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Question about alternators and electric fans

I honestly believe that the 12SI alternators don't have the capability to carry high load at low speed. I learned this from a friend and his Dad who own an alternator bussiness. I used to have dual electric fans on my '70, but it only has 2 headlights. I don't recall having a large drop when the fans kicked on at idle. I know that I had charging to the batterie all the time though. I really like Blue85's idea of a PWM controller, that way you could have the fans run constantly at a low speed to provide airflow through the condensor all the time. That was the main reason I pulled my electric fans and went back to mechanical as it evened out the A/C temps. Oh, I forgot, my truck has a CS130 with the Iceberg case halves.
 
We probably already covered this, but the ECM can't control both fans separately, and/or does not have a "fan anticipate" setting to bump up RPM at idle prior to the fans coming on?

The draw from fan *startup* is no joke. I do agree that with your setup you'd think there would be enough to not have a problem.

Can anyone comment on the later electric fans and how they are controlled? Many of the Ford setups I saw recently in the wrecking yard had some sort of "device" in the fan shroud that I can only assume had something to do with the fan motor control, as some were hooked into that circuit. I am unsure if it's PWM, or some sort of thermal resistor(?), but they were quite large, and do not look anything like just a temp probe.

As I recall, one I saw was in a Crown Vic Police Car. The piece is mounted right to the fan shrouds.
 
Maybe these fans just really weren't meant to be running wide open except in the most extreme cases. I didn't think I was breaking new ground with the windstar fan thing though... If I could find a 0.25 ohm big ceramic resistor, I could stop the surge and lower the steady state current, and probably have plenty of cooling capability anyway.

The ECM has a single fan control output line that goes on based on temp, or when the A/C comes on. The ECM also has a high idle input that I currently have hooked to a switch. It's set to 1200 RPM and I plan to use it for jump starting other vehicles, crawling, or maybe when I have a winch some day. I've be considering feeding the fan control line back in there so I'd have a higher idle when the fans are on. I'd lose my switch capability though, since I'd pick maybe 900 RPM max.

EDIT: http://www.newark.com/ohmite/tap800kr33e/resistor-power-0-33-ohm-800w-10/dp/05T6782 :doah:
 
If you were going to use an idle kicker, the ECM would have to bump idle up before the load hit, not simultaneously.

I should turn my "kicker" off and see what happens.

Can you go to a different ECM that has this all available? I know by 1990 dual fan control was in the TPI ECM, not sure what, if any, TBI ECM's had that available.
 
dyeager535 said:
If you were going to use an idle kicker, the ECM would have to bump idle up before the load hit, not simultaneously.

I should turn my "kicker" off and see what happens.
Click to expand...

I don't think the surge problem is a charging system problem exactly. It happens even when cruising down the highway. Well, it's a charging system problem in that the regulator needs time, but if two 850cca batteries can't handle the surge... Anyway, I was going after the issue of low charging voltage at idle. The alternator is doing great at 850 RPM even with all the loads on.

Can you go to a different ECM that has this all available? I know by 1990 dual fan control was in the TPI ECM, not sure what, if any, TBI ECM's had that available.
Click to expand...

Probably, but I'm not in the mode to spend a ton of money and completely rewire my sensors. The whole electric fan things was just a project. As usual one thing leads to another. I could just buy a fan controller, but I spent so much time getting all this relay wiring nice and clean. :doah: Bringing the fans on sequentially is probably my easiest solution. I can get a high temp switch, like 225, and wire the smaller fan to that. It would be the emergency backup, probably seldom needed.
 
I have a degree in electrical engineering
Click to expand...

Cool, I don't have to explain the explanation....

The problem is with your 8 gauge wire. I don't know the locked rotor amps for one of those type motors.
Normal AC motors can run 6 or 7 times full load amps.

You probably don't need the poor diagram I posted, but it helps me explain. The red "wires" have no resistance. R1 is the resistance of the 8 gauge wire.
R2 is the ECM, R3 is the fan motors.

