How to kill your fuel pump with a Merc kill switch

**MODVP22** · 05-04-2004, 11:28 PM

Can it be done? I've learned that the Mercury kill switch grouds out the ignition to kill the motor, which will not open the circuit as is needed to kill the fuel pump also. Anyone help here? Thanks in advance.

**Burke Kilgour** · 05-04-2004, 11:31 PM

How about a double pole switch? one to the ignition and one to the pump.
I'm thinking of doing the same thing.

**MODVP22** · 05-04-2004, 11:39 PM

Burke,
I looked at that option, it will work. But then what do you do with the Merc switch that's already wired in the harness? These things are more complicated than I thought, LOL.

**Superdave** · 05-05-2004, 01:46 AM

I replaced the single pole single throw (SPST) (2 position) toggle switch with a double pole double throw toggle switch (DPDT) (2 position). I wired the ground to the center pole and the engine kill to one side and the fuel pump ground to the other. In the "UP" position the motor is not grounded, but the fuel pump is, when in the "DOWN" position the motor is grounded and the fuel pump isn't. Fits in the factory hole in the control box.
DAve

**Charlie M** · 05-05-2004, 08:44 AM

Simple, use a 5 prong relay, like relay used for the tilt and trim. Some of those relays have the diagram on the back. What you have to do is use the normally closed side of the relay to run your fuel pump, when you kill the motor the kill wire activates the relay shutting the power off to the pump.

This system works good for the OMC guys running electric fuel pumps that can't get a toggle switch to fit in the control box. Or when you can't find a double pole switch to handle the amperage draw of the fuel pump.

**liberatorx2** · 05-05-2004, 09:09 AM

You can get a kill switch for a mercruiser. It has 4 prongs out the back, 2 for a "make" circuit & 2 for "break" circuit. Works great , i installed one on my boat yesterday. sierra part #mp-40970 , cost about 22$ with lanyard included.

**Jeff "Yammer"** · 05-05-2004, 01:50 PM

Supedave is right on the money !!!

JW

**Techno** · 05-05-2004, 03:25 PM

Ignition switch or safety switch?

Wiring a switch to select isn't a safety switch, only a selector. For testing? ignition off but pump on/ pump off but ignition on.
Nice option.
If you only want to shut the pump off with ignition off use the purple wire, its switched as a color standard. Anything going on with igniton on can be hooked to the purple wire. Goes off with the key. Might have to check it for power on since they kind of bleed over to other functions.

A safety switch with normal open and normal closed is best. Hook up ignition to closed and pump to open. Safety switch is tripped, pump goes off-engine goes off.

I don't like the relay method since I'm anal about redundant systems. 2 faults with this. Your placing the relays coil into the ignition grounding circuit which if it blows you can't shut the engine off. 2nd is the relay has to have power to shut the pump off when it should do the opposite, lose power to shut it off.
Trivial but worth mentioning. Also the relay has to be pretty sizable for the pump. Most of them only have 4 legs not 5. Those types are usually low amp rated.

My pump is getting wired to a dash switch through the saftey switch. I have to turn the pump on. Safety switch kills it. Ignition Don't do anything with the pump. New ignition is engine on/off. And a seperate Crank switch. No key.

**ShorePounder** · 05-05-2004, 05:07 PM

I'm going to have to disagree with Techno on his summary of relays and their operation.

I can and have used a relay to kill the ignition and the fuel pump simultaneously. By choosing the correct relay, the wiring can be configured so that it requires power to turn ON the fuel pump and open the ignition kill circuit and do the exact opposite when power is removed from the coil via either the killswitch or the ignition switch. This can be done with one DPDT relay. It was also pointed out to me by Gordie Miller that the same function can be achieved using the three wire merc kill switch.

When most(not all of course) relay coils fail, they fail open resulting in the relay contacts returning to their normal state as if they were simply turned off. If on the odd chance that a relay coil develops a short, it will a)not be able to produce enough electromagnetic force to keep the contacts pulled in therefore returning to normal state. and b) create a short in the power circuit that should pop the fuse (you did fuse the circuit didn't you?)

Take a look at the attached schematic. It worked very well in my last boat and will be put in place in my current boat when I get ready to rewire.

**steve** · 05-05-2004, 05:13 PM

Use Merc's 3 wire kill switch--green, white and black wire, red button . One wire goes to ground, thats the control wire . Another is normally open and goes to the blk/ yel wire for killing the sw boxes. The other is normally closed and goes to the grd wire on the fuel pump relay. The other exciter wire for the fuel pump relay goes to the purple key on sw. and the relay gets hooked to the battery and pos term. on the pump. The grd side of the pump goes straight to the grd on the battery.

