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Alternator Question

Old 09-18-2018, 05:24 PM
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stilambo07
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Slow starts lately on my Z06 until I came out yesterday and it wouldn’t turn over. Gave it a jump and came right on. This morning l, same thing. Jumped it again and right on. i thought it was my battery but did a battery test and it is ok. Turned the car on and checked and charging system said low. I have an order in for an alternator but wanted to double checks. How do you check the starter to the battery cable? I heard that is the place to start? Can someone help me with this and go in on depth in how to check this or link me. Any help would be appreciated. Much thanks in advance!
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Old 09-18-2018, 05:32 PM
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Clancy209
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With the car running disconnect the battery cable. If you have bad alternator the car will die.
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Old 09-18-2018, 05:39 PM
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Originally Posted by Clancy209 View Post
With the car running disconnect the battery cable. If you have bad alternator the car will die.
Does it matter negative or positive?
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Old 09-18-2018, 05:41 PM
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What kind of test did you do to the battery?
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Old 09-18-2018, 05:44 PM
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stilambo07
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Originally Posted by JR-01 View Post
What kind of test did you do to the battery?
Stopped by a mechanic and had battery tester. Off there was a good charge on the battery. Car on the charging system read low.
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Old 09-18-2018, 05:49 PM
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Originally Posted by stilambo07 View Post


Stopped by a mechanic and had battery tester. Off there was a good charge on the battery. Car on the charging system read low.
He didn't load test it? That takes some time. If you are satisfied it isn't the battery and the connections are all clean and tight then it probably is the alternator. Make sure you get the right one or have yours rebuilt. See other threads for that. Do not exchange yours for a core.
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Old 09-18-2018, 05:52 PM
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The disconnecting of the battery while running is the best test to do to make sure it is alternator?
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Old 09-18-2018, 06:18 PM
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Originally Posted by stilambo07 View Post
The disconnecting of the battery while running is the best test to do to make sure it is alternator?
I don't know... but I found this:
If you're tempted to test an alternator by disconnecting the negative battery cable, don't do it. A good alternator may indeed keep the engine running, but it was never a good test. In the pre-computer days, you could pull it off without damaging anything. Today, you risk frying every electrical device in your vehicle.
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Old 09-18-2018, 06:20 PM
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Mickeyrx70
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I’d suggest you NOT disconnect the battery with the engine running. No telling what all you may fry. Just do a quick google search on the topic.
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Old 09-18-2018, 06:25 PM
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stilambo07
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Thought it sounded incorrect, thanks. Appreciate the input. Guess I’ll pick up the alternator.
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Old 09-18-2018, 06:32 PM
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Originally Posted by stilambo07 View Post
Thought it sounded incorrect, thanks. Appreciate the input. Guess I’ll pick up the alternator.
Keep the old one. The new one might not work since these alternators are special.
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Old 09-18-2018, 07:05 PM
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As mentioned above, two things, whatever you do, DO NOT:
  • Turn-in your alternator as a core
  • Disconnect the battery with the engine running

Keep your alternator, even if it seemingly fails tests. Plenty of horror stories here, and keeping it you can always have the options of rebuilding it or replacing the voltage regulator yourself.

​​​​​The alternator provides a 3-phase full-wave rectified output, as can be seen in the following graph (green trace):



The battery acts as a capacitor to "smooth" it out. When the battery is disconnected, the filtering effect is lost and the "wavy" output plus any surge or spike that may occur is passed to all circuits, which could result in damages.

To be able to troubleshoot this problem you will need a multimeter. With the engine running, check voltage at the alternator's output terminal it should be above 13.5v (typically around 14v).

Fully charge your battery and have it load-tested at an auto parts store.

Check the voltage drop from the alternator to the battery. Place the multimeter's positive lead at the alternator's output terminal and the multimeter's negative lead at the battery's positive post. This reading shouldn't be more than 0.2v (typically 0.1v or less). If it is more than 0.2v, the connection point at the starter solenoid needs to be checked, as well as the cables in the alternator-solenoid-battery path.

Last edited by GCG; 09-18-2018 at 07:21 PM.
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Old 09-18-2018, 07:30 PM
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Listen to this man.^ He knows his stuff.
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Old 09-18-2018, 07:51 PM
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Originally Posted by Clancy209 View Post
With the car running disconnect the battery cable. If you have bad alternator the car will die.
Originally Posted by stilambo07 View Post
The disconnecting of the battery while running is the best test to do to make sure it is alternator?
Don't do this. It's already been mentioned, but I'll repeat that.

It's a good way to fry a modern alternator, and it risks frying all the electronics in the car. When you disconnect the battery cable, you've got about a 50/50 chance that you just destroyed the alternator. The sudden loss of load makes for a huge voltage spike from the alternator and it usually fries the diodes in the rectifier section of the alternator. The voltage spike is short in duration (it's shorter when it fries the diodes), but it can send the system voltage into the 50V to 70V range for several milliseconds (perhaps up to a tenth of a second or 100 milliseconds).

I wasn't going to quote @GCG for his entire post, but why not? It's good information and worth repeating. Everything he says here is good info:
Originally Posted by GCG View Post
As mentioned above, two things, whatever you do, DO NOT:
  • Turn-in your alternator as a core
  • Disconnect the battery with the engine running

Keep your alternator, even if it seemingly fails tests. Plenty of horror stories here, and keeping it you can always have the options of rebuilding it or replacing the voltage regulator yourself.

​​​​​The alternator provides a 3-phase full-wave rectified output, as can be seen in the following graph (green trace):



The battery acts as a capacitor to "smooth" it out. When the battery is disconnected, the filtering effect is lost and the "wavy" output plus any surge or spike that may occur is passed to all circuits, which could result in damages.

