Battery Meter Fluctuations at Idle
 

I have a 1996 Mustang GT Convertible (stock, no performance mods). I have been trying to clean it up a little/tune it up so that it can keep running well for me (it's only got 72,000 miles on it... need it to keep going though).

I recently replaced the alternator and serpentine belt, as well as charged/cleaned up around the battery.

A problem that existed before all of that, and still exists now, is that the battery meter fluctuates during idle. It's not fluctuating to the point that the car is going to shut off or anything like that (as in, it stays between the "A" and "O" of "NORMAL" on the meter itself)... but you can clearly see the meter bouncing back and forth. Along with that, when there are lights on, you can see them faintly dim a little during some of these fluctuations (again, it isn't drastic.. but you can definitely see them flicker... this is true with the headlights as well as the interior lights on the mirror). The bouncing meter is also evident when the turn signals are on... the meter fluctuates quite a bit while they are on, actually. The meter still fluctuates when nothing is on (as in, the car is stopped at a stoplight and I don't have the lights, AC, radio, etc. on).

I am pretty sure that this isnt' a huge problem, as everything seems to be running well enough... but it would be nice to figure out what is going on. I guess it is possible that the ground wires could be corroded on the inside... but they look ok from the outside. Other than that, I really have no idea what is causing this. The only other known problem with my car at the moment is a leak in the water pump (and a small radiator leak), which are going to be taken care of soon. I don't think that those problems would have anything to do with this, though.

Let me know if you all have any ideas... they would be greatly appreciated. Thanks.

It either a bad ground connection (check the ground to the block just aft of the oil filter adapter); or that the battery is getting tired.

Have a high amperage load test done on the battery (or buy a tester) to make sure of it's status...

Sorry it has taken so long to update this, but I finally was able to get some work done on it.

Along with the above mentioned fixes, we were able to replace the negative/ground battery cable, as well as get new terminals on both the positive and negative cables. The battery is fully charged and working well, so there isn't a problem with it. These fluctuations, however, are still occuring.

All the connections are tight/secure as far as I can tell. Is there anything that we're missing? Like I said, these fluctuations are most obvious while using the turn signals in idle (but they can occur any time when the car is idling... they just occur most consistently with the turn signals flashing). Oh, and the car idles around 600-800 RPM's I would say, judging by the gauge.

Any other thoughts? This is really annoying.

I dont know about where your at but in my town Advance Auto in my town can test your altanator. And its free os it isnt much to worry about.

Sears can conduct a full battery/starting/charging test on you're car without removing any parts, and that can pinpoint you're problem for you.

Or you can buy the tools for <$75, do it yourself, and then have the tools so you don't have to go to the FLAPS or Sears next time.

Here's a link to the generator wiring diagram, you can see on the right that the charge indicator in the instrument cluster is driven directly from the voltage regulator, which is internal to the generator.

It would seem therefore that any fluctuation of the indicator would be originating in the generator/regulator.

How was the generator tested? Just because it charges well at cruising speeds and normal loads does not mean it is providing significant current at low load/speeds--it could be that at idle it's capacity has diminished. A real auto-electric shop can test this, the roll-around "quick-check" stuff at a FLAPS or Sears would likely not detect it.

Was a high load drain test performed on the battery? Batteries can check out fine if only voltage and low load tests are performed.

Have you checked the voltage at the battery while the engine is running and the charge indicator is fluctuating--does the voltage reading at the battery also fluctuate?

Alright, once again, sorry for the long-delayed post... finally got in there to have a check done by an electrical shop. Anyway, everything checked out fine... the only recommendation I was given was that the wiring to the battery meter might not be working completely right (he said since I've replaced the battery (3 years ago), alternator (a few weeks ago), negative battery cable and both terminals (also a few weeks ago), and belt (also a few weeks ago), that it was possible that the wiring to the meter might be working incorrectly/sending the wrong signal).

Of course, to have the wiring checked... that is probably going to be a pretty big job (ripping out the dash and following wire trails doesn't sound too easy/fun... and at $65/hour, I don't think I can justify having them do it for such a seemingly small issue).

So I guess I'm probably SOL unless there are any other possible options. Like I said, it's annoying, but really isn't causing any problems that I can see. It would be great to get rid of it, but I don't know if I want to spend around $250+ to just take a chance on it.

Maybe belt slippage do to a weakened tensioner arm?

Yeah my meter will do that to. It seems that it will only do it if the engine rpm goes below 600, which is idle in gear for me. I have check it to see if it is actually drain the battery or just loosing + voltage from alternator. Turned out for me that the alternator was only giving me about 0.5V over the battery voltage. So it was still dipping but not really a problem at all. If it isn't drawing from the battery I would say your fine.