Electronics Q's

[Fire67]

Im about to wire up my new fuel pumps... Yeah pumps, Im running two Walbros.
I want to basically hotwire these pumps so the get maximum possible voltage.
My plan is to run a fairly heavy gauged wire from the alternator, through an inline fuse, and then to the relay in the back. I will trigger the relay with the stock on/off wire. I will split the power feed after the relay to the two pumps.
The Walbro 255 pump says right on it that it needs a 20amp fuse.
Now for my questions:
Since im running two pumps that each need 20A fuses, what amperage single fuse should I use?
Would a 10 gauge power wire to the relay be enough?
What gauge wire should I use from the relay to each pump?
What amperage should the relay be rated for?

If you guys have anything you'd like to point out, or suggest please do. I havent bought any wiring stuff yet, so If I need to change the whole plan Its not a problem.
Thanks in advance

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[Fire67]

We need some more electronics guys in here!

['68SuperchargedLT1Camaro]

Which relay will you be using? Is it the factory relay for the single factory fuel pump? If so, I would recommend switching to a bigger relay. If Walbro recommends a 20 amp fuse, and you will be running two of these pumps, you'll need to find a 12 volt automotive style relay with a current rating of 40 amps or more.

There are a couple different ways you can run the wiring. I recommend running two separate positive wires, and two separate negative wires. All the wires (positive and negative) need to be 14 gauge. The wiring to the switch can be much smaller because your using a relay. You'll need to run your positive wiring straight from the positive battery terminal. Don't run it off the alternator. You'll need to have two separate 20 amp fuses for the fuel pumps. One for each positive wire. You want these fuses as close to the positive battery terminal as possible. Run the two negative wires to a GOOD, I repeat GOOD grounding point. You can ground these two negative wires to the same point. As you know, the entire chassis of the vehicle is negatively grounded. Factory grounding points are typically good grounds. Just make sure that ALL paint, rust, dirt, etc, is removed for the metal that these wires are grounded to.

Do you have a pic of the pumps that you will be using? Id like to see the electrical contacts.

[fast93z]

A standard relay is 30 amps so two pumps drawn off of one relay is not a good idea in my opinion. I suggest running two relays and two hot wires like suggested. This will keep the pumps cooler and kinda act as a safety barrier if one relay goes......you still have another pump working.

['68SuperchargedLT1Camaro]

Fast93z is correct. You should run two separate relays. Let me explain why.

I have a relay in front of me that I'm looking at. It has two sets of poles, for two different circuits. It's basically two relays in one, that work off of one switch. I'm not thinking, and I'm assuming you'll be using this style relay............ but you won't, so now I'll recommend two individual 20 amp relays.

A large 40 amp relay with a single set of poles would work just fine, but wouldn't be ideal because a short would have to draw 40 amps of current through a single 14 gauge wire to blow the two 20 amp fuses, or if wired correctly with a single relay, a single 40 amp fuse.

['68SuperchargedLT1Camaro]

I'm going to draw up a schematic, and see if I can't come up with a good circuit with some additional safety circuits that will let you know when the individual fuel pumps are on or off. I'll get back with you.

[Fire67]

admin said:

I'm going to draw up a schematic, and see if I can't come up with a good circuit with some additional safety circuits that will let you know when the individual fuel pumps are on or off. I'll get back with you.

Cool, definitely looking forward to that schematic.
So basically it will be SAFER to run two individual circuits for each individual pump. The pumps themselves just have threaded studs and nuts to hold a ring terminal.

Im gathering that I need to run it like this:
Two 14gauge positive leads off the battery terminal, both fused inline - 20amps each.
Two relays using the factory "trigger" wiring to switch the relays.
Two individual 14gauge grounds
I will splice into the factory wiring to trigger the relays, and cap off the factory voltage supply and ground wiring.
Thanks for the replys guys!

[Fire67]

Ray, for safety circuits, are you meaning warning lights?
That would definitely be interesting...

Whats the reason for not installing the voltage supply wires on the altenator?
To me it wouldnt be much different from mounting to the factory remote mounted " + post".

['68SuperchargedLT1Camaro]

Sorry I haven't gotten back with you sooner.

Yes, warning lights. It's very simple actually. You'll just need to wire in a 12 volt light (or an LED with a resistor) in parallel with each pump. Connect this light to the contacts on the pump it self. If a fuse blows, or a relay doesn't engage, or if the circuit opens anywhere, the pump won't turn on. This warning light won't turn on either, letting you know that the pump isn't getting power.

You'll wind up with two little lights (or LED's) somewhere inside your car that will both light up whenever your fuel pumps are on. If one light doesn't light up, one of your pumps isn't on. If neither of the two light up, both pumps are off.

Keep in mind, this only let's you know if the circuit has opened. THIS DOESN'T LET YOU KNOW IF A PUMP HAS GONE OUT. I'll try to come up with a circuit that will let you know if a pump has gone bad.

[Fire67]

That would be simple enough... But you've gotten me even more interested. How could a warning light be hooked up to tell you if a pump went out? A pressure switch at the outlet or something?

['68SuperchargedLT1Camaro]

I don't have a plan as of yet, but I know it can be done. I'll need to think about it some more. If I can't come up with a good way of doing it by tommorrow, I'll call my buddy Darwin (my high school electronics teacher) and we'll brainstorm.

I did forget about your alternator question. Running the pumps directly off the alternator causes a few problems.

