battery becomes fully charged, to maybe 13. or 13.5 volts, which is just enough to keep the battery charged with the engine running.

The interaction with the AC may be due to the sophisticated computer electrical power management vehicles use these days. Not sure if it applies to this one, but many newer cars have a computer monitor which devices are on, which aren’t, what the battery charge is, and decide how much load to put on the alternator. For example, if the AC is on, the computer may decide to reduce the engine load by not demanding much from the alternator. Then when the AC is off, it may decide the battery has run down a bit, so since the engine now has plenty of extra power, it’s time to fully load the alternator and top the battery off.

Turn on your AC, your blower motor on high and your headlights on high beam and measure your voltage. If it is 13.3 volts or higher, your alternator is good. In fact at idle, a lot of alternators will not get that high. If yours doesn’t, raise your engine speed to about 1400 rpm. If it’s 13.3 or higher, you are good.

If you have an aftermarket high powered audio system, then your current alternator may not be adequate, most are not for that. You need a high amp alternator for some of those systems.

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Based on what I’ve read here, your battery and charging system are just fine

Have you been having problems starting the engine?

Has the red “charge” indicator been coming on while driving the car?

How old is that battery, by the way?

I tend to think that you’re fine although I also am curious about the age of the battery.

The roughly 12.6 volts is good but what is it after it’s been sitting overnight?

Just wondering if the battery could be getting a bit shaky which could be causing the alternator to work a little harder and the more amps required the lower the voltage.

Those sound like pretty normal voltages for a charged battery on a warm day. That is about what I get on my vehicle. If you’re worried about the battery, have it load tested.

Here’s another vote for normal voltages. I see nothing to be concerned about.

In fact, I just replaced the battery earlier this week as the old one gave out on me rather suddenly. Monday morning I drove to work just fine…went out to lunch just fine…then I went to leave work and it was dead. Got a jump and made it to a nearby parts store, and once I shut the car off I couldn’t start it again. Naturally it failed a load test at that point.

Thanks for the reassurances everyone. The last Panther platform car I owned (2000 Crown Vic) let me know loud and clear when it truly needed an alternator: battery charge warning light came on steady and voltmeter just went down, down, down. Replaced that one and it lasted till I sold the car three years later, so I’ll not worry about what I’m seeing here.

Thanks for this tip been trying to figure out why i go thur so many alternators

How old is the serpentine belt?