Charging system

Tiny
DAAA1993
  • MEMBER
  • 1996 BUICK LESABRE
  • 6 CYL
  • FWD
  • AUTOMATIC
  • 73,652 MILES
Car starts fine and runs fine to it just started discharging while stopped and driving did test on it it said the battery is good and the alternator is to now the check engine light just came on it was not on before this all started happing
Saturday, March 8th, 2008 AT 1:25 PM

23 Replies

Tiny
SERVICE WRITER
  • MECHANIC
  • 9,123 POSTS
See what codes are in the computer, my guess is the alternater is on it's way out and is intermittanly not charging right.
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Sunday, March 9th, 2008 AT 4:49 PM
Tiny
BYKERBOI_85
  • MEMBER
  • 7 POSTS
  • 1994 BUICK LESABRE
  • 3.8L
  • V6
  • FWD
  • AUTOMATIC
  • 76,012 MILES
I bought it and the PCM was fried, not allowing it to start. It would turn over, but never fire. Once I replaced the PCM, it fired right up with no problem and drives well. I soon realized that the charging system was not working properly. I replaced the battery (multiple times) the alternator, (multiple ones) and the cable that runs from the alternator to the positive post. It still is not charging. I checked if it's properly grounded by using the volt meter and testing in the ohm setting. I still don't have a reading in voltage when I test from the positive post of the battery to the alternator post that connects to the battery cable at the positive post. It's been replaced as well. I don't know what else it could be. I'm stumped. I didn't replace the 78 inch positive cable. I made sure it was grounded well. Do you have any suggestions? I'm at the end of my knowledge. Any help would be appreciated.
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Thursday, January 2nd, 2020 AT 8:43 PM (Merged)
Tiny
CARADIODOC
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"I still don't have a reading in voltage when I test from the positive post of the battery to the alternator post that connects to the battery cable at the positive post."

That needs to be clarified and understood. You're measuring at the two ends of the same wire so you must read 0.0 volts difference between them. It's when you DO find some voltage there that you have a problem.

So far you've spent all your time in the generator's output circuit, but you haven't said anything about the input circuit that makes the unit work. On most GM vehicles that "turn-on" signal comes from the instrument cluster through the "Battery" warning light bulb. Does that bulb light up when the ignition switch is turned on?
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Thursday, January 2nd, 2020 AT 8:43 PM (Merged)
Tiny
BYKERBOI_85
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When I turn the ignition switch, the battery symbol on the dash comes on. When the engine starts, it goes off like normal.
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Thursday, January 2nd, 2020 AT 8:43 PM (Merged)
Tiny
CARADIODOC
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Dandy. That means the generator is getting the turn-on signal and it is turning the light off. At that point you should be getting output current going back to the battery. Measure the voltage on the large output stud on the back of the generator when the engine is running. If it's between 13.75 and 14.75 volts, the system is working. If it's much higher than battery voltage, there's a break in that wire.
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Thursday, January 2nd, 2020 AT 8:43 PM (Merged)
Tiny
BYKERBOI_85
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That cable has been replaced and I am only getting around 12.5 from the stud on the back of the alternator. The light in my dash has always done that, but comes on when the battery is extremely low.

Also, when the car is started, I'm only reading about 12.5 volts from post to post on the battery instead of the recommended 14 or so. So could it be a bad alternator? (This one is new from O'Reileys) or does the whole 78 inch cable need to be replaced. I ride a motorcycle and it's getting cold in Kentucky so I'm trying to fix this issue ASAP. Lol. Thank you again for the help.
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Thursday, January 2nd, 2020 AT 8:43 PM (Merged)
Tiny
CARADIODOC
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Oh, I feel sorry for you! I was in Louisville a month ago, then had to come back to Wisconsin. I won't be happy again until it's 75 degrees next spring.

