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Let me try explain what I meant with that statement. If you are adding a 30 amp load being pulled from the starter lug and you add a higher output alt. you are creating a potential situation where more those additional amps would be pulled through the wire going from the alt to starter, therefore it should be beefed up with this wiring configuration whereas the larger load would never possible be pulled through that 10awg wire if the fans were tapped directly from the alt-unless the alt failed but then fusable links should do thier job.
And alternators are not designed to handle the spikes for instant-on transient loads. Which is why GM does not hook up anything directly to the alternator. The alternator feeds the battery (via the starter lug for C3 Corvettes). The battery feeds the fusebox which distributes the power. It also reduces the impact of the spikes. Start up spikes for dual fans can hit 100A.
Hooking your fans directly to your alternator can save some wiring but will increase the chances of killing the diodes in the alternator. Seems someone was talking about diodes failing earlier in this thread.
Hi JD21476, lots of interesting responses in this thread - with some experienced, well-meaning people trying to help. I have a question, and this is a real question, not a shot or commentary or whatever. You have a '68 with a crate 350 in it. My question is: what problem or opportunity are you trying to solve or achieve with the use of the dual electric fans? The reason I ask is that I've used both for different reasons depending on the application. I also have a '68 with a 427/435 and a proper stock cooling system that does a terrific job of keeping it cool on the hottest days in traffic, etc. But, you may be wanting the electric fans for other reasons and I totally get it. Just wondering. Best, Paul
Steve is spot on. Think about it this way. An alternator produces alternating current, the diode bank then rectifies that current into direct current, but it has significant ripple (voltage variations riding on top of the desired voltage), particularly at idle or low rpm. The output wire that runs to the battery via the starter connection has resistance and the battery is a voltage stabilizer (actually like a large capacitor) that dampens this ripple. The ripple riding on the desired voltage is filtered out across the wiring resistance and shunted by the battery providing much more stable voltage at the starter lug. That is as lay an explanation as I can provide. Now, let’s talk about electric motors. At start up, they draw maximum current, but as the motor begins to spin the magnetic field induces a counter current that reduces the current draw. Some people mistake the momentary dimming of lights as an indicator of insufficient power when the reality is that you are experiencing the manifestation of Ohm’s law visually. The faster the fan motor spins, the less current it draws to the point of maximum rpm. If you put this initial current draw right at the alternator without then the benefit of the voltage dampening of the battery, you run the risk of damaging the diodes in the alternator. I believe this is what those in the know have been trying to explain over now four pages of posts.
Steve is spot on. Think about it this way. An alternator produces alternating current, the diode bank then rectifies that current into direct current, but it has significant ripple (voltage variations riding on top of the desired voltage), particularly at idle or low rpm. The output wire that runs to the battery via the starter connection has resistance and the battery is a voltage stabilizer (actually like a large capacitor) that dampens this ripple. The ripple riding on the desired voltage is filtered out across the wiring resistance and shunted by the battery providing much more stable voltage at the starter lug. That is as lay an explanation as I can provide. Now, let’s talk about electric motors. At start up, they draw maximum current, but as the motor begins to spin the magnetic field induces a counter current that reduces the current draw. Some people mistake the momentary dimming of lights as an indicator of insufficient power when the reality is that you are experiencing the manifestation of Ohm’s law visually. The faster the fan motor spins, the less current it draws to the point of maximum rpm. If you put this initial current draw right at the alternator without then the benefit of the voltage dampening of the battery, you run the risk of damaging the diodes in the alternator. I believe this is what those in the know have been trying to explain over now four pages of posts.
There was zero mention of diodes being damaged until Steve brought it up but it does provide a good explanation. I was under the strong impression the rather large factory terminal block located on the inside of my 74s fender that fed other accessories under the hood like the alarm, horns and possibly headlights was being wired from the alternator post a foot away buts its hard to tell because the wires are wrapped. I see theres a number of larger gauge wires coming off of it in any case thats a good point about the brushed motors creating noise assuming the connected battery would not provide dampening unless the current was forced to flow from it which means these fans should only be wired directly from the battery and not the starter lug where it could still be and issue right?
