Voltage question

My 57 Chevy has alternator and AC. Because it gets a little hot in traffic, I added an electric fan. The fan is manually switched when needed. At idle the car makes 13 volts but when you turn on the fan it goes down to 10. Is that normal? At speed it goes back up but at speed I don't need the fan so I am only concerned with idle.
Thanks in advance

 

What amp alternator is it? It could be getting weak or not turning fast enough at idle.

 

where are you checking the voltage? At the battery or looking at a gauge?

 

Depending on the capacity of your alternator and how many amps the fan draws, yes it could be normal. Some electric fans take a LOT of power, and with it and the a/c running you may be drawing more current than the alternator can produce at low speed. You may need a higher capacity alternator as well as upgraded wiring to match.

 

Measure voltage at the battery & the alternator, the dash gauge might have voltage drop thru the harness

 

The voltage drop sounds to be a little excessive to me. It depends on a number of factors such as wire gauge ,length of wire and draw of the fan. Take a look at the link below ....They have a calculator where you can plug in the numbers and see what the drop should be based on those factors.

Voltage drop Calculator

 

Ok to answer a few questions. It has an aftermarket harness and is pretty new but not installed by me. The alternator is also new or at least rebuilt from Autozone. Nothing special just a stock replacement one so I do not know the amps . I am looking at the dash gauge and have not actually put a meter anywhere but can do that. Thanks for the quick suggestions.

 

Will do. Thanks

 

Gauge probably is not correct.

A new alternator, with a known good battery, should be 14.2 at the battery, no load applied like lights, or fan etc.
If you don't have a meter. HF has a pretty accurate, pretty good one for $5 or so, last time I knew.

 

With the ac on you are using power for the compressor clutch and the cabin air fan so the auxiliary engine cooling fan could put it over the top. You might need to speed the alternator up if it handles the load at higher speeds. You can put a smaller pulley on the alternator or a larger one on the engine which would also speed up your water pump and fan if it runs off the same pulley. That might be enough increased airflow so you won't need the electric fan or you can put a larger pulley on the water pump to keep their speed the same.

 

Could be wiring or alternator. The fan should be wired thru a relay and make sure the power wire is big enough. You may need a larger alternator too. AC equipped cars draw a lot of power and get larger alt from the factory. Find out what size alt a V8, air conditioned Chev uses and get one. Your local auto electric or rebuilder can advise you.

 

A healthy alternator should charge 14 + volts, 13 will barely keep your battery charged.

 

Sounds like at lower rpm @750 the alternator is putting out only a portion of what it can at higher rpm @ 1500. I would try a smaller pulley on the alternator. The other option is to find a alternator capable of putting out more amps at lower rpm.


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Your alternator is too small. It's not the wiring, or the output wouldn't go back up when the motor is speeded up if the load stays the same.

 

1. Smaller pulley.
2. Upgrade to 12SI alternator, close to 100 amp.

 

If using the old style v belts and pulleys, make sure they are not slipping at idle, and then hooking up at higher rpm. Not hearing any squeals, squeeks, or chirps are you?

 

No the belt it tight. Will look into the smaller pulley and the 12SI as well. Thanks for the help


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At higher speed, the fan is being helped (less load on fan motor) by the air being forced thru it by going fast. DD

 

Along with a higher amp alternator, use as large capacity battery as possible.

4TTRUK