Do I have a bad alternator?

Hi,
I did a datalog of the car and noticed that my bat volts were at 12.6 at 6300rpms:


I never thought anything of it because my car idles at 14 volts all the time. It's just at WOT, it'll hit 12.

I ran out with my battery tester and the battery is fine, pass the load test.

But when I turned the car on to check the charging system I saw this at idle:

This looked fine to me too.

Then I put a load on it, and I got this:

The needle fell right out of the charging system range.

The car starts up fine, I don't have any battery lights or headlight flicker. The only problem I see is the bat volts at WOT and....also take a look at my injector duty cycle and fuel pump duty cycle. They're in the 90's on a stock supercharger pulley. I'm wondering if the fuel pump is not getting enough volts and due to that, the duty cycle is going up and the fuel injectors are also going up to compensate for this.

What do you guys think?

OH, I forgot to mention....mods.

The car is stock except for exhaust but the car does have an aftermarket stereo in it. I don't use it (I'm not a stereo type of guy) but the prior owner put it in.

It might not be bad, bad. But I'd say it's on it's way out. Does the voltage output change when it's cold versus hot? And low voltage at WOT can cause higher duty cycles for pumps and injectors. It can also affect the range of the MAF.

slo984now said:

It might not be bad, bad. But I'd say it's on it's way out. Does the voltage output change when it's cold versus hot? And low voltage at WOT can cause higher duty cycles for pumps and injectors. It can also affect the range of the MAF.

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I think that's what's happening. now that's it's cold in NYC, my MAF range, injector DC and FP DC have all shot up. I was wondering if it was the cooler air making more power, but it must be the alternator on its way out. Over the summer, I was able to run a 2.76 fine, then as it got cooler, I had to drop down to the 2.93, then the 3.1 and now it's stock, just to keep my fuel system in range.

Usually when the alternator is cold it charges better. That was the issue I had with my last alternator. It would charge fine at first start up. Then slowly drop as everything warmed up.

But yeah, cold air means more power which ups the load on everything else to keep up. Since you're not making a street brawler out of the car I would look into having it rebuilt locally. A lot of shops can up the amperage output as well. I imagine just going up to a 130amp would make a difference.

slo984now said:

Usually when the alternator is cold it charges better. That was the issue I had with my last alternator. It would charge fine at first start up. Then slowly drop as everything warmed up.

But yeah, cold air means more power which ups the load on everything else to keep up. Since you're not making a street brawler out of the car I would look into having it rebuilt locally. A lot of shops can up the amperage output as well. I imagine just going up to a 130amp would make a difference.

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Great idea. Thanks. I can just remove it and get it rebuilt at a shop. The car will be sitting for the winter so that's a perfect time.

Also, you're right. When I first start it up, the car is strong at 14 volts. Then, it'll go down to 13's. At WOT, it'll drop down into the 12's

HPLouis said:

It's just at WOT, it'll hit 12.

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I would replace/rebuilt the ALT if ever dropped below 13. Do you have the charge wire upgraded? Have you checked/cleaned the grounds?

I checked the grounds and wiring and it's all good and clean. I took the car out for a drive earlier and it started out at 14v when it was cold and driving. As the car warmed up, it slowly dropped down to 13. When I hit WOT, it would then drop to 12.6 so it looks like the alternator is weak. I'm just gonna remove it while the car is stored for the winter and get it rebuilt.

Not sure what you're looking to buy or who will get it rebuilt, but I have the Mechman 170a with big 3 upgrade for almost 2 years (5k miles). It never dips below like 13.8 even with my 1000w system and ac running.

Midnight_Cobra said:

Not sure what you're looking to buy or who will get it rebuilt, but I have the Mechman 170a with big 3 upgrade for almost 2 years (5k miles). It never dips below like 13.8 even with my 1000w system and ac running.

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This proves it. I'm just gonna get it rebuilt while it's stored for the winter.