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Old 12-26-2011, 03:39 PM
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Question on where to pick up power for electric fan & headlights.

I was recently reading a car wiring book from the book store, appears to have been recently written.

It said not to wire the coolant fan from the alternator. In other words, not to have the power from the alternator that would have the fan and charging the battery in parallel. He said that the alternator regulator would sense the fan load, and would increase the voltage to up the current, and this was detrimental.

I dont see that this would be any different than wiring the coolant fan to the battery, as all 3 items, the alternator output, the battery, and the coolant fan would all still be in parallel.

The reason I ask, the battery is in the trunk. Rather than running a long length of large wire to run the fan, I can use one smaller size and run a shorter length from the alternator output. Isn't this electrically the same thing?

Please don't confuse this issue with fusing or relay switching, as those areas are covered, and that is not what this question is about.

I am thinking of adding the switched power of the headlight relay to be picked up from this same power lead for the coolant fan, ie. a power lead from the alternator to a second fused relay panel at the front of the vehicle, for the coolant fan and headlights. This would take a large load off the initial fuse panel.

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Old 12-26-2011, 04:02 PM
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What kind of electrical setup is this based on, Ford, GM, Chryco?

If using a Ford setup: You could pull power off the starter solenoid on the battery lead in wire (not the one going to the starter). You have to pull a large gauge wire from the trunk to the starter anyways and the ALT wire goes there also.

Pickup a copy of Tex Smith's book on wiring, it will give you all three systems.
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Old 12-26-2011, 04:20 PM
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Thanks for responding.

I'm not sure how the GM and Ford are greatly different, other than the GM solenoid is on the starter, and the Ford was remote until recently. I know nothing of the Chrysler.

As described in the initial post this would have the alternator charge lead going to the battery feed to the solenoid. Whether the solenoid is remote like the Ford, or on the starter like GM. The question is why can't the power for the coolant fan, and the headlights, be run directly from the alternator itself?
All three items (battery, alternator power out, and power to front of vehicle) are in parallel no matter in what order they are attached.

I don't understand the author's reasoning, and I not only want to power the coolant fan from the alternator, I want to add the headlights to it.
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Old 12-26-2011, 08:14 PM
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The issue is wire size and overcurrent protection.
The alternator, while charging can produce, for example, 50 amps if the battery has been partially discharged. And the wire connecting the alt. to the battery is #10. 50 amps is a full load for #10 wire. This wire should be protected by a fuse, a fusible link or simply the capacity of the alt.
If you add another load, such as cooling fan, to this wire it is overloaded and begins to get hot. When it gets hot the resistance increases and voltage drops. The alt. will then attempt to increase it's output because it sees a reduced voltage. Everything is now well beyond it's ampere rating, such as connection terminals.
So the basic protocol for wiring a vehicle is: Run a properly sized AND fused wire to each load.
The fuseblock should be located as near as possible to large ampere loads to keep larger wires as short as possible.

vicrod

Last edited by vicrod; 12-26-2011 at 08:18 PM. Reason: more info
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Old 12-27-2011, 08:24 AM
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vicrod,thank you for your response. You are correct in what you are saying. However what you described is not what is proposed.

There would be two wires, one running from the alternator to the starter solenoid to act as the battery charge (and feed for the primary fuse block), and a second wire from the alternator to the fuse/relay block at the front of the car for the coolant fan/headlights. There would be two wires, each #8. Overcurrent protection could be fuses, or fusible link.
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Old 12-27-2011, 12:17 PM
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What kind of alternator are you using? How is it wired? Is it a GM "single wire"?

The most important function of the alternator is to keep the battery charged. If the battery is dead, you cannot start the car, so cooling the engine becomes unnecessary, and if you are not driving the car, you do not need headlights.

OK, you knew that already.

With a remote battery, the issue become getting the right voltage and current to the battery, in the trunk. Every wire has resistance, even 4-0 gauge cable. The alternator has to put out more voltage than the "ideal" voltage needed at the battery to charge the battery. Depending on the wire (or cable size) the alternator may need to have around 15 or 16 volts AT THE B+ TERMINAL ON THE ALTERNATOR to get 14 volts at the battery. This is called voltage drop. Consider the ground side of the alternator wiring also. Especially with the trunk mounted battery.
One thing people do not realize the battery also functions to regulate the voltage, for a while. When the battery is electrically close to the alternator, the battery is able to better control, or damp any voltage spikes made by the alternator. With a remote battery the voltage at the alternator will vary more. it you start the car, and drive it at night, I could see a situation where the alternator is trying to charge the battery, and puts out 15+ volts to bring the battery up to charge, and you are also running the excess voltage straight to the headlights. They will be really bright. For a short period of time.

If you are using a "one wire" alternator, the alternator does not really know what the battery voltage. The internal regulator only knows the voltage at the B+ terminal of the alternator. Your battery will be most likely be slightly undercharged.

