84 voltage issues
Thread Starter
Melting Slicks
Joined: May 2009
Posts: 2,772
Likes: 176
From: Decatur IL
2025 C4 of the Year Finalist - Unmodified
2022 C4 of the Year Finalist - Unmodified
84 voltage issues
I just had an altercation with a fuel pump i replaced earlier this year in my 84 Z51. it was put in in April and 6 months and a thousand miles later the pump shot craps. when it went, I think the altenater took a hit before it blew the fuse. I had one of my C4 buddies say the voltage regulater could be the culprit. could he be right? Can anybody shed some light on a low voltage 84?
Thread Starter
Melting Slicks
Joined: May 2009
Posts: 2,772
Likes: 176
From: Decatur IL
2025 C4 of the Year Finalist - Unmodified
2022 C4 of the Year Finalist - Unmodified
Before all of my problems, 14.1 when it charges overnight, 13.5 when hot, 13.2 with the fan on. Now 12.8- 13.2 when chargws overnight. Anywhere between 12.2 and 11.5 when the red square comes on when hot. I dont think any of the accesories are drawing a charge, I think its in the alt. The voltage regulater.
Team Owner
Joined: Mar 2001
Posts: 54,124
Likes: 30
From: Cincinnati, Oh USA
When the battery symbol light on the dash comes on, your alternator is not charging the battery! Normal alternator output voltage across the battery terminals is 14.7 volts cold and this drops to 13.7 volts alternator hot. The dash voltmeter reads 0.3 volts low because it reads the voltage after the ignition switch which has 0.3 volts drop across it, so your dash voltmeter should read 14.4 volts cold, 13.4 volts hot. Car batteries are considered discharged at 12.0 volts and should be charged up by a battery charger and not the alternator!
A fuel pump blowing up should do no harm to the alternator!
A fuel pump blowing up should do no harm to the alternator!
Thread Starter
Melting Slicks
Joined: May 2009
Posts: 2,772
Likes: 176
From: Decatur IL
2025 C4 of the Year Finalist - Unmodified
2022 C4 of the Year Finalist - Unmodified
Could it be the battery? It's a yellow top Optima that's almost three years old? The alt was replaced at the same time. All I Know that before the pump went out the voltage was normal. Did I mention that I did send the pump back to Corvette Central and they sent it back rebuilt? After all it did blow the fuse.
Team Owner
Joined: Mar 2001
Posts: 54,124
Likes: 30
From: Cincinnati, Oh USA
It could be the battery causing low voltage with the engine running. The alternator voltage regulator also limits the current output to protect the alternator and it limits the current by reducing the output voltage. A battery with one cell shorted is a 10 volt battery and the alternator will try to charge this defective battery with 100 amps and cause the alternator to put out low voltage, protecting itself. This low voltage will turn on the dash battery symbol light. 3 year old batteries are always suspect. Measure the battery after it sat overnight, it shouldn't be lower than 12.0 volts and at 12.0 volts it is discharged. Charging the battery up with a battery charger and letting it sit for 24 hours, it should read 12.9 volts (fully charged). Batteries with some age on them and charged overnight may only read about 12.5 volts because they no longer have full amphour capacity but will still operate a car. If you can swap a battery known good from another car, that would be a good experiment. Normal dash voltmeter readings are 14.3 volts cold, 13.3 volts hot.
That a fuse was blown has no bearing on either the alternator, or the battery.
That a fuse was blown has no bearing on either the alternator, or the battery.
Last edited by jfb; Dec 21, 2009 at 09:44 PM.
Le Mans Master
Joined: Sep 2009
Posts: 6,320
Likes: 1,131
From: Vermilion OH
i would pull out the alternator and the battery and take them to your local napa or advance auto and have each tested. often times a bad battery will cause the alternator to overcharge..not good. the last yellow top i had lasted for seven years, but i would definatly have everything tested.
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Drifting
Joined: Aug 2000
Posts: 1,758
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From: Medford, Or
I would recommend taking it to an auto electric (alternator/starter) shop. That's all they do. They are getting harder to find these days but it will be worth it. Do not buy a rebuilt alternator from auto parts stores. Have the original rebuilt.
Good luck!
Good luck!