To be able to troubleshoot this problem you will need a multimeter. With the engine running, check voltage at the alternator's output terminal it should be above 13.5v (typically around 14v).

Fully charge your battery and have it load-tested at an auto parts store.

Check the voltage drop from the alternator to the battery. Place the multimeter's positive lead at the alternator's output terminal and the multimeter's negative lead at the battery's positive post. This reading shouldn't be more than 0.2v (typically 0.1v or less). If it is more than 0.2v, the connection point at the starter solenoid needs to be checked, as well as the cables in the alternator-solenoid-battery path.
I'll add that your issues sound more like an aging and weak battery or corroded or damage cables between the battery and the starter. Also check (all) your ground strap(s) between the engine and the frame. Those can corrode and cause issues for the starter before they affect other systems.

Last edited by C6_Racer_X; 09-18-2018 at 07:51 PM.
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Old 09-18-2018, 08:49 PM
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Moved to C5 Tech.
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Old 09-19-2018, 06:13 AM
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Disconnecting the battery to check an alternator is, as stated above, a really bad idea. This method of checking the charging system dates back to when cars had generators and no electronic components. It works fine for those vehicles. This is a case where a little knowledge can be a dangerous thing.
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Old 09-19-2018, 07:02 AM
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Originally Posted by stilambo07 View Post
Thought it sounded incorrect, thanks. Appreciate the input. Guess I’ll pick up the alternator.
Please remember the advice given in post #6.....Don't give up your alternator for a new one...consider a rebuild of your original.
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Old 09-19-2018, 09:22 AM
  #18  
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Here's the "why" part of why our alternators are a little different:
(From THIS Thread )



Originally Posted by Evil-Twin
If you have the original alternator, it is clutch-less. Newer C5's with automatic ( A4) came with a clutch at the pulley to address belt squeal when putting in and out of gear. You have a Valeo 110 amp alternator made in France... it is a good unit. Your specs seem to be OK. The alternator is not your everyday alternator, it communicates with your PCM, the PCM that came with your car.,. GM came up with a clutch type A4 alternator with 140 amps, made in Mexico... the alternator works OK, but does not communicate well with your pre 2000 PCM. Many people experience charging system fault messages with this unit..The PCM thinks it is a 110 unit.
AS our Resident Electrical guru has mentioned, and I concur, if you have a problem with your alternator, the PCM will flash you a message stating you have a "charging system fault".,.,.If you do not have this, then I suspect you are ok...
The latest word I heard from the people who sign my pension check is that they have a fix for this: and its is the new Mexican made clutch driven 140 amp alternator, and a PCM reflash.... But to be honest... this is from the same group that said they have a fix for the column lock disaster... If I were you, I would never give up my original alternator.
You have a smart alternator, here is how it works:

The L-terminal circuit from the generator is a discrete circuit (a discrete circuit has no splices and only one Src and destination) into the PCM. The PCM applies ignition voltage to the generator L-terminal circuit. A small amount of current flows from this circuit through the generator windings to ground to create a magnetic field which starts the generator process. When the generator is at operating speed and producing voltage, a solid state switch for the L-terminal circuit in the generator opens and the PCM detects that the initial startup current flow has stopped.
The PCM expects to detect low voltage on the L-terminal circuit prior to the generator rotating at operating speed and conversely expects the circuit to be at ignition voltage potential when the generator is operational. When the PCM detects a fault (circuit shorted to ground, or circuit shorted to voltage), the Driver Information Center will display Charging System Fault.

The generator has an input to the PCM called the F Terminal to indicate the percentage of total capacity that the generator is producing. This signal is detected by the PCM as a duty cycle from the generator and displayed on the scan tool as a percentage. The PCM can monitor the generators output under all conditions to determine if it is functioning normally.

When there is low demand from the electrical system on the generator, a low duty cycle percentage will be displayed. As more accessory load is placed on the generator, the duty cycle output detected by the PCM will approach 100 percent. A normally functioning generating system will never reach 100 percent as indicated on the scan tool.

The L and F terminals are the red and grey

I tried to make this as simple as possible so those thinking 1960's/70's alternators/gen will throw all that old stuff away, clear their head and rethink smart alternator/gen. systems.

Thanks Bill Curlee for the vote of confidence...

Good Luck
Bill aka ( ET )

Last edited by joezr2; 09-19-2018 at 09:22 AM.
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Old 09-19-2018, 09:24 AM
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Originally Posted by Clancy209 View Post
With the car running disconnect the battery cable. If you have bad alternator the car will die.
NOOOOOOOOOOOOOOOOOOOO !!!...NOT good !!....if you had a '57 Chevy that would have been OK...NOT for todays computer controlled cars !!!
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Old 09-19-2018, 09:28 AM
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Originally Posted by stilambo07 View Post
Slow starts lately on my Z06 until I came out yesterday and it wouldn’t turn over. Gave it a jump and came right on. This morning l, same thing. Jumped it again and right on. i thought it was my battery but did a battery test and it is ok. Turned the car on and checked and charging system said low. I have an order in for an alternator but wanted to double checks. How do you check the starter to the battery cable? I heard that is the place to start? Can someone help me with this and go in on depth in how to check this or link me. Any help would be appreciated. Much thanks in advance!
FIRST...get your battery load tested !!...if good you should do a voltage drop test on both battery cables while cranking...circuit must be loaded to perform...you can find a few You Tube videos on how to do it !!...would save me time typing what to do !!
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