When you are done with wiring your car, you will need to check the voltage drops in the charging circuit, and probably the starting circuit. It would be a good idea to check what the voltage is at the headlights, also
Here is a web page that explains checking voltage drops much better than I could, and it is already done.
http://www.vernco.com/Sparks/id606.htm
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Old 12-27-2011, 01:30 PM
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Not sure where everyone is going with this.

The alternator supplies all the voltage when the vehicle is running, as well as sufficient to charge the battery.

If you have to output 16 volts to get 14 to the battery, something is wrong. Not sure why you are bringing that up.

It is a 3 wire alternator, so I can put the sense point anywhere.

I understand voltage drops. That's why it makes more sense to run the coolant fan/headlights from the alternator, rather than from the battery at the rear. When the car is running, the alternator is the voltage source, not the battery. The voltage drop to the battery from the alternator will surely be less than 1/2 volt.


Please explain why it is better to run the front fuse block from the battery, rather than from the alternator, when all 3 items are in parallel. How would it be different?
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Old 12-27-2011, 02:59 PM
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"I understand voltage drops."

"If you have to output 16 volts to get 14 to the battery, something is wrong. Not sure why you are bringing that up."

I am going to politely suggest that you do not understand voltage drop, if you cannot understand why you would need an alternator to put out more voltage than you get at the battery with current flowing.
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Old 12-27-2011, 03:58 PM
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Quote:
Originally Posted by DanielC

I am going to politely suggest that you do not understand voltage drop, if you cannot understand why you would need an alternator to put out more voltage than you get at the battery with current flowing.

Pretty sure that if it takes 16 volts at the alternator to get to the battery with 14 volts something is indeed wrong. I'm willing to stand by that. Something is wrong for a 2 volt drop, should be always be definitely less than 5% (or even better 3%) of applied voltage.

Also stand by this: The voltage drop to the battery from the alternator will surely be less than 1/2 volt. (less than 3%)


Pretty sure that they are both correct, and follow with current flow, with properly sized cable and adequate components. The alternator would indeed be producing a higher voltage than the battery.

Sorry if I didn't explain this in a better fashion in my earlier post.
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Old 12-27-2011, 05:07 PM
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Quote:
Originally Posted by grobb284
Pretty sure that if it takes 16 volts at the alternator to get to the battery with 14 volts something is indeed wrong. I'm willing to stand by that. Something is wrong for a 2 volt drop, should be always be definitely less than 5% (or even better 3%) of applied voltage.

Also stand by this: The voltage drop to the battery from the alternator will surely be less than 1/2 volt. (less than 3%)



Pretty sure that they are both correct, and follow with current flow, with properly sized cable and adequate components. The alternator would indeed be producing a higher voltage than the battery.

Sorry if I didn't explain this in a better fashion in my earlier post.
Voltage drop will be minimal with properly sized cables. It certainly should not exceed .2-.4 volts. To answer the original question the connections should be as close to the battery as possible and you should never connect anything directly to the alternator. In a trunk mounted battery a good distribution block is essential.
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Old 12-27-2011, 08:55 PM
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coolant fan

why dont u just run a wire from the wiper ignition hot lead to a relay then to ur fans ur making this more complicated than it is.u can also use that for ur headlights.then u can run a switch in ur car also to run the fan,thats what i did.
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Old 08-21-2012, 08:53 PM
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Quote:
Originally Posted by T-bucket23 View Post
and you should never connect anything directly to the alternator.
Not to dig up an old thread but,

The new wire kit I just installed came with wiring to run a 10g jumper with fusible link from the starter solenoid post to the battery lug on the alternator. From there another 10g lead goes from the alternatorlug to supply power to my main fuse panel in the cab of the vehicle. Are you saying running the power wire from the alt to my fuse panel is wrong?

By the way it is a rebelwire 9+3 kit.
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Old 08-22-2012, 04:14 PM
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Well to follow up on this I contacted Bob at Rebelwire and learned that my harness was mislabeled. Apperantly someone installed an "Alternator +" lead where they should have put a "Solenoid +" lead. What this means is I have to spend time re routing my harness this weekend. It's a pain because I maticulously group, wrap and shrinktube all my wires at exact lengths so I will need to cut the wire out to re route. To make things more difficult I use high end adhesive lined shrink tubing on many of the wre groups. Not easily removed.

This is kind of a PITA. I did much research before buying my wiring kit and I had heard so many good things and not a single knock aganst rebelwire. Well, here's the first knock against them.
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Old 08-22-2012, 06:40 PM
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Not trying to cause a row, but I really don't see the common lines being any more that a buss, that is why I originally posted this topic.

How does the item know where it is at on the buss, it doesn't usually except for negligible resistance of the buss? If you draw a line (buss) and tap off for the battery, the starter solenoid, the alternator, and the take off for the fuse block, and then subsequently drew another line with a mix up of the location of the take offs, how would the buss be any different electrically?
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Old 08-22-2012, 08:50 PM
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Put in a distribution block just like this...
Catalog
and you will be golden... Very simple. Pull your power from the alternator just as you were planning.
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