is there a standard. rule of thumb amp draw when a dual cooling fans set up turns on to cool the radiator. I am asking as I am charging around 13,8 volts without the fans on and drop to 12 maybe 11,8 when they turn on. Now let me say when they come on my engine temp drops very quickly. My fan setup is out of a dodge product not sure year or make. Also the gauge wire running from these fans is larger then what these new aftermarket fans have. Some say get a bigger alt other say get a new fan setup that draws less amps. I had the alt tested is a 110 amp and is working fine. THOUGHTS????

 

only way to know for sure is to put an amp clamp on it and check.

Do the engine RPM's drop when the fans come on?

If so, you might have to add some airflow in the tune for when the fans kick on to keep the RPM's up so the alt is spinning fast enough to charge.

What crank/alt pulleys are you using?

 

all Pullys are factory. whatever came on a 2002 firebird ls1 is what is on it, as far as the rpms the motor doesn't miss a beat. It runs at the same rpm as before they come on.
My mechanic and I are in agreement we are on the border line on if we are discharging or not with everything turned on. If I am idling with nothing on I am at 13,8 when I hit the lights it will drop on the gauge but then go back up some as it looks like the pcm is allowing the alt to make more volts, Then when I turn on the fans its drops again but will recover some. If I revere it up it will go up some. I think the alt is doing everything suppose i just not sure if it is enough. I am running some type headlights that i know pull my juice then factory.

 

all Pullys are factory. whatever came on a 2002 firebird ls1 is what is on it, as far as the rpms the motor doesn't miss a beat. It runs at the same rpm as before they come on.
My mechanic and I are in agreement we are on the border line on if we are discharging or not with everything turned on. If I am idling with nothing on I am at 13,8 when I hit the lights it will drop on the gauge but then go back up some as it looks like the pcm is allowing the alt to make more volts, Then when I turn on the fans its drops again but will recover some. If I revere it up it will go up some. I think the alt is doing everything suppose i just not sure if it is enough. I am running some type headlights that i know pull my juice then factory.

 

I would put the amp clamp on the big wire off the alternator and see what it's actually doing.

How big is the wire coming off the alternator? Does it run directly to the battery to a BJB?

 

I am running the original alt wire direct to battery. But i had to extend it I didn't Soder it So I ran a second 4 gage cable from back of alt to starter which then is connected to battery via factory starter cable. So I think I am pretty sure I am not losing any volts.

 

30 to 40 Amps is typical for electric fans. While your alternator might be rated at 110 Amps, it generally needs more RPMs than idle to put out that much. A larger alternator may put out much more at idle.
Many sets either use 2 fans (at 15-20 Amps each) or a 2-speed fan. Most ECU's can run a dual/2-speed setup, starting one fan at e.g. 190F and the 2nd at 200F. Perhaps your fan are full blast at the lower temperature. Of course max fans are needed when the car sits in traffic idling on a hot day.

 

Do your fans list a wattage on the fan housing? Say, maybe "80 watts"? Volts x amps = watts. Thusly, 80 watts divided by 13.8 volts = about 5.8 amps. This is what the small, eight inch fan for my transmission cooler consumes. My two radiator fans are in the neighborhood of 400 watts each so about 29 amps each.

Rick

 

The fans should pull 10-20A each. Where are you measuring voltage? Either your voltage reading is from a point where voltage drops, or your alternator sens point might be in a spot where voltage is good. Try measuring the voltage at the alternator output