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Old 04-20-2021, 01:46 PM   #1
Ken Arms
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Default charging question

Hey folks, forgive me for asking this on the barn but its were I am comfortable.
For reasons too long to explain I put an electric fan in my 34. The car is converted to 12v and I'm using a typical GM alternator. All works well except I notice when the fan starts and I am at idle the amp gauge drops dramatically. Once I step on it a bit it comes up. I always thought an alternator put out the same amps at slow speed or high. Do I need a higher output alt? Better batterie? I am using a group 35 batterie like a small Honda might use to make room for dual exhaust. The alt is three wire with a diode. I cant recall the fan amp draw but its fairly high. Ken
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Old 04-20-2021, 02:17 PM   #2
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Default Re: charging question

Look up the performance curve for your particular model GM alternator and you will see that the output curve does not flatten out until about 5000 shaft rpm. Just search for your model of alternator performance curve and you should find it.
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Old 04-20-2021, 03:07 PM   #3
JSeery
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Default Re: charging question

Something like this.
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Old 04-20-2021, 07:09 PM   #4
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Default Re: charging question

Not a good idea to have the fan main power cable connected through the ammeter. Connect directly to the main battery cable at the starter solenoid. Less voltage drop there and wont effect other electrical items as much as being through the ammeter. The fan should also be connected through a relay. The alternator will charge at engine idle speed but not any where near its full output. Need some revs up to obtain higher output. Regards, Kevin.
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Old 04-21-2021, 07:02 AM   #5
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Default Re: charging question

Couple of thoughts on what I did;
(1) figured total amperage load
(2) sized the alternator accordingly
(3) used the smallest pully I could find on it

I too had a early GM (42 amp) alternator. When I added a late 90s Taurus SVO 2 speed fan and shroud (basically a bolt on), a 800watt stereo amp, halogen headlights and AC I had a huge deficit. Modified the GM mount to fit a later Ford 120? amp alternator and problem solved. The small GM pully I was using fit the late model Ford. Lot of different configurations of the Ford alternator, I forget which one fits (almost) the GM bracket I used on the Merc flatty.
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Old 04-21-2021, 07:37 AM   #6
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one other thing, your system alternator is functioning as it should. If your driving is constant stop and go traffic you should think about an upgrade. I wouldn't be concerned about a fall off sitting at a stop light once in a while. If you attend "cruises" you are going to need an upgrade.

Just my $0.02
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Old 04-21-2021, 10:35 AM   #7
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Default Re: charging question

Quote:
Originally Posted by Ken Arms View Post
Hey folks, forgive me for asking this on the barn but its were I am comfortable.
For reasons too long to explain I put an electric fan in my 34. The car is converted to 12v and I'm using a typical GM alternator. All works well except I notice when the fan starts and I am at idle the amp gauge drops dramatically. Once I step on it a bit it comes up. I always thought an alternator put out the same amps at slow speed or high. Do I need a higher output alt? Better batterie? I am using a group 35 batterie like a small Honda might use to make room for dual exhaust. The alt is three wire with a diode. I cant recall the fan amp draw but its fairly high. Ken
Ken you are going in the right direction but only have way there. You need
to have a 24 volt statter installed. This isn't expensive. I had it done to two
GM 12 volt alternators and they charge at idle with a standard 39 Ford pulley.
They can do it for 6 or 12 volts, either polarity. Contact Chestnut Hill Electric
in Denver or Lancaster Pa. They also have a place in Ohio. G.M.
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Old 04-22-2021, 09:47 AM   #8
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Default Re: charging question

G.M., what is a statter? I did some searching of the internet and could not find the answer. Just curious as I am experiencing a similar issue.
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Old 04-22-2021, 10:26 AM   #9
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Originally Posted by CaliforniaBorn36 View Post
G.M., what is a statter? I did some searching of the internet and could not find the answer. Just curious as I am experiencing a similar issue.
Statter is the windings inside the outer case or shell. G.M.
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Old 04-22-2021, 10:36 AM   #10
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The spelling is wrong. It's STATOR. Look it up.
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Old 04-22-2021, 10:42 AM   #11
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Default Re: charging question

Stator-
The stator is the stationary part of a rotary system, found in electric generators, electric motors, sirens, mud motors or biological rotors. Energy flows through a stator to or from the rotating component of that system.
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Old 04-22-2021, 02:22 PM   #12
Ken Arms
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Default Re: charging question

Thanks all, I ordered a smaller pulley to start out with. As near as I can tell my alternator is rated at 60 amps. I will try the pulley first and post what happens. Ken
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Old 04-22-2021, 09:13 PM   #13
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Thanks all for the info on stator
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Old 04-30-2021, 12:47 PM   #14
Ken Arms
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Default Re: charging question

Just an update, I tried a smaller pulley and although I do see an improvement the amps still drop when the fan kicks in. I think I may bite the bullet and purchase a high output alternator. Ken
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Old 05-26-2021, 12:51 PM   #15
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Default Re: charging question

So, Its fixed, I put I high output alternator on from powermaster but the symptom remained. Once I powered the fan from starter solenoid all is good. I still don't get a charge at idle but I think its just that the flatheads can idle so slow. Thanks Kevin, you were on the money. Ken
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Old 05-26-2021, 12:57 PM   #16
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Default Re: charging question

Most likely it looks better because you have now bypassed the ammeter, LOL. It's the same as it was, you just can't see it now. The ammeter shows the current flow to and from the battery, but to show that everything has to pass through it.
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Old 05-26-2021, 03:57 PM   #17
Wrenchead
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Default Re: charging question

How old is your battery? It may be part of the reason/solution.
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