When the fans are running at speed, the voltage drop across R1 is probably less than a volt.
Since R2 and R3 are in parallel, they both see the same voltage. Most of the full battery voltage is being dropped across the R2/R3 resistance.

When the fans kick in from a standing start, the resistance of R3 suddenly drops to much less than R1.
The 10 gauge wire going to the fans limits this resistance, but there are two wires in parallel, so the combined resistance is less than R1, and the fans' internal resistance is really low.

This causes R1 to suddenly be the primary voltage drop.
Plus, remember Eli the Ice man. That piece of 8 gauge also has inductance which delays the current delivery. And those fans are trying to suck current.

If you had a recording fast scope, you could record the voltage drop across the 8 gauge when the fans kick in.

I think you would be surprised how much is dropped for a fraction of a second.

I suspect that increasing the size of the supply wire might cure your problem.

If not, the circuit on the right side of the picture should. The resistor should be about 50 ohms, which will need about a 5 watt resistor. The diode should be a fairly heavy one, 30 amps or so just in case.

Hook the whole works up to the ECM circuit.

Or you could contact this outfit and maybe get one of their units.

http://www.dccontrol.com/selector.htm

Fan Problem.JPG
 
That all makes sense. 8 gauge seemed overkill at the time, given the lug size on the fuse block and the short run. But now that I think about it, if the fans take 80 amps for a fraction of a second, it only takes about 0.01 ohms to drop a volt. The current could be temporarily higher, and who knows what the resistance really is. Resistance at the connections may be more of an issue than the wire itself.

I can't lose the shared wire without abandoning my fuse block altogether. At least for the ECM. I thought I was doing the right thing by getting rid of a bunch of those little in-line fuse holders dangling around underhood.

What size cap do you suggest for the hold up circuit?
 
It's not too hard to filter the ECM supply. Just use a simple LC or buy a pre-made "noise filter".
 
I actually have an old Radio Shack noise filter in my electrical bin (270-051B). Maybe I'll wire it in. I'm a little uncertain how to wire up the thing since it has ground connections on both sides. The instructions are long gone. Unlike audio, I'll never really know if it's doing anything.

EDIT: This guy: http://www.aeroelectric.com/articles/filter/RS_Noise_Filters.pdf, has the circuit diagram. Looks like I could just clip one of the ground leads and ignore it.
 
Well, I made a slight error on my diagram.
I blame turkey season.
Today wound it up, so maybe I will get some intelligence back soon.

You need another diode in the power line going to the ECM, between the capacitor and the fans.
It will only see the ECM current and the charging current of the capacitor, so it does not need to be all that big.
With it in there, the capacitor does not have to be as big, the other diode can be smaller also.
I was thinking that the capacitor was going to have to help supply the fans as well as the ECM, which was stupid.
If the fans see low voltage for a fraction of a second, so what?

In fact, just putting a diode in the line to the ECM might be enough. I would expect it to have filter caps inside to handle some voltage sags or noise. If so, then they might keep it from seeing the voltage drop if they did not have to try to supply the fans.

Of course, if I were designing the unit, I would have a diode in the input to prevent a problem if the battery got hooked up wrong, or someone jumped it off wrong.
But you never know.

Either way, you do need to isolate the capacitor from the fans.
 
Using a choke on the ECM supply has the same effect of isolating the ECM during a short transient, which is what we're concerned with. It also allows the filter cap to be smaller. The advantage is that it doesn't give a steady state voltage drop like a diode does. This helps the datalog voltages be more accurate and keeps the ECM alive to a lower battery voltage, for cranking with a low battery or cold temperatures.
 
I considered a choke, but I was not sure if the "frequency" of the voltage drop was fast enough for the inductance to help.
Wouldn't hurt though.

I considered the .7V drop of a diode, both from the minimum voltage needed to run the ECM with a weak battery, and from the point of some inputs to the module being .7V higher than the powersupply voltage.
But, I figured that it would not be unusual for those conditions to occur in normal usage, and the module was probably designed to handle it.

Don't forget, even with a good battery, its not unusual for the voltage to drop to 9 volts during cranking.
 
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