2G Very odd low voltage issue.

[_97tsiawd_]

Here is the deal: My volt gauge (tested to be accurate) will from day to day, hour to hour read different. At times, 13.5-14v. Pretty normal. However, other times will be as low as 9.5-10v. The weird part is that until lately I have had no drive-ability issues. But, then yesterday it turned over very slowly. It started, volts read 12 and within a block were at 9 and the lights dimming terribly. I have had both the alternator and battery tested. Both good. My thought here is that it is the regulator. But can the regulator that is built into the alternator go bad while the alternator still tests good?
The alternator was purchased 13 months ago and is a 90 amp.

Tension on ALT belt checked and good as well.

 

[Calan]

Both the battery and alternator can be bad while still testing "good", depending on how they are tested and who does the testing.

Sounds like a dead cell in the battery.

What does the voltage read while cruising on the highway?
What does it read at idle, and what does it drop to if you turn on the AC, roll down a window, etc.?
Does the voltage change as the outside/under-hood temperature changes?

 

[miguelmcv]

Our alternator fail like that, If your alternator was tested good on the shelf at an autorpars, then that test is not valid.

The alternator needs to be in the car and the cars has to be at least warm, after the cars is warm the alternator will have some load and that's when they fail.

However you just have to check and clean all connections, like battery terminals, grounds, etc etc, if the problem persist, then you might have a failing alternator.

 

[_97tsiawd_]

Both the battery and alternator can be bad while still testing "good", depending on how they are tested and who does the testing.

Sounds like a dead cell in the battery.

What does the voltage read while cruising on the highway?
What does it read at idle, and what does it drop to if you turn on the AC, roll down a window, etc.?
Does the voltage change as the outside/under-hood temperature changes?

Great questions. over the last few weeks I have been looking for some consistency in what external factors might be playing a part (like weather) and could find none. The voltage does not drop significantly when various accessories are turned on or off. The voltage has been anywhere between 9 and 14 in ANY driving situations. i.e freeway or surface street and stoplights.

Our alternator fail like that, If your alternator was tested good on the shelf at an autorpars, then that test is not valid.

The alternator needs to be in the car and the cars has to be at least warm, after the cars is warm the alternator will have some load and that's when they fail.

However you just have to check and clean all connections, like battery terminals, grounds, etc etc, if the problem persist, then you might have a failing alternator.

I tested the alternator both in the car with it warm, turning on everything I could including high-beams, and with an in-store test. In-store it tested at on output of 15v. In the car 12.5v, but at that time that's exactly what my volt gauge was reading. It's hard to tell in a situation like this because the problem is so random. It would go for a couple of weeks reading fine, no issues at all and then all of a sudden for a couple days down to 10 or 11v.

 

[Calan]

Where are you sourcing the voltage gauge from?

Do you have any kind of logger that you can use to log ECU voltage, and then compare that with the voltage gauge?

 

[stars]

Hook your volt gauge directly to the battery, there your reading will be more consistent.