1979 - Alternator Issue ?????
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From: Tennessee
1979 - Alternator Issue ?????
I noticed today that when the car starts, the voltmeter is at 12v and after a couple minutes of driving, it bumps up to 14v. The previous owner told me that he was told that the alternator has a high side and low side. It will stay low until more charge is needed then will go high. I've never heard of that. That's a new one on me if that is correct. In this case, I'm assuming that this may be a high output alternator.
Last edited by Wolfman1979; Jun 16, 2018 at 07:43 PM.
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From: Tennessee
It may be something like that. If it just sits there and idles after first starting, then it's only at 12v. I have not paid any attention to rpm's. I also have not noticed if it falls back down to 12v after it bumps to 14v. I can rev it up, but that does not seem to make a difference. It only bumps up after I take off, or at least that is what it seems. Guess I need to look to see what type of alternator I have. Once I'm on the road, then it seems to stay at 14v. Just really weird to me.
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That high/low thing is hogwash on a 1979 vehicle. The alternator isn't monitoring the battery or electrical demand so it has no clue if more charge is needed.
Does the alternator (charging) light in the center cluster work properly? It should light with the key on, engine off and then go out when you start the engine.
A varying output does happen on newer vehicles. It is PCM controlled since the PCM and BCM together know loads on system and current driving conditions. It is not a 2 level system, but voltage varying.
Does the alternator (charging) light in the center cluster work properly? It should light with the key on, engine off and then go out when you start the engine.
A varying output does happen on newer vehicles. It is PCM controlled since the PCM and BCM together know loads on system and current driving conditions. It is not a 2 level system, but voltage varying.
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Joined: May 2018
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From: Tennessee
That high/low thing is hogwash on a 1979 vehicle. The alternator isn't monitoring the battery or electrical demand so it has no clue if more charge is needed.
Does the alternator (charging) light in the center cluster work properly? It should light with the key on, engine off and then go out when you start the engine.
A varying output does happen on newer vehicles. It is PCM controlled since the PCM and BCM together know loads on system and current driving conditions. It is not a 2 level system, but voltage varying.
Does the alternator (charging) light in the center cluster work properly? It should light with the key on, engine off and then go out when you start the engine.
A varying output does happen on newer vehicles. It is PCM controlled since the PCM and BCM together know loads on system and current driving conditions. It is not a 2 level system, but voltage varying.
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This car has a voltmeter. The indication is the output upon starting the vehicle is 12v and stays there and even after revving the engine I do not see a change in voltage. After driving for just a minute or two, the voltage then jumps to 14v which to me indicates the alternator is charging. I've always thought that an alternator charges the system no matter what. By the voltage jumping to 14v after maybe a perceived load on the system seems strange. The car seems to always behave in this manner. I'm just trying to determine if this is a normal reaction for this alternator, which to me doesn't seem correct, or a symptom of a possible failure or intermittent problem.
So you see no light? There is a light in the center cluster. I believe it's right under the low fuel light. You not mentioning it's there probably means it's burned out or the printed circuit for the center cluster is damaged and not connecting it or the brown wire going to the alternator plug is broken.
Without the light properly connected, the alternator doesn't self excite as soon as it starts rotating hence it doesn't produce the correct 14V when the engine first starts.
Last edited by lionelhutz; Jun 19, 2018 at 03:13 PM.
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From: Tennessee
So you see no light? There is a light in the center cluster. I believe it's right under the low fuel light. You not mentioning it's there probably means it's burned out or the printed circuit for the center cluster is damaged and not connecting it or the brown wire going to the alternator plug is broken.
Without the light properly connected, the alternator doesn't self excite as soon as it starts rotating hence it doesn't produce the correct 14V when the engine first starts.
Without the light properly connected, the alternator doesn't self excite as soon as it starts rotating hence it doesn't produce the correct 14V when the engine first starts.
Last edited by Wolfman1979; Jun 19, 2018 at 03:40 PM.
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Now we finally get the whole story. EVERY 1979 Vette came with a voltmeter so pointing out it has a voltmeter means nothing about what gauges are in the car.
The standard 10SI alternator needs a light bulb or resistor connected between ignition power and the #1 terminal. And your thinking is wrong, a voltmeter does not work as a replacement for this light or resistor.
The standard 10SI alternator needs a light bulb or resistor connected between ignition power and the #1 terminal. And your thinking is wrong, a voltmeter does not work as a replacement for this light or resistor.
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From: Tennessee
Now we finally get the whole story. EVERY 1979 Vette came with a voltmeter so pointing out it has a voltmeter means nothing about what gauges are in the car.
The standard 10SI alternator needs a light bulb or resistor connected between ignition power and the #1 terminal. And your thinking is wrong, a voltmeter does not work as a replacement for this light or resistor.
The standard 10SI alternator needs a light bulb or resistor connected between ignition power and the #1 terminal. And your thinking is wrong, a voltmeter does not work as a replacement for this light or resistor.
Thread Starter
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Joined: May 2018
Posts: 52
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From: Tennessee
Now we finally get the whole story. EVERY 1979 Vette came with a voltmeter so pointing out it has a voltmeter means nothing about what gauges are in the car.
The standard 10SI alternator needs a light bulb or resistor connected between ignition power and the #1 terminal. And your thinking is wrong, a voltmeter does not work as a replacement for this light or resistor.
The standard 10SI alternator needs a light bulb or resistor connected between ignition power and the #1 terminal. And your thinking is wrong, a voltmeter does not work as a replacement for this light or resistor.
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From: Tennessee
Problem solved.
I had purchased an indicator light that had a protective diode in it. The diode prevented the ignition voltage from reaching Term #1 of alternator. Replaced light with a standard bulb fixed the problem.
I had purchased an indicator light that had a protective diode in it. The diode prevented the ignition voltage from reaching Term #1 of alternator. Replaced light with a standard bulb fixed the problem.
