Switched power source for fuel pump install

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cudajames

cudajames

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I am not much of an electrical wizard and need some help. Car is a 66 Barracuda with stock electrical with just a higher amp alternator

I am in the process of installing an electric fuel pump and need to identify and tap into a switched power source. What should I be looking for and anything to look at for?

here is the instructions to the relay kit
upload_2021-3-8_12-53-21.png
 
There is only one ignition "run" source and it comes from the key. It appears in the engine bay comes out of the bulkhead connector and splits off to run the ballast resistor, alternator field, voltage regulator, electric choke if used, and smog doo-dads on some cars

IT IS NOT FUSED and should NOT be used other than to trigger a relay. So fuse the relay source, and consider getting yourself a "rollover switch." There are several available came off factory cars. You can also tie it into an oil pressure switch if you get "tricky."

I'm not trying to make fun, not at all. but some guys seem to have a confused idea about magic sources in the wiring
This is all that comes from the key/ ignition switch:

ACCESSORY: This is hot in either "run" or "accessory"
IGNITION RUN This is hot ONLY in "run" and goes dead in "start" and of course "accessory"
START: This is normally yellow, goes out through the bulkhead and fires the start relay
BYPASS: This is also called "IGN2" and is hot only in start. Normally brown, it feeds power to the coil during cranking

NONE OF THESE is fuse protected
 
Absolutely, and you DON'T want to pull it from the resistor.
To your point, the power wire has a circuit breaker, he would just need to install an inline fuse from the ignition source.
Now as far as making fun, not sure what you are referring to?
 
I run a Pertronix igniter and low ohm coil so I removed the resistor.

I powered the fuel pump relay from that resistor circuit. Power from the battery to the relay is fused.

So far I haven't added a fuel pressure switch. Maybe some day!
 
Another little tidbit about a pump relay. You can add an "unobtrusive" switch in the relay coil circuit and create a simple anti-theft device. Even if there is fuel left in the float boat, it won't run long
 
Another tip. With the factory wiring it is better to draw the power for electric pumps direct from the alternator.
upload_2021-3-8_19-36-57.png


That's a much more direct path when the engine is running.
upload_2021-3-8_19-55-22.png
 
@Mattax that is very interesting, if I understand what you are saying, instead of tapping into the switched power, I connect to the alternator instead? I prefer, that route. And it seems that would still be a constant source of "keyed" power. I see other benefits as well, with out tapping into old wiring
 
cudajames said:
@Mattax that is very interesting, if I understand what you are saying, instead of tapping into the switched power, I connect to the alternator instead? I prefer, that route. And it seems that would still be a constant source of "keyed" power. I see other benefits as well, with out tapping into old wiring
Click to expand...
You are confused.

The alternator output is not switched. It is constant. That feeds to the relay contacts

You still need a switched source to key the relay. Re-read what I posted. There is only once source from the key. It goes several places. Any one of those sources can key the relay as they are just branches of the original---from the key
 
I'm saying that the power for running the fuel pump should flow from the power source, which is the alternator.
The relay still needs switched power. The relay is an electrically powered switch. A small amount of current is used to turn on the switch. When the switch is on, it completes a high current circuit between the power source and the whatever you are powering.

Ideally the relay for a fuel pump would switch on with the key in start and run and have a roll-over switch which will open in a serious accident.

The power requirements to close the relay is small. Thats why I did not show the current flow in the second diagram.
Randomly an automotive relay off the internet, this data sheet shows it will close when supplied more than 7 Volts. Thats why you can trigger it on either side of the ballast resistor.
It uses around 1.4 Watts. So will draw around 0.2 amps.
The fuel pump will draw what? 5 amps? 10 amps? 15 amps? depending on the pump.
 
Mattax said:
I'm saying that the power for running the fuel pump should flow from the power source, which is the alternator.
The relay still needs switched power. The relay is an electrically powered switch. A small amount of current is used to turn on the switch. When the switch is on, it completes a high current circuit between the power source and the whatever you are powering.

Ideally the relay for a fuel pump would switch on with the key in start and run and have a roll-over switch which will open in a serious accident.

The power requirements to close the relay is small. Thats why I did not show the current flow in the second diagram.
Randomly an automotive relay off the internet, this data sheet shows it will close when supplied more than 7 Volts. Thats why you can trigger it on either side of the ballast resistor.
It uses around 1.4 Watts. So will draw around 0.2 amps.
The fuel pump will draw what? 5 amps? 10 amps? 15 amps? depending on the pump.
Click to expand...

Not agreeing or disagreeing with you, other than to say this would be the same as connecting directly to the battery, which should have a line run directly to it (with fusible link, natch) from the alternator output stud, if you are inclined to skip past all the bulkhead connector problems.
 
Just picking a Walbro pump because that's what Cudafever is using on his racecar.
We looked up the power needs Holley/Demon Carb stumble
Assuming this is for a carburator, its going to take minimal pressure and power. So lets say 17 amps. That's 17 amps flowing all the time the car is running.
Headlights themselves draw about 9 amps.
Tail and instrument lights draw about 3 amps
Ignition and field if original 3 to 5 amps.

Lets say only the fuel pump is on a relay. The current flow with lights on would be like this.
upload_2021-3-8_22-44-15.png


Compared with the current flow in the same situation with the power connected at the battery.
upload_2021-3-8_22-57-10.png


So even with the battery fully charged, there will be 32 amps going through the wire connection at the bulkhead.
Then 15 amps will branch off and flow through the two connections at the ammeter, the connections for the fusible link, the 16 gage fusible link, the connection at the starter relay and the one at the battery before getting to the relay.

But of course after starting, the battery is going to need recharging. So adding that, the total current through is exceed the capacity of the factory circuits.
Wiring will degrade, resistance will increase, eventually something will fail.
upload_2021-3-8_23-20-34.png

Yes in theory one could wait until the battery was fully recharged before turning the lights on, reducing the load some.
Also a pump that used less power would be better. I just picked one.
Regardless, an electric pump is a big load for the factory system.
This is primary reason why I suggested that when keeping the factory wiring, it is better to connect to the alternator.
Better assuming the alternator can supply the power needed at idle rpms.

If this is all too much of a project, then give the need/desire for an electric fuel pump some serious reconsideration.
 
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