Aircon and electric fan don't mix on a 79. - Team Camaro Tech
Electrical & Wiring Troubleshooting electrical.

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post #1 of 5 (permalink) Old Jun 4th, 03, 10:32 AM Thread Starter
Join Date: Oct 2001
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I have a 1979 with a 305. I added an electeric fan to get a few extra HP, more space, and the cool look. Well a week ago I charged up my aircon, because it is hot down here in South Texas. Well if I turn on the A/C my battery goes dead. At idle with nothing on the Alt kicks out 13.50 volts. With the fan on it drops down to 12.50 volts. And with the A/C on who knows I havent checked it. But why does my fan eat so much juice. I don't want to put the manual fan on, but I want A/C. I have it wired with a relay. From the bat to the relay with a fuse. And to the positive side of the fan. Ground which is attached to the fender part of the car. And the fan has the groud attached to this place too. And the wire that goes to the fuse box that turns the fan on when I turn the key. I though a relay was suppose to help draw less juice fron the battery. So whats up. Do I need to wire the relay different. Maybe grab it to the back of the alt or something. Or do I go back to using the old manual fan?
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post #2 of 5 (permalink) Old Jun 4th, 03, 11:06 AM
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The relay is only a high-current switch of sorts. I prevents the need for heavy wiring throughout the electrical system and expensive switches etc. Relays are designed to handle high current loads, but do nothing to reduce current consumption.

I'm not sure what alternator you have, so I presume it's stock. I also don't know what the 1979 vintage alternators were rated for. But consider this estimate... Fans draw about 25amps each. The engine probably consumes 5 or 6 amps without lights or other electrical loads. Tunes can draw a fair chunk depending on what you've got. A battery draws more than 10 amps charging after startup and usually 5 amps or so running. In summary, with electric fans, you can easily demand 60-70 amps from the alternator without lights and another 10 amps or so with lights. Most factory alternators that I know of are rated for 50 amps or less. You might need to upgrade to a 100 amp alternator and increase the size of the wire from the alternator to battery. BTW: Under-drive pulleys can compound the problem you're having.

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post #3 of 5 (permalink) Old Jun 4th, 03, 11:49 AM
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There are two different alternator types listed for '79, 37 amp and 61 amp. How much you wanna bet you have the 37 amp.

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post #4 of 5 (permalink) Old Jun 4th, 03, 12:04 PM Thread Starter
Join Date: Oct 2001
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I called Pep Boys up because that's where I bought the thing and they said it was a 63amp alternator. The guy said there are two diamiters for the alt. 5 1/2 and 6 5/8. The 6 5/8 has 100amps. But will one of those fit on my car if my car has the 5 1/2? What is the difference?
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post #5 of 5 (permalink) Old Jun 5th, 03, 09:26 AM
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That's the difference between the GM 10SI and 12SI alternator case.

When you buy a rebuilt alternator, there is no guarantee that it will put out the original rated current. Rebuilders have a bad habit of swapping stators and rotors and basically mis-matching components out of ignorance. If you talk to the right parts guy, you should be able to buy the proper 10SI alternator tested at 65A output for your car.

If it's still a problem child, you can upgrade to the larger alternator. I did my brothers Monte Carlo years back 'cuz it kept burning out the 65A alts for no good reason. (Actually, he runs all the accessories with the car idling a lot!) You may have a little bit of trouble fitting the larger 12SI case in your car, but it can be done reasonably easily if you really need to do it.

Paul D.
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