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VS Commodore Voltage drop

HoldenHoon

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Hi guys. I have a 1996 VS BERLINA V8 and I have noticed a big voltage drop. On idle it stands at around 14.20V. With headlights on it drops down but comes back up to 14.20. But when I leave highbeams, headlights and A/C on it’ll drop down to 12.6-12.9v. I heard anything below 13.5 is not good. When I rev the car it instantly shoots up to 14.30. but on idle the voltage is low at around 12.9 with everything on. Is that normal?
 
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jeepster

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I had a look at mine. I get 14.1 volts with high beam and aircon. I have a cheap Chinesium alternator. The output will drop off when they get hot, i.e. city driving on 40-degree day. You might have a blown diode, so the alternator is still charging the battery if you don't put much load on it.
 

Immortality

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I think that is fairly typical. Alternators don't put out it's full current at idle, far from it, turning on high draw accessories is going to pull the voltage down until the engine revs come up. As long as it maintains 14 volts + once you are up to about 1800rpm it should be fine.
 

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Yeah it just depends on what I throw at it. I have a twin AU thermo fan setup running twin relays. I notice that when they come on the voltage drops down to 13.2. When I throw everything at it. (A/C, Highbeams, Headlights, Thermofans) it will drop down to 11.9 and with a rev it will go up to 12.6. Then when everything is turned off it will go back up to 14.2. Maybe a bad alternator?
 

HoldenHoon

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I had a look at mine. I get 14.1 volts with high beam and aircon. I have a cheap Chinesium alternator. The output will drop off when they get hot, i.e. city driving on 40-degree day. You might have a blown diode, so the alternator is still charging the battery if you don't put much load on it.
Is it possible to replace the diode yourself? And where abouts is it located?
 

HoldenHoon

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I think that is fairly typical. Alternators don't put out it's full current at idle, far from it, turning on high draw accessories is going to pull the voltage down until the engine revs come up. As long as it maintains 14 volts + once you are up to about 1800rpm it should be fine.
It’s just weird that when I have a/c and highbeams it will drop to 13.2. With revs it stands at 13.5. Maybe It’s normal? I’ve had some problems with charging in the past and I’ve replaced the original alternator with a genuine Bosch one. But it’s second hand.
 

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Yeah it just depends on what I throw at it. I have a twin AU thermo fan setup running twin relays. I notice that when they come on the voltage drops down to 13.2. When I throw everything at it. (A/C, Highbeams, Headlights, Thermofans) it will drop down to 11.9 and with a rev it will go up to 12.6. Then when everything is turned off it will go back up to 14.2. Maybe a bad alternator?
Thats completely normal considering you have those huge thermofans that draw a lot of current on an electrical system that was never designed to handle electronic thermofans.

My suggestion would be to fit a bigger alternator to handle the higher loads.
 

HoldenHoon

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Thats completely normal considering you have those huge thermofans that draw a lot of current on an electrical system that was never designed to handle electronic thermofans.

My suggestion would be to fit a bigger alternator to handle the higher loads.
So it’s a good alternator then? I only installed thermo fans to help with the heat. I’ve heard some people say that the clutch fan adds strain to the pulley system since the fan is so big. It’s on a sensor that’s tapped into the thermostat housing. It comes on when the temp reaches just over a quarter on the cluster and brings it back down to half way in between C and the first line. Also, where would I be able to get an alternator that can handle the loads that is being thrown at it?
 

vc commodore

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I have an 80 amp alternator fitted to my VC.

With that, I run 2X 130W spot lights, a lightbar, 2X 100W fog lights, an amp, stereo, an electric fuel pump, UHF radio, 2 thermo fans, electric oil pressure, water temp and taco....On idle, I have 14.2 volts....Turn everything on, it has 12.3 or4 volts on idle....

Still haven't had a flat battery with it all running driving down the highway.....So I think you'll be fine
 

BlackVXGTS

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As other members have said, your situation is fine and you are unnecessarily worried. If you really, really, really, really, think you have a problem, you can always install a 120 amp alternator. Check out the link below if you want to go that way.

 
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