[ponjohn]

74 Ventura
Bone stock, 350 Pontiac 2bbl with AC.

OK...

• The atlernator light was on, naturally replaced the alternator. not charging
• Found the fuse blown for the "gauges"
• Replaced the fuse let it idle no issues.
• Took it for a ride a week later, no issues until...
• I accerlated like taking off from a light, alternator light comes back on, fuse blow.
• Today, I changed the fuse, verified it was charging 13.7v with DVM on Battery
• I gently went for a ride, probably not over 15 mph, no issue.
• Stop the car, hit the gas pedal as agressively taking off - bingo.
• Fuse blows.
• Measure voltage at battery 13.7 - appears to still be charging with the alternator light illuminated.
• Bring it back. change fuse, alternator light on.
• Let it sit for a 5 minutues, start it up alternator light off.

My theory:
1. Igition coil breaking down ( car sat for 10 years) and causing excessive current.
2. Movement of engine under hard load causing ground issues?

I am open to ideas or a better undertanding of the function of the alternator light. [ my understanding is, the guage fuse supplies voltage to "idiot" light which in turn provides excitement voltage for the alternator]

Thanks.

[omarsphatt]

I'd start by checking your wiring harness very carefully, sounds like you've got an intermittent dead short in the circuit that fuse protects, likely happening with the movement caused by hard acceleration.

[george kujanski]

I also would say a circuit short when accelerating. Engine may be shifting in mounts and pinching a wire someplace.


George

[ponjohn]

I also considered critters have gotten in somewhere.

Thanks for the input.

[jwcfbd]

I don't know if it would cause your problem or not but check the routing of your positive battery cable. If it only happens under heavy throttle maybe you're pinching the cable under the oil pan to the frame???

[FHummel]

A bit puzzling, but the alternator should be putting out 14.2 volts, not 13.7 volts. Could be a problem in the regulator. Have you looked in the distributor? It's possible that when the advance kicks in a short is created momentarily that causes your problem. I'll check back later to see how you are progressing.

FHummel

[punkin]

Possibly the regulator...curious if you have a voltmeter that you can connect when running up the RPMs. Any chance the output voltage is going way high when RPMs climb?

[VCho455]

This may help you understand what's happening.

http://www.madelectrical.com/electri...witworks.shtml

You can disregard your first theory.
My theory:
1. Igition coil breaking down ( car sat for 10 years) and causing excessive current.

The ignition circuit is on a different branch of the harness and if something shorted directly to ground (not like the points which are an intermittent short to ground) it would be cause the fuseable link to blow which would shut everything off.

You didn't mention if you have guages or warning lights.

Does it have tachometer?

Not having seen your car, has someone added electrical load to the guage circuit?

Since the guage fuse is blowing the problem will be in the circuit after the fuse. Some part of that circuit is going to ground and overloading the fuse. The generator light may just be a symptom of the fuse blowing and not part of the problem. I'm not as familiar with your year of car as I am with the earlier years. So I can't say with certainty how the alternator light is wired and what causes it to light.

[SamB937]

Originally Posted by ponjohn

74 Ventura
Bone stock, 350 Pontiac 2bbl with AC.

OK...

• The atlernator light was on, naturally replaced the alternator. not charging
• Found the fuse blown for the "gauges"
• Replaced the fuse let it idle no issues.
• Took it for a ride a week later, no issues until...
• I accerlated like taking off from a light, alternator light comes back on, fuse blow.
• Today, I changed the fuse, verified it was charging 13.7v with DVM on Battery
• I gently went for a ride, probably not over 15 mph, no issue.
• Stop the car, hit the gas pedal as agressively taking off - bingo.
• Fuse blows.
• Measure voltage at battery 13.7 - appears to still be charging with the alternator light illuminated.
• Bring it back. change fuse, alternator light on.
• Let it sit for a 5 minutues, start it up alternator light off.

My theory:
1. Igition coil breaking down ( car sat for 10 years) and causing excessive current.
2. Movement of engine under hard load causing ground issues?

I am open to ideas or a better undertanding of the function of the alternator light. [ my understanding is, the guage fuse supplies voltage to "idiot" light which in turn provides excitement voltage for the alternator]

Thanks.

The alternator light receives 12V from the ignition circuit through the printed circuit in the cluster. It grounds through the alternator when it is not spinning. When you start the vehicle 12V is fed back to the light from the alternator. When the light has both legs positive it won't turn on. If there is a difference in the voltage on either side of the bulb (e.g. Battery or alternator failure) the difference in the voltage will turn the light on. This is on an internal regulator style alternator. External regulator style works similar with the addition of the external regulator. Any kind or intermittent electrical issue ALWAYS go back to the basics. Good positive feed, good engine ground, and most importantly good body ground. I cannot tell you how many old cars(and new) I repaired by the addition of a good heavy body ground wire.

[Squidward]

Originally Posted by SamB937

The alternator light receives 12V from the ignition circuit through the printed circuit in the cluster. It grounds through the alternator when it is not spinning. When you start the vehicle 12V is fed back to the light from the alternator. When the light has both legs positive it won't turn on. If there is a difference in the voltage on either side of the bulb (e.g. Battery or alternator failure) the difference in the voltage will turn the light on. This is on an internal regulator style alternator. External regulator style works similar with the addition of the external regulator. Any kind or intermittent electrical issue ALWAYS go back to the basics. Good positive feed, good engine ground, and most importantly good body ground. I cannot tell you how many old cars(and new) I repaired by the addition of a good heavy body ground wire.

This.

I suspect that the IP fuse blows for some as of yet undetermined reason, but there is still a parallel path to ground on the supply side of your alternator light. My guess is your instrument cluster is shorting out somewhere, blows the IP fuse, but the ground that blew the fuse is allowing power to back feed from the alternator, through the light, and to ground. Maybe pull your cluster and check your PCB - that flimsy collection of laminated copper strips that separates after 50 years. All your instrument cluster bulbs plug into it.