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Thread: Choke, Fuel Pump, and Coil...same relay? Reply to Thread
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  Topic Review (Newest First)
08-21-2015 06:35 AM
daginsu
Re: Choke, Fuel Pump, and Coil...same relay?

Ok, thanks again for all of the info. I was able to view the picture you sent so that will help. I'll also plan on running 12 gauge for the main feed to the components instead of 14 gauge just to be safe.

There is a lot of good information in this thread and hopefully it will find it's way to others who have similar questions
08-20-2015 12:44 PM
PSIG
Re: Choke, Fuel Pump, and Coil...same relay?

Be aware, I'm not picking on you for the large amount of info to follow. I have had several requests to outline how I determine the wiring in projects I do. So, I'm using your situation as an excuse to outline my process to others as I answer your situation, with how I would figure it. Remember, this is just how I do it on my stuff, and everyone must use their own judgment when it comes to electrical stuff.
Quote:
Originally Posted by daginsu View Post
I believe I understand what you are saying about the relay, but I think the image you linked is being blocked from work because I didn't see anything. Is the attached image wired the way you are recommending?
Almost. The trigger wire for the relay would normally be (+) on 86, and ground 85. Otherwise, yes.

Here is the photo I linked to at another hosting site, or the direct link is <http://i.imgur.com/opnNldx.jpg>. Let me know if you can see it here:


Quote:
Originally Posted by daginsu View Post
Also, from the charts I'm looking at since I'm only drawing a max of 15A I should be able to use 14 gauge wire instead of 12 gauge correct?
That depends on your wire length and whether it is intermittent (choke) or constant duty (fuel pump). Only the primary wire to and out of the relay needs to be heavy, and then the wires to each device can be suitably smaller. So, I figured 12A max, and you're saying 15A, so let's go with 15A.

While trying to avoid info overload; here are a couple helpful aids and guidelines I use for good, safe and durable wiring. Remember, when designing a circuit, you are looking to satisfy three primary requirements:
  • Safety
  • Performance
  • Reliability
Safety The gauge size is commonly selected to limit the wire heating, and while this is not the only concern, it certainly impacts safety. The photo below of FAA recommended practices is a very handy reference. If in-doubt, I follow what it says. Note that the top section is for temperature, and a 14 gauge wire will max-out a wire rated at 105°C at 12.7 amps. That's right - with less than 13 amps, that wire will be over 221°F. While that's not enough to melt the wire or start a fire, it is enough to burn you or affect sensitive things touching it. Hotter means the insulation may melt and allow a short and possible fire. Of course, you could use higher-rated wire at 150°C/302°F and it would hold it fine, but the wire will be that much hotter as the amps get higher.

That gauge choice is up to you, but the chart indicates to me that 12 gauge would likely be a better long-term choice for the primary connection when I think about what that wire may be running near, or in a bundle, or under my carpet. If it's running in free air in the engine compartment, and it's rated for compartment temperatures with the added load temperatures, then it may be fine from a temperature and safety standpoint. If I'm out of 12 gauge wire, then perhaps I'll pair two 16 gauge wires to handle the load instead. Again, a personal judgment call, based on the specific situation and requirements.



Performance Voltage drop due to wire resistance means whatever you attached will get less voltage than it might need for best performance. The length of your power path in both directions causes this resistance to increase. A calculator to help determine voltage drop can be found here: Voltage Drop Calculator

One rule-of-thumb I use is that intermittent-duty devices can often get-by with 5% voltage drop. Constant-duty devices usually perform best with less than 2% drop. A looser set of rules some folks use are 1-volt and 0.5 volt drops. I use the percentage rules so that the system is still within the looser rules as the wiring ages, to maintain reliability and performance. Your horn volume probably isn't a concern, as much as your headlights dimming by 22% with only a 1-volt drop from the rated 14 volts to 13 volts.

While most spec's call for a 15.5 volt max system voltage, typically the highest you will see in normal operation is about 14.7 volts. That's the number I use in the calculator. Running the calculator with the following numbers give these results for performance:



As you can see, the results are over my limits, and very close to exceeding the looser rules with 14 gauge at 14.7 volts and 15 amps. My choice would be the next larger gauge.