All of the current required to run the fuel pumps will have to be drawn across the single wire that runs from the alternator to the positive terminal on the battery. I don't know what gauge that wire is (I don't have a 4th gen F-body, so I can't check), but regardless, the engineers at GM intended for that wire to charge the battery, and possibly any other circuit thats powered from that point that I'm not aware of. I assure you, they didn't intend for it to power your Walbro fuel pumps. Somewhere in the neighborhood of 20 to 30 extra amps of current draw wasn't factored in when designing that portion of the circuit.

Your fuses won't be in the propper location of the circuit either. The main purpose of a fuse is to keep wire, or anything in close proximity of the wire from catching on fire. For fuses to be effective in an automotive application with a negatively grounded chassis, they must be as close to the positive battery terminal as possible. This will ensure that you leave an extremely short run of wire that isn't fused. If any part of the wire after the fuse comes into contact with ground, it will create a short, and blow the fuse. If you wire in the fuses far away from where the circuit begins (actually where it ends, electron flow moves from negative to positive) then you increase the possibility of a fire in the event of a short. If a portion of wire before the fuse contacts ground, that section of wire between the positive battery terminal, and the short will have to get hot enough to melt through itself to open the circuit. It takes alot more heat to melt the wire, than it does to melt a fuse. At the same time, a fuse that melts is contained inside a glass cylinder, and not a piece of rubber or plastic that could potentially catch fire, or catch something else on fire.

There are a few other reasons, but at this point people won't stop calling me, and I can't remember what I'm supposed to be thinking about.

[Alex94TAGT]

Not sure if this will be of any use, but this is essentially how my pump is now wired (Racetronix Harness), using the factory fuel pump wiring as the relay trigger. In this case, if you wanted to run dual relays, you'd just add a secondary relay to the first relay's ground wire (if you follow me -- wiring the relay's trigger circuit in-series). hometown.aol.com/alex94tagt/wpwiring.jpg


I wired in two warning lights for the electric waterpump -- I don't see why you couldn't do the same with the fuel pump. I used one large LED for the positive wire, and one smaller LED for the ground wire. Just pretend the pump in this diagram is a fuel pump: www.akmcables.com/2ledmez.jpg

-Ignore the 86 relay terminal stuff, since you'd be using a different switched/trigger power source (using factory fp wiring to maintain factory on/off settings), and you probably won't be needing a toggle switch.

['68SuperchargedLT1Camaro]

You may have made my life a little easier. Thanks Alex!

Let me get back to looking at that schematic.

[Alex94TAGT]

Eh, I had it handy. Justin might not want to use it...

['68SuperchargedLT1Camaro]

That should work, but I think it would be better to run the "bad ground" diode and 1K resistor across the fuel pump so that it will give you an indication of whether or not the circuit is open anywhere, and not just at the ground connection.

I'm going to dig around right now and see if I have all those LED's and diodes. I'd like to mock up that circuit, and see how it functions when I disrupt certain areas of the circuit.

[Fire67]

Alex, that second diagram is exactly how I wired my Meziere, minus the warning lights I used to have both those diagrams somewhere, just lost and forgotten by now.
Ray, I think Im a bit confused... At first it you say not to wire directly to the altenator. Then in your next reply that first paragraph is scaring me away from wiring to the + terminal on the pass. fenderwell.
Or Im taking this all wrong and you mean that the altenator wont be able to supply the pumps by itself, and therefore some of the amperage would be drawn from the battery through the charge wire that links them together.

[fast93z]

Looks like you guys have it figured out. I like the LED warning light idea!! I think I will do that with some things on my car.

['68SuperchargedLT1Camaro]

fire67 said:

Ray, I think Im a bit confused... At first you say not to wire directly to the altenator.

Correct.

Then in your next reply that first paragraph is scaring me away from wiring to the + terminal on the pass. fenderwell.

If this terminal on the passanger side fenderwell is there for the purpose of jump starting the vehicle, then an engineer or two intended on a lot of current to pass through this wire. I think this might be a good spot to run the positive wires from, but the location of the fuses is still somewhat of an issue if wired to this terminal.
The BEST spot to supply (actually return) power from for a critical component like a fuel pump in a supercharged engine is straight from the positive battery terminal.

Or Im taking this all wrong and you mean that the altenator wont be able to supply the pumps by itself, and therefore some of the amperage would be drawn from the battery through the charge wire that links them together.

The pumps can probably be powered just fine from the alternator. But running power directly from the alternator could potentially cause a few problems. Why take that chance?

I can assure you that the absolute 100% best place to draw power from is the positive terminal of the battery. Why not run it from there?

[Fire67]

Ok, no altenator!
The terminal on the passenger fenderwell is where the factory connects everything that needs a battery source. The positive terminal on my side post battery is too close to the fenderwell to add anything to safely.
Basically the factory has the usual huge gauge wire going from the battery to the starter, the 12ish gauge from the altenator to the battery, and one more 12ish gauge wire from the battery to the terminal on the fenderwell. It is the terminal to use for jump starting, and its were I already wired the MSD 6BTM box and meziere pump to. Ive had no problems with these components, so I think well be ok using it for the power source.
So as a final layout in general:
>2 - 14gauge wires from fenderwell '+' terminal to relays, with 20amp fuses inline on each as close to the terminal as possible.
>2 - 30amp relays, weather protected
>factory relay trigger wire, wired to both relays in series
>2 - 14gauge wires from relays to pumps
>2 - 14gauge wires from pumps to good body ground
= End result; two hotwired fuel pumps, ready to rock!
Indicator lights sound very interesting and cool, but I think they'll wait untill later. At this point, Im so buried in stuff that needs to be done the simpler the better.
Thanks again guys, this would have taken me forever to figure out on my own!