Last edited by augiedoggy; Mar 5, 2021 at 05:51 PM.
Exactly- thanks for proving my point- the Vette's electrical system was not designed to handle double the stock current draw.
The stock alternator in my car was 62amps.... I no longer have ac so that would mean the current amp load would be equal to the model without ac. Those had a 42 amp alternator. I'm fairly certain they both had the same size wiring so I fail to see how the extra 10 amps (or lets say 20 amps for good measure) translates to double the factory capability? as you said earlier having the correct numbers does matter and it seems your exaggerating now.
These fans have been tested by other in another thread and have been shown to vary in amp draw but the highest momentary peak reported was just over 40 amps with the operating current after that being reported to be as low as 24 amps and as much as 32 amps. (im sure the voltage and whether they are spals or clones comes into play here as well) I just dont see how the 30 amp load justifies NEEDing to go from a 42 or 62amp alternator to 144amp model.
Last edited by augiedoggy; Mar 5, 2021 at 05:32 PM.
The stock alternator in my car was 62amps.... I no longer have ac so that would mean the current amp load would be equal to the model without ac. Those had a 42 amp alternator. I'm fairly certain they both had the same size wiring so I fail to see how the extra 10 amps (or lets say 20 amps for good measure) translates to double the factory capability? as you said earlier having the correct numbers does matter and it seems your exaggerating now.
These fans have been tested by other in another thread and have been shown to vary in amp draw but the highest momentary peak reported was just over 40 amps with the operating current after that being reported to be as low as 24 amps and as much as 32 amps. (im sure the voltage and whether they are spals or clones comes into play here as well) I just dont see how the 30 amp load justifies NEEDing to go from a 42 or 62amp alternator to 144amp model.
Well, my stock alternator couldn't keep up with my high beams. I replaced it with a 3-wire 12si 150 Amp unit. I'm not building a bridge, so I'm not trying to find the smallest alternator that will barely work. But I certainly don't want to have to think about alternators again in the future when I throw another electric load at the car.
The stock alternator in my car was 62amps.... I no longer have ac so that would mean the current amp load would be equal to the model without ac. Those had a 42 amp alternator. I'm fairly certain they both had the same size wiring so I fail to see how the extra 10 amps (or lets say 20 amps for good measure) translates to double the factory capability? as you said earlier having the correct numbers does matter and it seems your exaggerating now.
These fans have been tested by other in another thread and have been shown to vary in amp draw but the highest momentary peak reported was just over 40 amps with the operating current after that being reported to be as low as 24 amps and as much as 32 amps. (im sure the voltage and whether they are spals or clones comes into play here as well) I just dont see how the 30 amp load justifies NEEDing to go from a 42 or 62amp alternator to 144amp model.
The 42 and 62A are what I call- "OGD" ratings - On a Good Day
The fans don't come on when your cruising down the road- but at IDLE- see the real world ratings below...
Peak power is NOT what you are looking for- but rather output at IDLE...
Originally Posted by lionelhutz
Stock 10SI alternators can't support dual fans. You don't get to rated output on a 10SI until about 7000rpm at the alternator. With the typical 2.5x to 3x pulley ratios in use, this is 2300-2800rpm at the crank. - A 42A 10SI produces around 10-14A at idle.
- A 63A 10SI produces around 20-27A at idle.
Neither of these can support the fans plus the other vehicle loads at idle.
Originally Posted by Factoid
Steve is spot on. Think about it this way. An alternator produces alternating current, the diode bank then rectifies that current into direct current, but it has significant ripple (voltage variations riding on top of the desired voltage), particularly at idle or low rpm. The output wire that runs to the battery via the starter connection has resistance and the battery is a voltage stabilizer (actually like a large capacitor) that dampens this ripple. The ripple riding on the desired voltage is filtered out across the wiring resistance and shunted by the battery providing much more stable voltage at the starter lug. That is as lay an explanation as I can provide. Now, let’s talk about electric motors. At start up, they draw maximum current, but as the motor begins to spin the magnetic field induces a counter current that reduces the current draw. Some people mistake the momentary dimming of lights as an indicator of insufficient power when the reality is that you are experiencing the manifestation of Ohm’s law visually. The faster the fan motor spins, the less current it draws to the point of maximum rpm. If you put this initial current draw right at the alternator without then the benefit of the voltage dampening of the battery, you run the risk of damaging the diodes in the alternator. I believe this is what those in the know have been trying to explain over now four pages of posts.
