1994 Buick Lesabre Charging problem

Tiny
BYKERBOI_85
  • MEMBER
  • 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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Tuesday, October 7th, 2014 AT 8:36 PM

12 Replies

Tiny
CARADIODOC
  • EXPERT
"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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Wednesday, October 8th, 2014 AT 12:36 AM
Tiny
BYKERBOI_85
  • MEMBER
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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Wednesday, October 8th, 2014 AT 7:01 AM
Tiny
CARADIODOC
  • EXPERT
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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Wednesday, October 8th, 2014 AT 11:20 PM
Tiny
BYKERBOI_85
  • MEMBER
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, October 9th, 2014 AT 6:34 AM
Tiny
CARADIODOC
  • EXPERT
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, October 9th, 2014 AT 5:32 PM
Tiny
BYKERBOI_85
  • MEMBER
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, October 9th, 2014 AT 5:57 PM
Tiny
BYKERBOI_85
  • MEMBER
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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Wednesday, October 15th, 2014 AT 6:12 AM
Tiny
CARADIODOC
  • EXPERT
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, October 16th, 2014 AT 12:11 AM
Tiny
BYKERBOI_85
  • MEMBER
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, October 16th, 2014 AT 3:36 AM
Tiny
CARADIODOC
  • EXPERT
I think you just ran into some bad generators. It's not that uncommon.
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Thursday, October 16th, 2014 AT 3:47 AM
Tiny
BYKERBOI_85
  • MEMBER
Is there any part store you suggest over another?
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Thursday, October 16th, 2014 AT 4:24 AM
Tiny
CARADIODOC
  • EXPERT
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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Saturday, October 18th, 2014 AT 2:40 AM

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