Unless there's something I'm missing, you have a bad generator. If you have 12 volts on its output terminal, that circuit to the battery is okay. If the dash light is turning on and off appropriately, that circuit is okay too. All that's left is a slipping belt.
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Thursday, January 2nd, 2020 AT 8:43 PM (Merged)
Tiny
BYKERBOI_85
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Thank you. I will take it back tomorrow. I replaced the belt too. Lol if it's not one thing its another. I can see the belt turning it, even at high RPM's. It's tight. So I'm REALLY hoping that will fix my problem for good. Thank you again for all the help and suggestions.

And the summer is what I look forward to. My motorcycle hasnt failed me yet.
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Thursday, January 2nd, 2020 AT 8:43 PM (Merged)
Tiny
BYKERBOI_85
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So I changed the alternator and had it tested. It was bad. I got a new battery as well. Once I replaced them, I tested the charging system and was getting 14.5 at the battery while the car was running. It was working correctly. 30 mins later the alternator wasn't putting anything out and I was back to 12v at the battery while the car was running. What could be shooting the alternator so quickly? What would be your next thing to check in a situation like this?
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Thursday, January 2nd, 2020 AT 8:43 PM (Merged)
Tiny
CARADIODOC
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This is so common that normally I paste a copy of a standard reply but in this case I'm suspicious there's something going on with those rebuilt units. A new, inexperienced worker might not be soldering an internal connection properly. There might not be heat sink grease applied to the back of the voltage regulator. Things like that. This is almost always solved by switching to a more expensive or less expensive generator from a different rebuilder, but handled by the same parts store. My friend ran into this with his Suburban a few years ago. He bought a generator with a lifetime warranty for $200.00 and had three in a row fail about two weeks apart. Next, he "upgraded" to a $250.00 generator with only a one-year warranty, and hasn't had a problem in years.

The more common issue is GM had what I considered the second best generator in the world up through 1986. They redesigned them for the '87 model year and turned them into the world's worst pile, and they have no intention of fixing that design. Due to how the internal voltage regulator switches field current on and off about 400 times per second, the output circuit, which is similar in operation to an ignition coil, develops huge voltage spikes that can destroy that regulator, the internal diodes, and they can interfere with computer sensor signals. Elusive running problems on GM vehicles often clear up when the smaller plug on the generator is unplugged to disable it.

The battery is the key component in dampening and absorbing those voltage spikes, (and is related to why we must never ever remove a battery cable while the engine is running). As any battery ages, the lead starts to flake off the plates and that reduces its ability to absorb those spikes. Once it gets to be more than about two years old, those voltage spikes start to become a concern. My standard reply, when someone finds the generator needs to be replaced, is to replace the perfectly good battery at the same time unless it is less than about two years old.

To be fair, all manufacturers switch field current on and off this way because a very light-duty switching transistor can handle the job without overheating. (Vcrs and newer tvs also use these "switching power supplies" for the same reason. I don't know why, but this voltage spike problem only involves GM's generators. Their '86 and older models worked the same way but never caused a problem.

Since you beat me to the punch and already replaced the battery, you either have an intermittent connection or the generators are failing due to a mistake at the rebuilders. To check for a bad connection or corroded wire, remember that the voltage on the large output stud on the back of the generator, and the voltage between the two battery posts, must always be the same. If that fat wire has a break in it, you'll find 0.0 volts on the generator's output stud when the engine is not running, and considerably more than 14.75 volts, as in 16 to 18 volts when the engine is running.
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Thursday, January 2nd, 2020 AT 8:43 PM (Merged)
Tiny
BYKERBOI_85
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I'm suspicious of the alternator as well. I changed the "fat wire" that connects from the stud on the generator to the positive post. I even put a thicker gauge wire to aid In proper current flow. I put a different terminal at the positive post to see if that may have been the problem. Do you think the whole 78" positive cable may need replacing? That there may be some corrosion in the cable and shooting the generator during a power spike? I'm getting pretty desperate. The car sat still for about 4 months. Before it sat still, it just needed a new PCM. Now this a problem. When I bought it in sept it had an undersized battery on it but the owner said they never had a problem out of the charging system.
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Thursday, January 2nd, 2020 AT 8:43 PM (Merged)
Tiny
CARADIODOC
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I think you just ran into some bad generators. It's not that uncommon.
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Thursday, January 2nd, 2020 AT 8:43 PM (Merged)
Tiny
BYKERBOI_85
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Is there any part store you suggest over another?
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Thursday, January 2nd, 2020 AT 8:43 PM (Merged)
Tiny
CARADIODOC
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Nope. My daily driver is a 25-year-old Grand Caravan. Thanks to Wisconsin salt, I've replaced rusted out brake lines and fuel lines, but other than a fuel tank and straps, nothing in about the last five years. Even the battery, which is on its last legs, is over 8 years old. Oh, and one head light bulb.