Reliability The same process is used to choose the wiring for the smaller runs to each device. All wiring should only be automotive stranded wire or better - never solid wire. For simple robustness in daily handling, vibration, etc., minimum wire gauge for automotive use is typically 18 gauge. For anything. Lighter gauges are avoided as too easily damaged or broken in normal use, just due to their small size and low relative strength.

Hope that helps!

David
08-20-2015 06:35 AM
daginsu
Re: Choke, Fuel Pump, and Coil...same relay?

David...thanks for all the information. I'm glad I was at least on the right track, most of my wiring experience comes from hooking up car stereos when the need arises

I believe I understand what you are saying about the relay, but I think the image you linked is being blocked from work because I didn't see anything. Is the attached image wired the way you are recommending?

Also, from the charts I'm looking at since I'm only drawing a max of 15A I should be able to use 14 gauge wire instead of 12 gauge correct?
08-19-2015 02:50 PM
PSIG
Re: Choke, Fuel Pump, and Coil...same relay?

Welcome to the Forums.
Quote:
Originally Posted by daginsu View Post
Am I missing something, or would this work?
Your concept is fine. Your choke amps are a bit off. For example, the typical Holley or Ford choke element is around 8-10 ohms. The formula for amps is Volts / Ohms = Amps. That means the max draw—even at over 14 volts—is less than 2 amps. The pump is about right at 2 amps, and the ignition module at 8A is fine also.

So, that means a combined max draw of less than 12 amps, when cold, and revving beyond redline. By the same token, your fuse sizing should be a bit lower for best protection, at perhaps 15A for the circuit. That's enough to handle it all, and yet it should pop quickly if any one of them or their wire runs short-out.
Quote:
Originally Posted by daginsu View Post
I thought I read some where that the coil should be on a separate feed, but I don't understand why.
Most 12V factory points and early electronic distributors (of any brand) had coil power feeds with a resistance wire or ballast to reduce amperage to the coil. Ford used the infamous fat pink wire under the dash. This prevented coil overheating and damage or burnout if the key was left on the ON position, but with the engine not running, such as after a stall.

This meant the power to the coil was already limited and adding something else to the resisted circuit would reduce spark energy even further. Obviously not good for performance or anything else. Now you know. Today's electronic ignitions and EFI systems do not have the coil overheating issue, so no resistors are used, and therefore no problem with sharing the power source - if the circuit is designed properly. Yours if fine.

On a separate note, be aware that "Bosch" style relays normally have (+) power in on even-numbered terminals, and out on odd ones. So, BAT+ in on 30 and key power in on 86, with relay coil ground on 85 and power to the devices on 87 and/or 87a. See image here. While this is not a requirement for most standard relays that use internal resistors for voltage spike suppression; some have an internal shunt diode instead, and wiring 86 and 85 the other way will fry that diode. Likewise, power in on 30 is most common as it leaves normally open (87) and normally-closed (87a) contacts available, and some relays (especially solenoid and fan/motor versions) have diodes as well. Unless necessary for a specific purpose, I'd suggest wiring in this fashion just to play it safe, so any relay of that type can be used.

David
08-19-2015 11:58 AM
daginsu
Choke, Fuel Pump, and Coil...same relay?

So I'm new to these forums, but neither my brother or me are wiring experts and need a little help finishing up the 302 swap into my 70 Maverick. Right now we have the coil, choke, and fuel pump all wired up to a temporary toggle switch just so we could hear the car run and make carb and timing adjustments as needed.

I've been researching relays and it seems a lot of people use them for fuel pumps and for electric chokes. So that got me thinking...why couldn't I use one relay to control/power all 3? I even came up with a (handrawn) diagram that seems to make sense as far as amps goes. See the attached pic.

Am I missing something, or would this work? If not, could I make some minor tweaks and get it to work? I thought I read some where that the coil should be on a separate feed, but I don't understand why.

In case it matters...I'm running an MSD ready to run distributor, a typical cannister style coil (unsure of brand), Holley 600 4bbl with electric choke, and a Holley Mighty Mite fuel pump.

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