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.