I keep this thing on the road because it has most of the same toys that people demand on newer cars, but none of mine need a stupid computer. My friends with newer vehicles spend more on repairs every few months than I spend on repairs and gas for the whole year.

My rubber fuel hose came from O'Reilley's because they were the least expensive. NAPA always seems to be higher, but it depends on what you need. Carquest is usually higher than everyone else too. My friend with a repair shop prefers Auto Zone. Don;t know anything good or bad about Advanced Auto Parts. For parts online, check out the Rock Auto web site. I never ordered from them but friends have and they were very happy. I use the site every day for reference.
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Thursday, January 2nd, 2020 AT 8:43 PM (Merged)
Tiny
INDIANOUTLAW72
  • MEMBER
  • 3 POSTS
  • 1995 BUICK LESABRE
  • 122,835 MILES
Still no luck I tried the thick red wire theory and it's got power dont know what else it could be any ideas? I have checked the little lead off the back of the alternator and the lead that goes from the alternator to the battery and the wire from the battery to the starter and the ground wire everything tests out fine any other ideas?
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Thursday, January 2nd, 2020 AT 8:43 PM (Merged)
Tiny
CARADIODOC
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No idea what the problem or symptom is. What tests did you do? What are you trying to solve?
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Thursday, January 2nd, 2020 AT 8:43 PM (Merged)
Tiny
INDIANOUTLAW72
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I had the battery and alternator tested they both came back good the battery and alternator are both new I checked and tested all wires and the alternator is not charging or doing anything any ideas what else I can do?
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Thursday, January 2nd, 2020 AT 8:43 PM (Merged)
Tiny
CARADIODOC
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First check that the battery light on the dash lights up when you turn on the ignition switch. That is the turn-on circuit to wake up the voltage regulator inside the generator. If it doesn't light up, leave the ignition switch on, unplug the connector from the side of the generator at the rear and measure the voltage there. As I recall it will be a brown wire. If you use a digital voltmeter you should find 12 volts. If you use a test light you will find the light is dim indicating around 6 volts. If that voltage is missing there is a break in that circuit.

If that voltage is present, check the terminal to see if it's stretched and not making contact.

If the battery light does turn on and it goes off once the engine is running, measure the voltage on the large output stud, again, while the engine is running.
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Thursday, January 2nd, 2020 AT 8:43 PM (Merged)
Tiny
INDIANOUTLAW72
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  • 1995 BUICK LESABRE
  • 122,835 MILES
I have a 95 buick lasbre. It have a new alternator and a new battery in it, and the battery light still keeps coming on. I took the alternator off and had it tested and it test good but when I put it back on the light still comes on so I took the car and had it tested on the car and the test says the alternator is not charging so what else can it be.I have checked all the wires and they all look fairly new. Any ideas what it is and how I can fix it.
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Thursday, January 2nd, 2020 AT 8:43 PM (Merged)
Tiny
ASEMASTER6371
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There are fuses for the alternator. Check all with a test light.

Make sure the heavy red wire is hot all the time

Roy
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Thursday, January 2nd, 2020 AT 8:43 PM (Merged)

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