What's going on with my alternator?

charlieruu said:

It doesn't seem to be bouncing, I do have serpentine belt wear though. Now another mystery. The last time I started my car was about 4 hours ago and it barely started. I just went outside to charge the battery so I can still use it for getting to work in the morning and while i was at it i measured the voltage at the battery...11.5?! How is that possible. The instrument cluster lights would dim even if i turned the fan on yet the battery is almost at a full charge. Maybe 11.5 is considered low but to me it doesn't make sense.

Click to expand...

11.5 is not a low battery, it’s a dead battery. I used to think a 12v battery had....well 12v’s. Not so, a brand new fully charged battery will be in the 12.7-12.8 range, with 12.6 being pretty much normal for a battery that has had some use but is still healthy.

When your battery charge goes below 12.2-12.3 on a chilly morning your car may not start, or just barely start. Which is the reason up north here the first cold snap in the fall brings about a spike in battery sales. When a battery is this low and will not charge back to 12.6 and hold that charge it’s time to start thinking, replacement.

Tdijarhead said:

11.5 is not a low battery, it’s a dead battery. I used to think a 12v battery had....well 12v’s. Not so, a brand new fully charged battery will be in the 12.7-12.8 range, with 12.6 being pretty much normal for a battery that has had some use but is still healthy.

When your battery charge goes below 12.2-12.3 on a chilly morning your car may not start, or just barely start. Which is the reason up north here the first cold snap in the fall brings about a spike in battery sales. When a battery is this low and will not charge back to 12.6 and hold that charge it’s time to start thinking, replacement.

Click to expand...

No it charges back to 12.8. I guess I was just mistaken. I thought that 11.5 was still pretty high. Thanks for the insight.

12.65V is correct voltage for a charged, flooded lead acid, battery. 12.8V has a "surface charge" . Same way you can check battery voltage after shutting off the engine and see 13.1V at the battery.

Serpentine belt "flutter" and noises can be because of pulley on alternator is damaged.

Charging the battery every night and discharging it driving to school/work every day is false economy. If you didn't need a battery before, you will now. And I have seen starters fail from repeated starts with a weak battery.

Time to get this thing figured out or start bumming a ride with someone until you can.

Jason

Well, now that we know what a dead battery is, has anyone had a "for sure known" dead battery to cause the ALT light to stay on? Didn't think so!

I believe the OP's problem is a Dead ALT and he's trying to hobble along with hopes there'll be some magic correct the problem. And, yes in the meantime he is challenging the operational limits of what apparently was a good battery!

AndyBees said:

Well, now that we know what a dead battery is, has anyone had a "for sure known" dead battery to cause the ALT light to stay on? Didn't think so!

I believe the OP's problem is a Dead ALT and he's trying to hobble along with hopes there'll be some magic correct the problem. And, yes in the meantime he is challenging the operational limits of what apparently was a good battery!

Click to expand...

I think we're all saying the same thing

By continuing to drive the car as is lessens the possibility of saving the alternator should the wires in worn out brushes be wearing into the slip rings.

Got any auto wrecking yards in your area? Used alternators usually go for about $25. I will take them if they look clean and the bearings run smooth. You might even get one with a recent overhaul decal on it.

These figures taken from a chart on battery facts on another forum.

12.65 100%
12.45 75%
12.24 50%
12.06 25%
11.89 or less - discharged

There is a limit on how far the brushes will be pushed toward the slip rings. Thus, it is an impossibility for the brushes to wear down to the wires.

The brushes below on the right (new on the left) are from a 2002 Jetta TDI with 401k miles on it. They are from the OE Bosch Alt that come on the car (I've done maintenance on this car since about 70k miles). The Alt light was on and the battery voltage was at 12.35 volts. It required a boost to start the engine.

Anyway, new brushes put the light out and the battery has rebounded to 12.55 volts. Sure, the battery's useful life has no doubt been reduced. But, with spring and summer coming, I'm sure it will take the car into the fall without any problems.

Again, the OP needs to fix his ALT.

I suppose alternator technology has improved a lot over the years but I have seen on my early sixties American made car where the wires did in fact start eating into the slip rings at 145k miles. I've since converted it to an alternator with an internal regulator and interestingly the battery in that car that I borrowed from another vehicle (Ford Windstar mfd fall of 97) is now over twenty years old. Voltage? I'll have to check.

Pretty good that a VW alternator can get up to 400k miles on it and still function. They must have very good bearings in them.

vincej said:

These figures taken from a chart on battery facts on another forum.

12.65 100%
12.45 75%
12.24 50%
12.06 25%
11.89 or less - discharged

Click to expand...

Temperature will also affect the voltage too.

vincej said:

Pretty good that a VW alternator can get up to 400k miles on it and still function. They must have very good bearings in them.

Click to expand...

Yep, the springs in 'em old generators would just keep pushing the brushes out until they were gone.

Well, actually this ALT's bearings are near the limit (not a smooth spin). The car is due a TB in 26k miles. The lady will be rounding up either a used one, reman or a new one by then. The OE ALT in my 2000 Jetta made it past 300k miles before the brushes wore out. I was in a hurry, so I installed a 90amp that I had picked up at a junk yard... Now, many miles later, no problems. The OE ALT just needs brushes.

The brushes come in the unit containing the Voltage Regulator.

Update: The weird noises I was hearing was alternator bearing. Swapped in a new shiny alternator and everything is all good. Thanks to everyone who left some advice!

Just curious, where did you purchase the replacement alternator?

JB05 said:

Just curious, where did you purchase the replacement alternator?

Click to expand...

It is a valeo from napa