Intermittent alternator?
Thread Starter
Drifting
Joined: Dec 2006
Posts: 1,771
Likes: 1
From: Cuyahoga Falls Ohio
Intermittent alternator?
I posted about the alternator. I had it checked at advanceauto, it checked out good. The car always starts and is always around 14.2v at speed. Once in a while, at idle, it will drop. Not always. Usually when the car is hot, and more often with the AC on, but I've had it happen with nothing on. I can watch the voltmeter... 13.9 13.7 13.6 13.4... 12.7 chug, chug, chug. Once I start driving, back up to 14.2v.
Has anyone seen an intermittent alternator? I don't want to throw parts at the car, but I want it to run reliabily.
Has anyone seen an intermittent alternator? I don't want to throw parts at the car, but I want it to run reliabily.
Team Owner
Joined: Aug 2004
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From: Sussex,Wi
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I wonder how much of a load they put on the alternator at test.
It's a quick test, and the unit isn't at "real world" temp, so I wouldn't put too much trust in it.
You can probably live with the 12.7V reading/charge, but if it were mine, I'd replace it. They're around $150.00 for a brand new unit.
Takes about 10 minutes to replace.
Cheap insurance.
It's always something.
It's a quick test, and the unit isn't at "real world" temp, so I wouldn't put too much trust in it.
You can probably live with the 12.7V reading/charge, but if it were mine, I'd replace it. They're around $150.00 for a brand new unit.
Takes about 10 minutes to replace.
Cheap insurance.
It's always something.
Melting Slicks
Joined: Nov 2004
Posts: 2,799
Likes: 2
From: Cape Coral FL
I posted about the alternator. I had it checked at advanceauto, it checked out good. The car always starts and is always around 14.2v at speed. Once in a while, at idle, it will drop. Not always. Usually when the car is hot, and more often with the AC on, but I've had it happen with nothing on. I can watch the voltmeter... 13.9 13.7 13.6 13.4... 12.7 chug, chug, chug. Once I start driving, back up to 14.2v.
Has anyone seen an intermittent alternator? I don't want to throw parts at the car, but I want it to run reliabily.
Has anyone seen an intermittent alternator? I don't want to throw parts at the car, but I want it to run reliabily.
Burning Brakes
Joined: Nov 2005
Posts: 908
Likes: 1
From: Colorado Springs Colorado
I posted about the alternator. I had it checked at advanceauto, it checked out good. The car always starts and is always around 14.2v at speed. Once in a while, at idle, it will drop. Not always. Usually when the car is hot, and more often with the AC on, but I've had it happen with nothing on. I can watch the voltmeter... 13.9 13.7 13.6 13.4... 12.7 chug, chug, chug. Once I start driving, back up to 14.2v.
Has anyone seen an intermittent alternator? I don't want to throw parts at the car, but I want it to run reliabily.
Has anyone seen an intermittent alternator? I don't want to throw parts at the car, but I want it to run reliabily.
Thread Starter
Drifting
Joined: Dec 2006
Posts: 1,771
Likes: 1
From: Cuyahoga Falls Ohio
Racer
Joined: Feb 2002
Posts: 390
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From: St. Louis MO
Most likely not your problem, but when mine started acting like this I disassembled the alternator and found the output post to be loose. Simply tightened the nut on the inside of the case which caused the loose electrical contact to be corrected and solved my problem. Needless to say I was 
Team Owner
Joined: Aug 2004
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From: Sussex,Wi
2023 Corvette of the Year Finalist - Unmodified
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Have the battery tested... this symptom is indicative of a possible failing battery. The function of the alternator is to provide power to the vehicle primarily, and provide a maintenance charge only on the battery as its secondary function. A battery in poor condition can lead to alternator failure as it's not designed to deep charge a battery fulltime.
If you measure the voltage at the battery with engine off, It should measure 12 to 13V. Anything less than 12V is a bad battery.
With engine running, battery voltage should be above 13V.
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Race Director
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From: Merritt Ils Fl
War Story 101:While doing time at a local parts house a very nice slim came in and asked for an intermittent alternator for her car. Said she had been to her mechanic and he told her she had an intermittent alternator and needed a new one. It was impossible to get her to understand what her mechanic meant. She left, said she would go somewhere else.
Team Owner
Joined: Aug 2004
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From: Sussex,Wi
2023 Corvette of the Year Finalist - Unmodified
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War Story 101:While doing time at a local parts house a very nice slim came in and asked for an intermittent alternator for her car. Said she had been to her mechanic and he told her she had an intermittent alternator and needed a new one. It was impossible to get her to understand what her mechanic meant. She left, said she would go somewhere else.
Thread Starter
Drifting
Joined: Dec 2006
Posts: 1,771
Likes: 1
From: Cuyahoga Falls Ohio
I just got this car in February of this year. If I need a new alternator, is it worthwhile to go with a higher output, (140A)?
Last edited by DaleD; Jun 27, 2007 at 01:09 PM.
Melting Slicks
Joined: Nov 2004
Posts: 2,799
Likes: 2
From: Cape Coral FL
Alternator output has nothing to do with the battery. It charges the battery. You can have the battery disconnected from the engine and still get a reading at the alternator.
If you measure the voltage at the battery with engine off, It should measure 12 to 13V. Anything less than 12V is a bad battery.
With engine running, battery voltage should be above 13V.
If you measure the voltage at the battery with engine off, It should measure 12 to 13V. Anything less than 12V is a bad battery.
With engine running, battery voltage should be above 13V.
Thread Starter
Drifting
Joined: Dec 2006
Posts: 1,771
Likes: 1
From: Cuyahoga Falls Ohio
I just had to run an errand. It's hot as heck, (for Ohio), so I ran the AC. At lights, it started doing it... 14.2... 13.7... 13.2... 12.7...
When it got under 13.0 I slide the car into neutral. It came up to 13.5. I have no idea what that means.
When it got under 13.0 I slide the car into neutral. It came up to 13.5. I have no idea what that means.
Team Owner
Joined: Aug 2004
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From: Sussex,Wi
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I just bought a brand New unit from Advance for my Blazer, and it came with a dyno sheet. The specs. said it started to register 14.3V at 230 rpms, so I'd say your low idle speed should be ok. Probably has more to do with the amount of accessories running when the car is hot and at low rpms (radiator fan is on). I've had my '85 stall out on normal take-offs on hot days with the A/C running, and the gauge reading below 14V. This is mainly due to all the 20+ y/o parts still in there that have seen their better days (A/C comp. still running R12, old PS pump, etc)
Team Owner
Joined: Mar 2001
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From: Cincinnati, Oh USA
Alternators cannot generate their rated current or even high current at engine idle and when you have the a/c on, the main fan is on, the compressor clutch solenoid is on, the a/c, heater fan motor is on, and all the other normal electrical load is added too. This can be too much of a load for the alternator to supply and its output voltage will be lower than at a higher engine rpm. Next time you notice the low voltage, increase the engine rpm with the accelerator and see if the voltage comes up to its normal 13-14 volts.
Alternators that are intermittent do so at any rpm, not just idle.
Alternators that are intermittent do so at any rpm, not just idle.
Last edited by jfb; Jun 27, 2007 at 10:45 PM.
