Alternator and battery question

[41magsnub]

Vehicle - 1995 GMC S1500 with the 4.3L V6.  106K miles.  Original damn near everything, except spark plugs.

New Napa battery installed July of 2018.

The battery warning light came on.  Voltage meter dropped down to about 10V.  I was on the way to pick something up, but turned right around and got it back to the driveway.
In the driveway, the truck started 4 more times, each time the battery was a bit lower.

The battery will take a charge from a trickle charger and start the truck a bunch of times.  When the battery was dead it would jump start successfully too from the charger.  I cleaned all the terminals and battery cables - they were very good already though since I did all that last July.

I think the alternator is dying and should be replaced.  While I am at it, it is really due for a timing chain and water pump.  That all sound about right?

[French G.]

Sounds about right. Do the serpentine idlers and tensioner while you have it apart too. I started running a secondary ground too from an alternator mount bolt straight to the battery. Cured a bunch of stupid Subaru tricks. Last go the alternator died, install new one, it's garbage, store says it is good, says installed alternator I got at another store is bad, yet it ran for two years after that with no issue.

[Hawkmoon]

Sounds like the alternator.

Do you have a multimeter? If so, put the meter on the battery with the engine off. A healthy battery should register about 12 volts. Then start the engine (if it'll start!). With the engine running and a healthy alternator churning away, the multimeter should now read between 13.6 and 14.2 volts.

[41magsnub]

Sounds like the alternator.

Do you have a multimeter? If so, put the meter on the battery with the engine off. A healthy battery should register about 12 volts. Then start the engine (if it'll start!). With the engine running and a healthy alternator churning away, the multimeter should now read between 13.6 and 14.2 volts.

Good idea, I was trusting the gauge.  It normally reads pretty much exactly what you said.

Edit - normally as in before yesterday!

[Brad Johnson]

Yeah, probably the alternator. Easy check. Just start it and check voltage at the battery with it idling. Should be somewhere 13.5-14.0 volt. At fast idle it should jump to 14.0-14.5 v and stay there no matter what the RPM.

Some shops still do in-house alternator testing. Shouldn't be a big deal removing the alternator on that year. I can't remember if it's two or three bolts, but it's no big deal either way. Biggest thing will be the electrical connector. Probably brittle as glass so take plenty of care getting it unplugged.

Brad

[41magsnub]

Thanks guys for the confirmation!

[bedlamite]

Yep, that's either the alternator or voltage regulator, which comes as part of the alternator.

I'd also replace the distributor cap and rotor, that's a pretty common problem on that motor.

[Frank Castle]

I would also replace serpentine belt.

[AmbulanceDriver]

On the other side of the equation, it looks like I get to replace the batteries in my '96 Ram diesel.   Charges right up after starting, but a couple times over the winter I was worried whether it was going to successfully turn over or not.   Doesn't help that the air intake grid heater (which kicks in on startup if the temp is below 50 F) draws 180 amps (2x90 amps) for 30 seconds before I start cranking.

[Silver Bullet]

Sounds like the alternator.

Do you have a multimeter? If so, put the meter on the battery with the engine off. A healthy battery should register about 12 volts. Then start the engine (if it'll start!). With the engine running and a healthy alternator churning away, the multimeter should now read between 13.6 and 14.2 volts.

I'm not current on auto electric, but I seem to recall that at one time (probably still) the alternators drove DC through diodes so that they wouldn't drain the battery when the engine was off.  That is, the diodes were to block the current that would otherwise flow back from the battery through the alternator.  If the diodes were shorted it was possible for the battery to drain back to the alternator.

In which case ... you could still see the 13.6 to 14.2 volts when you measure voltage with the engine running, but there would be the diode problem with the alternator.

[Brad Johnson]

I'm not current on auto electric, but I seem to recall that at one time (probably still) the alternators drove DC through diodes so that they wouldn't drain the battery when the engine was off.  

Bridge rectifier (aka diode bridge).

Unfortunately, even if it is just the rectifier the alternator will likely have to come off as the BR is usually an internal part. BR's can be sourced and replaced separately but disassembling the alternator is a giant pain (some require a bearing press). Easier and much, much faster to get a decent parts store reman and be done with it.

Brad

[Brad Johnson]

I'm not current on auto electric, but I seem to recall that at one time (probably still) the alternators drove DC through diodes so that they wouldn't drain the battery when the engine was off.  That is, the diodes were to block the current that would otherwise flow back from the battery through the alternator.  If the diodes were shorted it was possible for the battery to drain back to the alternator.

In which case ... you could still see the 13.6 to 14.2 volts when you measure voltage with the engine running, but there would be the diode problem with the alternator.

Wait a sec... just dawned on me that we haven't seen you for a while. Couple of years, actually. Welcome back!

Brad

[Silver Bullet]

I agree with everything you’re saying, except I wasn’t suggesting replacing the diode (although I didn’t realize they were tightly integrated into the rest of the alternator).  I was only suggesting that a passing measured voltage doesn’t completely exonerate the alternator, since a shorted diode might not prevent it from functioning while running, but it could drain the battery when the alternator (and engine) aren’t running.

Here's a very nicely done overview:

A Short Course on Charging Systems (courtesy of Car Parts)
https://www.carparts.com/classroom/charging.htm

[Silver Bullet]

Wait a sec... just dawned on me that we haven't seen you for a while. Couple of years, actually. Welcome back!

Brad

Thanks, Brad!  Browsing around, I'm surprised and gratified to see so many familiar names.