Is Alternator With Max Voltage 13.5 Defective?

[Fred Klintworth]

My wife's 2001 Pontiac Montana van has been displaying the red battery symbol on the dash display sporatically for months. It comes and it goes. Once though the battery drained so much that all the electrical devices in the van started shutting down.

I took it to a Pontiac dealer to check this problem, but they couldn't find it. At that time they couldn't get the problem to occur. I've also had Advance Auto test both the battery and the alternator. The serpentine belt is practically new.

I tested the battery this evening after my wife got the red battery symbol again. However, when I was testing it never displayed.

These are the voltages I got:
- Engine running, warmed up, at idle: 12.9 volts
- Same, except increased engine RPM: 13.5 volts maximum
- Right after turning engine off: 12.5 volts

In my experience alternators should put out at least 14 volts. So, I suspect that the alternator is going bad.

What do you think?

[GregA]

Hi Fred,

There could be a few things wrong. Battery going, generator going, bad connections, or combination of these.

But according to my manual, the PCM has to detect a generator output of LESS than 11 volts or MORE than 16 volts to turn on the Charging System Light (Battery Symbol).

Here is what I would do.
Visual Inspection:
First, I would look for any loose or dirty connections between the generator and the battery (both positive & negative). I have heard of several battery cables that had internal corrosion and failed.

Then I would look for a loose connection between the generator and the PCM (my schematic says it is Pin B on the generator - a small size RED wire - going to Pin 62 of Connector 2 of the PCM).

If all the wires & cables look (& feel) OK, go on to the next step.
Testing:
1) If you have a scanner, use it to read the level of "Ignition 1" voltage. With the engine running, it should be between 11 & 15.5 volts. This is the PCM input level from the generator. If you don't have a scanner, I would at least measure the voltage at pin B of the generator.

2) My manual says to run the engine at about 2500 RPM & measure the voltage between the positive terminal of the battery & the positive output terminal of the generator. There should be no more than 0.5 volts.

3) Do the same thing (measure) between the negative battery terminal and the metal housing of the generator. There should be no more than 0.5 volts.

You could also have the generator tested at a parts store (for proper current & voltage). There may or may not be a fee.

I hope this helps.

Take Care,

[Fred Klintworth]

GregA:
"2) My manual says to run the engine at about 2500 RPM & measure the voltage between the positive terminal of the battery & the positive output terminal of the generator. There should be no more than 0.5 volts."

Thank a lot for your post. I just did this test (although I ran engine at idle rather than at 2500 RPM) and got a voltage of 2.8 volts. Plus, the positive terminal on the alternator has white stuff on it that could be increasing the resistance; plus, the large red wire going from there to battery looks burned near the alternator.

Will post back with further results. Just wanted to say thanks first.

[1999montana]

Originally Posted by GregA

...First, I would look for any loose or dirty connections between ...the battery (both positive & negative)....

Hi Fred,

I'm with GregA on this. I would also certainly look at the connections to the battery. If there is corrosion between the cables and the battery (or if the battery is reaching end of life; - think they are original equipment with a 72 month lifespan) the alternator will not attempt to charge the battery because it will 'believe' it is fully charged even if it is not.

I've found that a visual on the cable connections isn't sufficient as the corrosion hides behind the rubber boot that covers the cable end that attaches to the battery. Physically disconnecting the cable from the battery will reveal quite a bit more. Be sure to disconnect negative first and reconnect negative last to avoid a nasty short to ground.

[Fred Klintworth]

PROBLEM RESOLVED!!!

After cleaning the positive battery terminal and the end of the red battery cable there, the voltage drop from there to the positive terminal of the battery dropped to 0.17 volts, and the idle voltage across the battery terminals climbed to 14.3 volts.

Interestingly, both the nut holding the battery wire at the alternator and the connector post (bolt) were pitted, an obvious sign of electrical arcing due to the corroded connection. I feel confident that this was also the cause of the intermittent red battery light problem we've had for months.

I hadn't done "voltage drop" tests before, so I also learned something useful for the future out of this.

THANKS AGAIN GregA!!! You probably saved me the cost and time of installing a new alternator.

[GregA]

Originally Posted by Fred Klintworth

PROBLEM RESOLVED!!!

Excellent!!!

Originally Posted by Fred Klintworth

I hadn't done "voltage drop" tests before, so I also learned something useful for the future out of this.

THANKS AGAIN GregA!!! You probably saved me the cost and time of installing a new alternator.

Isn't it cool to learn something new, even if its just a little troubleshooting technique?

And You're Welcome!
But YOU did the work, I just pointed you to a couple of possibilities. Good Job!

Now take your wife out to a nice dinner this weekend to celebrate!

Take Care,