My alternator and AC does this at idle. The problem is the alternator is not hardly charging anything at idle. We need a smaller pulley on our alternators to get our fancy modern alternators to work. I have my idle set to 1200rpm to combat this, but my AC will drop my engine to 750rpm at idle.Hello. I'm hoping to get some advice. I have a rebuilt 71 - 1.9 to 2,0. Combo cam, big valves etc. I have a Kenlowe electric fan drawing 28 amps. A bit more at start up. Also a Gm alternator 105 amps. With the Combo cam the Idle is somewhat loppy which I like. My problem is that when the fan kicks in it puts such a load on the alternator it wants to kill the Idle. Is there a way to wire it so it doesn't draw so bad on the alternator. I have put the fan back on and have the electric as back-up but I'd sure love to take it off.Thanks for any suggestions.
Exactly. Mine is done that way and I have yet to have my idle drop when my pusher fan comes on. I have to listen for the fan to cut on. It will cycle when the car is cut off, but no problems when the car is running. JarrellFor a high amp. draw like you are having (28 amps) you need to have the bat. wire from the alternator wired directly to the battery terminal (10 gauge) and not through the amp gauge. Use a voltmeter in place of your amp gauge.
HTH
I think you may be mistaken (but I’m open to correction).A smaller alternator pulley will make the alternator output higher at idle.. but that will make the power draw of the fan WORSE when it turns on. So that is moving this problem the wrong way.
OP, you might improve your idle response to loads like this fan by better tuning. There tends to be a lot of interaction between ignition advance and idle opening in the carb. Too much ignition advance at will result in a very small throttle opening at idle, and that will be much more effected by changing loads. The main point is that part of what you may have is in the engine's tuning.
Great info thanks. Wonder why my fan draws so much. Had to go to a 40 amp fuse too.TWENTY-EIGHT AMPS!!!!! Holy Bejeesus! That's ridiculously huge. You should be down around 15amps max. I run an excellent engine fan, same as PJ, and a separate auto tranny cooler fan on the same controller and fuse and I only use 15amps.
I use a GM SI-style one-wire 105amp alternator. It doesn't begin the charging cycle until 1500rpm. I was running undersized crank and oversized water pump/fan pulleys to increase cooling efficiency, but that makes your alternator run slow and yet I still only needed to rev the engine to 1500 to start the charging and then I can ease off on the pedal and it would continue to charge at the 1000rpm my side draft required. I have an auto tranny that squashes the rpms down to 800rpm when in Drive no matter what you set the idle at and I would still have some charging, albeit a little lower. My FI mod has forced me to go back to the stock pulley sizes, so I should have a faster spinning alt and even better charging. I retain the use of the engine fan and my electrics only kick on at 180*. Under normal circumstances cruising around my engine only gets up to 160*, so my fans seldom kick on. Only when I'm stuck in traffic, a fairly common occurance, does the temp start to spike and kick on the fans. My funny pulley sizes made the water pump turn slower for better cooling efficiency at higher rpms, but at idle this would cause the engine fan and alternator to turn slower than oem. Going back to the oem pulley sizes should make my fan spin a little faster and provide better radiator cooling, plus improve charging at idle.
When I had a Combo cam I also disliked how the lope would mess with my charging at idle. Very annoying. Every time it lopes you're on the verge of stalling.
If I were you I would get a fan that doesn't gobble so much power. I'll see if I can find the fan that me and PJ use....
Power draw is not a constant vs voltage in these motors... the example numbers are not correct. Don't confuse the rise in current draw when a motor stalls with running current versus voltage when the motor is running in the normal RPM range. Current in motors like these is related to torque which is related to fan blade, speed, etc. Increase the voltage (from the faster alternator as proposed) and the speed will increase....we all know this from hearing fans slow down when we drop and engine to idle and the fan(s) slow down due to alternator voltage dropping. Torque/load will increase with speed, and current has to increase; it may not increase much, but it won't go down. (This all comes out of the mathematical modeling for motors and involves back-EMF's, etc.)The alternator charges the battery to a higher voltage whenever the pulley is moving faster at idle (since it’s currently not with a stock sized pulley). The power draw from the fan does not change. An 80W motor is an 80W motor for example.
Since the battery voltage is higher the amp draw from the battery will be less.
For example at 12V an 80W motor will draw around 6.7 Amps
At 14V the motor will draw more like 5.5 Amps
Ah! Your response encouraged me to look at how DC motors worked. Good lord they are more complicated than I thought.Power draw is not a constant vs voltage in these motors... the example numbers are not correct. Don't confuse the rise in current draw when a motor stalls with running current versus voltage when the motor is running in the normal RPM range. Current in motors like these is related to torque which is related to fan blade, speed, etc. Increase the voltage (from the faster alternator as proposed) and the speed will increase....we all know this from hearing fans slow down when we drop and engine to idle and the fan(s) slow down due to alternator voltage dropping. Torque/load will increase with speed, and current has to increase; it may not increase much, but it won't go down. (This all comes out of the mathematical modeling for motors and involves back-EMF's, etc.)
With better voltage, the torque load on the engine is higher.
In other words:Power draw is not a constant vs voltage in these motors... the example numbers are not correct. Don't confuse the rise in current draw when a motor stalls with running current versus voltage when the motor is running in the normal RPM range. Current in motors like these is related to torque which is related to fan blade, speed, etc. Increase the voltage (from the faster alternator as proposed) and the speed will increase....we all know this from hearing fans slow down when we drop and engine to idle and the fan(s) slow down due to alternator voltage dropping. Torque/load will increase with speed, and current has to increase; it may not increase much, but it won't go down. (This all comes out of the mathematical modeling for motors and involves back-EMF's, etc.)
With better voltage, the torque load on the engine is higher.
Oh ya that cleares it right upIn other words:
Let's say, you have a 12V DC motor with 80 Watts. This means, the motor has a power consumption of 80W at 12V. Since power consumption P (Watt) = U (Voltage) x I (Current), as a result your current draw is P / U or 80W / 12V = 6.67A.
Also, the electrical resistance R (Ohm) = U / I, so your motor has a resistance R = 12V / 6.67A = 1.8Ohm. Since the resistance is constant (within our idealized example), you can see that you can rearrange the equation to I = U / R --> Since R is constant, if the voltage drops down, the current drops down in the same relation which will bring the power consumption down even further.
If the voltage drops by factor 2 (to 6V) and the current drops by factor 2 as well (to 3.33A), the power drops by factor 2[SUP]2[/SUP] = 4 from 80W to 20W (6V x 3.33A).
Dieter
Can you put this in laymans terms that relates to the issues at hand.
As the voltage to the fan drops the fan slows down, uses less power (watts), and draws less current (amps)Oh ya that cleares it right up