EXACTLY- x2 electric motors have very little resistance...a battery not only starts the car but stabilizes the voltage
You don't need to connect a wire right to the battery terminal to get the benefits of the battery helping stabilize the voltage. As you move a further distance from the battery, the ability of the battery is limited based on the wire resistance. The wire to the starter is a large gauge wire with very little resistance. So, connecting to the starter end of that wire is almost the same as connecting at the battery. The to the alternator is a much smaller gauge wire which has a much higher resistance, so connecting to the alternator end of that wire is not the same as connecting at the battery, or the solenoid.
You don't need to connect a wire right to the battery terminal to get the benefits of the battery helping stabilize the voltage. As you move a further distance from the battery, the ability of the battery is limited based on the wire resistance. The wire to the starter is a large gauge wire with very little resistance. So, connecting to the starter end of that wire is almost the same as connecting at the battery. The to the alternator is a much smaller gauge wire which has a much higher resistance, so connecting to the alternator end of that wire is not the same as connecting at the battery, or the solenoid.
You don't need to connect a wire right to the battery terminal to get the benefits of the battery helping stabilize the voltage. As you move a further distance from the battery, the ability of the battery is limited based on the wire resistance. The wire to the starter is a large gauge wire with very little resistance. So, connecting to the starter end of that wire is almost the same as connecting at the battery. The to the alternator is a much smaller gauge wire which has a much higher resistance, so connecting to the alternator end of that wire is not the same as connecting at the battery, or the solenoid.
Yup, which is one of the reasons that I upgraded my positive battery cable from stock #2 to #1/0 using cable from West Marine. I have a power stud by the starter where I pull power for my fan and MSD box.
For my EFI and fuel pump controller, I go straight to the battery to reduce any issues of noise on the the FAST XFI (pump controller is in storage compartment anyway). XFI is under passenger dash.
The only success to use an alternator stud as a power distribution point for fan(s) is to increase the output wiring capacitance to a minimum of 4 ga and move up to a 100 amp alternator. The goal is to have No voltage drops and utilize the reserve capacity of the battery. I have done it a few times with adding a relay for A/c-fan activation to boot with no issues over the years. I am in the same camp as others here to stay with a mechanical fan setup and will advise to stay that course unless its a race car or a modern efi engine.. As to why owners choose to run exclusively electric fans seams mostly is for the exceptional perception of "horsepower gains". The other is to make the car "modern and reliable". Which is a modest expense for little gain if any. Most times I see wrong choices for fan sizing and incorrect cfm rated fans. Then there is the wiring design choices! Good grief. Most all early alternators drew 5-7 hp loaded. So Doubling or tripling the usable output ups the negitave horsepower draw accordingly. What it is I dont know. Maybe engine masters could do an episode about it. IMHO
There is an old saying in the engine cooling world. If all components are working properly and the engine overheats at idle, but not at speed the fan is not drawing enough cfm, it is too small. If it overheats at speed, but not idle the radiator is too small. Here is the reality. You need a fan at idle more than at speed. Idle is when a mechanical fan is at its worst. Electric fans are superior in every way. Tom Dewitt has already run the comparisons and proven the fact.
I finally got around to moving my wires and checking everything out. I plugged the orange wire into the fuse box where it is labeled IGN but the weird part is the car still wont shut off with the key anymore.
I never had this issue until I messed with electric fans.
I have a constant power to the starter and the IGN wire to the ignition. The only other two are the thermostat plug and the temp wire. The temp is back to working normal and the fans work fine. It just wont shut off by the key anymore.
