Fan recommendations

[phat46]

While driving my old truck last Friday during our local Cruise Night I noticed the volt meter reading a little low. When I was driving home later and turned the lights on it dropped to 11 volts. I turned the electric fan off and it went back to 13 volts. When I start the truck it reads about 14 volts and when I turn the fan on it drops to about 12 volts. If I turn just the lights on without the fan it's fine. I'd like to know if anyone has a recommendation for a fan that pulls less juice.

[papastoyss]

Is the fan a recent addition or has this been an ongoing problem? I would start by double checking all connections & wiring including grounds. Most oem cars that use an electric cooling fan have at least a 100 amp alternator.My '40 coupe has the large Cooling Components fan & a 140 amp alternator. It holds at 13.7 volts w/ac & radiator fan on. If all your wiring & connections are up to snuff the alternator may not be up to the task at hand.

[wayne petty]

install a fan thermostat and relay to allow the radiator cooling fan to cycle on only when there is hot coolant in the radiator..

you will also need a thermostat to complete this operation...

this reduces the load on the alternator by 2/3 to 3/4 as the fans only need to run about 30 seconds out of every 2 minutes.

and if the air flow thru the grill is properly ducted and directed thru the core. instead of around it.  it will greatly help..
a thermostat with a properly installed bypass circuit allows the coolant pushed by the water pump around the cylinders.. up thru the back of the heads.. then forward thru the heads and across the closed thermostat.. again and again until it reaches the opening temp.. so the coolant can swap with the cooler coolant in the radiator.. this cooler coolant closes the thermostat and allows the stopped coolant in the radiator to loose the heat to the ram air or fan flow..



since i have not seen your truck... or the way its wired..

forgive me if i explain a few things that you have already done..

a decent sized wire from the alternator output to the battery positive..

a decent sized wire from the positive battery post to a terminal block where you have installed the radiator fan relay/s

a similar sized wire from the battery negative to the body or a terminal block where you have connected the fan to ground..  remember..  you can do a voltage drop test in a few minutes to see if your wiring is undersized for the horsepower your electric cooling fan is requiring..

engine running.. radiator fan/s on...

digital volt meter set to 20 volts DC...

probe positive battery POST to the positive wire on the fan close to the motor or at the last connection you can get to..

post results..

probe negative battery post to the negative wire on the fan as close to the motor as possible..

post results..

in a proper world.. less than 0.1 volts should be measured.. that 1/10th of a volt..

you might want to print this image..

http://imageshack.us/scaled/landing/534/voltagedroptesting.jpg

if all the wiring checks out.. and the alternator belt tension passes the thumb test...

belt tension thumb test.
engine off.. keys in pocket..
push alternator blade with thumb.. the alternator belt should be just tight enough to prevent any slippage..  if the alternator slips inside the belt without turning the crank.. tighten the belt slightly ..

you can also run the voltage drop test from the output stud on the alternator to the battery positive..

you might also want to look at increasing the alternator output with a different alternator..  most cases its easy to swap them or just change the rotor, stator windings and rectifier bridge for a heavy duty version.. that has 6, 50 amp 200 piv diodes instead of 6, 25 amp 100PIV diodes.

[phat46]

Is the fan a recent addition or has this been an ongoing problem? I would start by double checking all connections & wiring including grounds. Most oem cars that use an electric cooling fan have at least a 100 amp alternator.My '40 coupe has the large Cooling Components fan & a 140 amp alternator. It holds at 13.7 volts w/ac & radiator fan on. If all your wiring & connections are up to snuff the alternator may not be up to the task at hand.

I was also thinking the alternator might not be up to snuff,  I'll have to check and see how many amps it is. I didn't  mention in my original post that the setup is a '70 Ford 429 which I assume has the stock alt.  on it. I have noticed before that the volts dropped when the fan is on but never put two and two together because I rarely am driving at night with the lights on.

[phat46]

Wayne I will make those checks you suggest, and you're right about cycling the fan, I barely need a fan if I can keep moving more than a couple mph. The rad is rated for 700 h.p. and I'm in the 400 range.

[rumrumm]

If you are running a one-wire alternator, that is your problem. Switch it over to a typical three-wire connection and your alternator will sense the additional voltage and adjust for it. I will never use a one-wire alternator again.

[wayne petty]

If you are running a one-wire alternator, that is your problem. Switch it over to a typical three-wire connection and your alternator will sense the additional voltage and adjust for it. I will never use a one-wire alternator again.

just curious ... what problems did you have with a ONE wire alternator..

lack of charge?? slow to start charging???

one needs to have significant sized alternator to battery positive or alternator to starter /battery post cables with a one wire alternator to reduce voltage drop to a minimum...

same effect on the negative side... if the engine is not properly grounded to the frame/chassis/body.. where ever the battery and the rest of the accessories are attached to ground may not have enough current because of resistance or voltage drops..

the only difference between a one wire system and the 3 wire system is where the voltage is sensed at..  3 wires allow the system to sense voltage closer to the cable reducing voltage drop for the sense circuit..

on GM cars.. the alternator output and the sense wire are both connected to the top post on the starter solenoid...

only the lamp circuit is different on the 3 wire version..

i am not trying to sell you on a 1 wire alternator.. as when they blow out . they are hard to get fixed..  you have to carry spare voltage regulators as most alternator shops pretend they don't exist.

issues rise up with loose grounds.. where i have measured as much as 8.5 volts between the engine and the firewall...  and 1.1 volts between the battery and the engine block with the engine running and the headlights on... since i brought the voltage drop numbers down below 0.04 volts between the engine and the battery negative and 0.02 volts between the battery neg and the body and the engine and the body.. those cars don't have issues... nor do they burn out alternators...

[rumrumm]

With a 100 amp alternator, my voltage would fall below 12 v. when I turned on the A/C. That was with an electronic distributor, electric fan, and electric fuel pump. It would kill  the engine as there was not enough voltage to the distributor. When I switched the alternator to a 3 wire set up, the voltage would drop just .1 point when the A/C switched on and another .1 point with the fan on.

http://www.madelectrical.com/electricaltech/onewire-threewire.shtml

[wayne petty]

With a 100 amp alternator, my voltage would fall below 12 v. when I turned on the A/C. That was with an electronic distributor, electric fan, and electric fuel pump. It would kill  the engine as there was not enough voltage to the distributor. When I switched the alternator to a 3 wire set up, the voltage would drop just .1 point when the A/C switched on and another .1 point with the fan on.

http://www.madelectrical.com/electricaltech/onewire-threewire.shtml

as described several times... on the mad link page 2 and 3.

too much voltage drop in the circuits..


print this test sheet...




then run tests from the alternator output to the battery..

post results..