Radiator fan power draw on highway?

[EcoCivic]

Update- I extended one of the fan power wires into the car temporarily so I could measure current draw, but I was not able to because my alternator died (while stuck in a bad traffic jam in the middle of nowhere 160 miles from home. Good times). Needless to say I was more concerned with getting home before the battery died than the power draw of the fans. I could test the draw at different speeds now, but doing so at less than normal voltage is pointless. Testing will resume once I replace the alternator.

[EcoCivic]

So I finally replaced the alternator and went to check the draw but for some reason my amp clamp won't work, I think the batteries may be dead. But either way I feel pretty confident that the draw is significant because the voltage momentarily drops below 13V when the fans come on even when driving 70 and the blower fan speed momentarily drops as well, which tells me the draw is pretty significant. And with the alternator off, forcing the fans on while driving down the highway drops the voltage by a couple tenths of a volt. I will report back when I have a number.

I finally have a solution though. What I did is I replaced the 2 position fan override switch I had with a 3 way switch so I can select off, auto, and on as I see fit instead of just on and auto. Normally the switch stays in auto (the middle), but I turn it off when on the highway. Not sure about any fuel savings, but I feel confident that the electrical load was reduced because I no longer experience that random voltage drop when driving down the highway as the AC compressor/fans come on. If anyone wants a diagram of how I did this let me know. I'm not thrilled with the solution of having a manual switch to shut the fans off since it's one more thing to think about and remember, but life goes on.

[Piotrsko]

Of course it's going to sag. Motors have huge starting currents relative to running current. All inductors exhibit the same characteristic including the A/C clutch. Look at the current after the fan is up to speed.

[EcoCivic]

Originally Posted by Piotrsko

Of course it's going to sag. Motors have huge starting currents relative to running current. All inductors exhibit the same characteristic including the A/C clutch. Look at the current after the fan is up to speed.

You're right. But my thought was since the fans should have already been spinning from the wind blowing through them, there shouldn't have been much of a drop on startup. The AC compressor clutch may be the same way, but it only draws a couple of amps instead of 20+ amps and there is no noticeable voltage drop when it comes on as long as the fans are off.

[Piotrsko]

They may be spinning but I bet it isn't more than 1/2 the rated rpm.

[EcoCivic]

I was finally able to get a measurement. At idle with the alternator off, I see a draw of about 16 amps for both fans. At 70 MPH I see a draw of about 9 amps from the fans and there is definitely a spike in current draw when they start up, but there appears to be less of a spike than when idling. This seems to indicate that the fans do spin while driving down the highway, but at less than full speed. Not sure how the alternator being on affects this yet, but there are definitely power savings to be had from shutting the fans off on the highway. Although the reduction in alternator drag would be impossible to see on my MPG by itself, it will significantly save my battery when driving around with the alternator off and possibly extend the life of my alternator as well since it won't be working as hard.

[Piotrsko]

The main wear point to an alternator is the friction contact to the commutator by the brushes which wont be reduced by load, followed by bearing drag. There is some arc erosion, but it seems insignificant with a wise choice of brush carbon composition

[ksa8907]

Originally Posted by EcoCivic

I was finally able to get a measurement. At idle with the alternator off, I see a draw of about 16 amps for both fans. At 70 MPH I see a draw of about 9 amps from the fans and there is definitely a spike in current draw when they start up, but there appears to be less of a spike than when idling. This seems to indicate that the fans do spin while driving down the highway, but at less than full speed. Not sure how the alternator being on affects this yet, but there are definitely power savings to be had from shutting the fans off on the highway. Although the reduction in alternator drag would be impossible to see on my MPG by itself, it will significantly save my battery when driving around with the alternator off and possibly extend the life of my alternator as well since it won't be working as hard.

The fans will run at whatever their predetermined speed is, typically there are two speed settings. The current draw is due to the different amount of work the fans are doing.

As the fans are being assisted by the flow of air forced through the radiator, it reduces the power requirement for any given fan speed.

I didn't read the entire thread, just cruising through so I apologize if I missed it. One possible fix for keeping the A/C from running the fans would be to have a secondary relay that interrupts the circuit below a set speed. Or even a separate fan controller at a different temperature monitoring the hot side of the refrigerant.

[EcoCivic]

Originally Posted by Piotrsko

The main wear point to an alternator is the friction contact to the commutator by the brushes which wont be reduced by load, followed by bearing drag. There is some arc erosion, but it seems insignificant with a wise choice of brush carbon composition

I didn't know that, thanks for the information. It just seemed like less load and heat would be better for the alternator's longevity, but it sounds like any differences would likely be negligible.

[EcoCivic]

Originally Posted by ksa8907

The fans will run at whatever their predetermined speed is, typically there are two speed settings. The current draw is due to the different amount of work the fans are doing.

As the fans are being assisted by the flow of air forced through the radiator, it reduces the power requirement for any given fan speed.

I didn't read the entire thread, just cruising through so I apologize if I missed it. One possible fix for keeping the A/C from running the fans would be to have a secondary relay that interrupts the circuit below a set speed. Or even a separate fan controller at a different temperature monitoring the hot side of the refrigerant.

My car has a very basic 1 speed fan setup that runs at full speed whenever the AC compressor is engaged or whenever the coolant temp gets to 207 degrees.

I did add a secondary relay to shut the fans off with a manual switch above around 50 MPH, but it would be nice to have some type of automatic system so I don't have to think about it and so there's no chance of me forgetting to turn the fans back on, but I'm not sure how I would do that.

The Dakota Digital fan controller has an option to shut the fans off above the speed the user selects. They recommend disabling the fans above 35 MPH. But that setup would also cost $230 for the controller and the separate module and it's hard to justify spending so much on something that would make so little difference, even though it would be cool and fun to play with.