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post #2 of (permalink) Old 08-19-2015, 02:50 PM
PSIG
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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

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