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Any way to bump up my RPMs when fan comes on?

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TR6BILL

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When my Spal 16" fan automatically kicks in when I am in traffic, the big electric motor sucks down something in my electrical system that causes my RPMs to drop. I have to feather the gas pedal to keep the RPMs up enough to not hesitate or stall when stopping for a light or stop-n-go traffic. Now, I have a GM-style alternator that I had tested at 73 amps! What gives? I can physically feel and see the RPMs drop from 1200 idle to 900 idle, which my car won't run at. Any suggestions or explanations of what is going on?
 
Alternator draw is the first WAG. Look into "A/C idle compensator solenoids". Mebbe you can MacGyver one to the throttle linkage.
 
That's odd. I know and have seen what you are experiencing, but my 57 amp Bosch keeps up with the load with a drop of maybe 50 rpms top. I hardly even know the fan is on. Are you sure the alternator is putting out the proper amount?
 
Mine does the same thing.

The fan sucks a lot of juice. That forces the alternator to work harder. That forces your engine to work harder to turn the alternator. Without increasing the gas intake, the RPMs will lower. You can't get more out of the engine without putting more in. -The motor is doing more work and needs more gas.

The effiencency doesn't get better when the fan runs, and the alternator isn't going to work for free.

You'll have to step up the throttle a bit or turn the idle up a little higher than you'd prefer when the fan is not running.
 
DrEntropy said:
Alternator draw is the first WAG. Look into "A/C idle compensator solenoids". Mebbe you can MacGyver one to the throttle linkage.
Click to expand...

I think Doc has the right idea.

Also, if you haven't already you might try running your fan power direct from the battery connection. If you are connected closer to the alternator it will, at least in theory, draw more from the alternator than from the battery. Path of least resistance thing.
 
Just pretend you are 16 years old again, and rev that puppy at every stop. When I was 16, the girls loved it, especially if the car rocked back and forth. Oh, wait, that was 48 years ago, now they just flip me off. Never mind.

The A/C solenoid is the way to go, or go to a milder cam and dump that aluminum fly wheel.
 
Just how many amps is that fan pulling? My 16" permacool fan pulls 10 amps. moves over 2200cfm of air. My rpms barely move when the fan comes on, maybe 50 rpm like Paul, and I'm still using the stock alternator a 17ACR. Mine is wired like Tom suggested, draws power from the battery to/through the relay.

Just looked up the specs on my fan. It turns at 1800rpm, pulls 10.5 amps and moves 2950cfm of air.
 
I agree, most likely just the physical load from the alternator. But, wouldn't hurt to check how much your ignition circuit voltage drops; or an easier check might be to temporarily bypass the ballast resistor and see if that changes things.

Assuming it is alternator load, the questions become why is your idle so unstable (fast timing, lean mixture, vacuum leak, etc.); and why won't your engine pull from 900 rpm (800-850 is stock idle according to my book) ?

Note that your GM alternator is actually aggravating the problem, as it will put out enough at idle to keep up with the fan. The stock Lucas was so wimpy at idle that it didn't load the motor much (and let the battery take up the slack).

ISTR you mentioned having brake booster problems recently ... if the booster is leaking vacuum, it can cause idle problems like this.
 
assuming everything is ok, you could rig in a cable throttle controller that you just pull out to raise the rpm to a set point when this happens. I have one on my landcruiser that i use 4x4ing when my feet can't stay on the pedals from bouncing around.Just set it up so that when you pull it out(thottle controller) it gives you a precise rpm. At least you don't need to "work" at the rpm.
Rob
 
TR3driver said:
Assuming it is alternator load, the questions become why is your idle so unstable (fast timing, lean mixture, vacuum leak, etc.); and why won't your engine pull from 900 rpm (800-850 is stock idle according to my book) ?
Click to expand...

His engine is far from stock. I don't remember which cam he has but my GP3 won't idle smooth below 1100rpm.
 
Some of the problem may be the amps my fan is pulling. Like over 21 amps! No wonder I might be having a brown-out. Or making the alternator work harder. Now, my fan harness pulls directly off my Optima battery, not through the harness. And I just had the alternator checked, it is putting out plenty amps. I just may have to rig some kind of gizmo to bump the RPMs. A mechanical pull won't be convenient because the fan comes on at will. I now just watch my temp when I start in traffic and go ahead and use the manual override switch to kick the fan on and use my vroom-vroom pedal. Actually, I am getting quite good at left-foot braking, though not fun. Gotta be a better way. And, because the engine is heavily modified, ain't no way it will idle below 1200 RPM.
 
TR6BILL said:
And, because the engine is heavily modified, ain't no way it will idle below 1200 RPM.
Click to expand...
That explains it, then. With the engine just barely running, it can't pick up the extra load of the alternator powering the fan. Low-tech solution might be to just set the idle to 1200 with the fan running; and live with slightly higher idle without it.

I really hate having the throttle jump around; owned a GM car once that would 'help' by cracking the throttle wider when the clutch was depressed. Irritated me so much that I eventually ripped it out. If it were me, I'd look at either introducing some extra idle air (you're probably rich at idle anyway), or advancing the timing slightly when the fan is on.
 
If you are rich at idle and IF the introduction of extra air past/after the carbs gives you an RPM increase there is a very easy solution. Attach a GM boost solenoid to any port on the manifold and leave the other side of the solenoid vented to atmosphere. Then wire the solenoid into the same circuit as your fan and whenever the fan turns on it will open the solenoid and introduce more air and increase your idle speed. You can play around with how much air gets past the solenoid by inserting various restrictions or use a simple fish tank air valve.

This is all predicated on an increase in idle with no added fuel of course. So just pull off a vacuum line and see what happens.

The solenoid can be found from lots of sources and is not too expensive.

https://www.bmotorsports.com/shop/product_info.php/products_id/1422

This way you are not messing around with the throttle linkage or stepper motors and can easily be hidden out of sight.

Good Luck
 
I'd not be too keen on leaning it out at idle just to increase the RPM... I'm still suggesting the idle compensator from about any old A/C compressor'd car. You can mount the thing "creatively", certes. You're a professional!

The mods on that engine are such that it has NO low-end torque to speak of, so idle is likely overly affected by any load...

<Yoda Voice Mode> "Use the <span style="font-style: italic">solenoid</span>, Bill." </Yoda Voice Mode> :devilgrin:
 
That is the what the spec sheet reads, unless what they are calling "imput" is not what you mean. Almost 22 amps. My wife uses it as a blow drier for her hair.
 
Bill............which Spal fan do you have? I've got their 16" low profile puller which draws 3.8-4.2 amps per their spec sheet. However, the 16" high performance puller draws between 21 & 23.5 amps. That's a huge difference but in poking through the Spal site, it appears to be correct. At least I now know why I see no RPM impact when the fan kicks in! :driving:
 
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