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2005 chevy trailblazer_lt
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Discussion Starter · #1 ·
Looking for some ideas. My factory alternator went about 2 months ago. I replaced it with a rebuilt unit. After a 20 minute drive, the gauge needle dropped to below 14 and I felt a loss of power. I returned it and replaced it with a brand new alternator. It worked OK until I turned on the A/C, then the needle would go below the 14 mark, and I again would feel a loss of power and different shifts. I have also replaced the Big 3 as I did see corrosion in my ground wires, it helped overall, but not this issue. Does anyone else have the charging needle go down with turning on the A/C?

This new alternator tested at 14.02V - no load and at 13.92 with a load. The paperwork stated it was spec'd at 14.2
 

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2005 gmc envoy_slt_xl
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is your a/c working properly? i would check the system charge,and if that checks look at the amp draw on the compressor itself, system charge,noisy valves,bearings,etc. my guess would be the compressor is drawing abnormally high amperage. just a thought.
 

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Discussion Starter · #4 ·
Not many responses but I thought I would give an update. I put a third alternator in Saturday, drove the TB Sunday, and watched the gauge dive to around 7, then go back up. When I got back home I popped the hood and the windings were black.
Today I called an alternator shop and he stated black winding indicate a bad stator. he also suggested that the battery may be the cause of this problem, which I suspected, but did not want to fork out money to replace a 6 month old battery. Two alternator shops said the GM alternators are all rated in the 14.8 V range. The ones I have been buying are at 14.2. Tomorrow I will be putting in an alternator from a different supplier and a battery.
 

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2002 olds bravada
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IMHO i would stick with a GM Alternator..or something that is at least rated at 14.8. And try looking at your throttle body too. Make sure its clean since you probably disconnected the battery.
 

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Discussion Starter · #7 ·
I am still dealing with the issue. I will be putting in my 5th alternator tonight. I got a bad reman. The issue is coming down to an electrical issue(probably it) or an PCM issue(doubtful, because I don't think the PCM controls much with the alternator like Ford's do). The TB runs great without the A/c on. When I do turn it on it is fine, but the alternator slowly drops in voltage, in runs in the 13.5 - 14 range, and then the transmission acts up.
 

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2004 gmc
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The PCM has two signal wires connected to the alternator. One is an inhibit line where the PCM can suppress the alternator for the second or two that the engine requires to start and get a stable idle RPM. That prevents a cold engine from being loaded down by an alternator trying to recharge a half-dead frozen battery during the start process in Alaska in the wintertime. For instance.

The other wire carries the field PWM signal to the PCM so the computer can see how hard the alternator's working to generate the voltage it's generating, as a measure of how heavily loaded the alternator is. That can indirectly tell the PCM something about the battery's state of charge if it needs to do some load shedding.

Are you getting a good voltmeter on the alternator's output terminal when it droops, or are you trusting the PCM and voltage gauge to tell you what's happening?
 

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Discussion Starter · #9 ·
I have been relying on the factory gauge, but surprisingly when I have had it tested, this last time at an alternator shop, the gauge has always been very accurate. How should I check to see if the A/C is pulling too much when I turn it on?, It is not an immediate drop when it is engaged, but will slowly go down.
 

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2004 gmc
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The AC compressor clutch is fused at 10A, so I don't see that being an issue electrically. The blower fan is fused for 40A, but of course its draw depends on what speed it's running at. The secondary air injection pump can draw 30-35 Amps, and might run a diagnostic 20 minutes into a drive, but it wouldn't be correlated to the AC running.

When you see the voltage go down with the AC on, are you seeing the idle RPM be rock solid at 600 or does it droop as well? If the idle RPM is not steady, and you have more than 30K miles on the vehicle, a dirty throttle body is the most likely culprit.

http://forums.trailvoy.com/articles.php?do=viewarticle&artid=91

If the idle RPM is steady, and you see voltage drooping down to 13.5V, I recommend whipping out a meter while it's happening and checking the voltage output directly at the alternator output terminal. If it's above 14V, then you do have a massive electrical load somewhere, or a problem in that connector/wire.
 

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Discussion Starter · #11 ·
The idle RPM is solid. With all the alternators I have had, there was no variation in RPM's while at idle. This last alternator moves the gauge more than any other, and at idle w/o A/C on was at 13.9. He was going to give me another rebuilt unit and I asked him to bench test it, it only pulled 14.2, but was said to be 14.8. I will pull the left front wheel for access tomorrow after I replace the alternator, so I have better access to the wires. What reading should I look for from the back of the A/C unit? The TB has 52K on it and the throttle body was cleaned 5K ago.
 

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The shop manual doesn't have very tight specs for what the charging voltage should be. On my vehicle, I've seen 14.4 to 14.6 right after start, and 13.6 to 13.9 while running around after the battery was recharged in a few minutes.

I don't see anything wrong with an alternator that can put out 13.6 to 14.6 at idle.

When you pull the wheel, don't look at the AC compressor, but the alternator itself. The output terminal should be within 0.5V of the battery positive lead, and the same for the alternator's frame compared to the battery negative terminal.

Even a DEAD alternator shouldn't result in the vehicle's voltmeter reading anything less than 12.0 to 12.8V from the battery, and the charging light should be on at that point. For the system to read 7V, as you saw, then either the bad alternator is sucking 200A out of the battery (and that's why the windings turned black) or something in the VEHICLE is sucking 200A from the battery, which is very unlikely. I've got a 9000 pound winch that will draw 300A at full stall, and I use an Optima red top battery with the winch hooked directly up to the top terminals and I never see 7V during testing.
 

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Discussion Starter · #13 ·
After reading the post about the whistle sound, and thinking about my problem, and the fact that I could not find the origin of the whistle(stethoscope), I opted to change the high pressure sensor, along with the alternator. I took a drive to the parts store and all seems fine. The voltage drop no longer exists, and the whistle is also gone.

I can now swap out my alternator in less then 40 minutes.
 
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