**Techno** · 05-05-2004, 07:27 PM

ShorePounder
Your right to disagree since I thought Charlie was drawing a relay that operated off the safety switch not the ignition. Beside you put up a nice schematic which shows all the options.
But even so "when you kill the motor the kill wire activates the relay shutting the power off to the pump." The only way I see the kill wire black/yellow of operating a relay is to hook the relay to positive and then its energized whenever the switch is off. It blows and the pump turns on.
If this was a misprint by Charlie, which I'm sure it is now then its not the kill switch but the purple switched power that operates the relay. I don't have any problem with that, thats the good way like your schematic shows.

What you drew is a fuel pump relay operated from the switched power in the key switch. The same result can be had by not using any relay at all and running the purple wire to the pumps 3 way switch( keeping all those nice features). Bypass the relay and run straight to it. You could run straight to the pump but then can't play with it like the 3-way switch lets you.
Neither is right or wrong just different uses or requirements. Either way you don't need extra switchs(the ones trying to turn the pump on), the relay is an extra switch but doing a job by making the pumps power wires be as short as possible. The purple wire is already on a switch and no one needs to add any more.
What I don't understand is how youse guys are powering the pump now? From a battery supplied switch? You have it in the key switch already.

The Purple wire is off when the ignition is off. And battery + when its on. The only limit is not hooking too many things to it with high loads. The reason for a relay.

I did notice something odd though ShorePounder
The part of the relay grounding the black/yellow wire is completeley redundant, its grounded in the key switch when switched off. Everything else is nice but the use of the relay to open an already open wire and close an already closed wire for the black/yellow kill wire just adds complexity and parts.

The very best method places a lanyard operated safey switch to kill the motor and pump too. Everyone has at least a motor killing safety switch don't they?

BTW does anyone know what an EFI fuel pump draws in amperage?

**Charlie M** · 05-05-2004, 07:43 PM

A 5 prong relay is the only relays that provide both normally open and normally closed circuits dude. You can get these relays as big as you want. The ones used in tilt and trim units handle 15 amps easy. The way this wiring diagram is made (although I left out the diagram on the relay itself) only has current going through it when the kill switch is pulled. Normally closed means current flows normally open means break in the circuit (you must not understand this). By using the kill ground wire it activates the relay when the switch is pulled, by using the ign wire as hot for the relay, the relay does not stay activated when the key switch is off. The fuel pump feed wire will have to be the existing fuel pump wire (notice fuel pump wire in the diagram, did not say battery positive professor know it all), how in the hell would the pump come on anyways if anything blew.

I thought this was self explanatory by writing IGN WIRE

I realize that mercruiser uses the double pole switch but thought this was an easy way to wire without pulling your controls apart or some of the boats that use non-mercury kill lanyards (like the off shore boat switches). Omc powered boats can use this also.

You know the hell with I will just keep my mouth shut..........get slapped in the face for just trying to help out

**MODVP22** · 05-05-2004, 08:01 PM

Thanks for the info guys. Really don't want to see anyone getting inot a pissing match with anyone over this topic. I'm not an electronics guru by a long shot, that's what I am asking. As of now, All I have is the Mercury harness (didn't buy the kill switch itself yet) with the black & yellow wire for the kill switch. My fuel pump (weldon EFI) will be switched with a toggle, not on the key.

**Rickracer** · 05-05-2004, 08:32 PM

...as that circuit is not designed nor constructed to carry that much current. The purple wire is sufficient to power a fuel pump relay, thereby ensuring the pump will have the current it needs.

**ShorePounder** · 05-05-2004, 08:54 PM

Charlie, don't sweat it. You've got it right. The 5 pronger is a DPDT. I think we're all now more confused than Tom was originally.

Techno, to clarify, and to reinforce what RR said:

The 3 position switch was a perk I added for my particular application. I wanted a way to turn the fuel pump on without turning the key on (to pump out the tank or fill my weed eater

)and I wanted to be able to turn my fuel pump off so it wouldn't come on with the key while performing ignition troubleshooting or timing adjustments. A convenient option, but it sucked when you left the switch off. Nothing turns the fuel pump on then.

The 12VDC the fuel pump eventually sees comes directly from the battery, switched on and off by the keyswitch with the killswitch as an interlock. It's a better place to grab the 12amp max that my Mallory Comp 140 pump is capable of drawing.

As for the redundant ignition shunt, its not as redundant as it may appear. They are in fact seperate shunts activated in a different manner to produce the same effect. Yes the keyswitch does shunt the ignition when the key switch is off, but how many guys you know turn off the keyswitch as they're being ejected from the boat?

That's what the killswitch is there for in the first place. The schematic shows that the ignition will be shunted if EITHER the killswitch is pulled OR the key is turned off.

The only thing I don't like about the KISS method is that it controls the fuel pump via the ground wire. Not that its wrong or bad or anything, just a personal quirk. My theory is that anything can find a way to ground, and it's alot harder to accidentally find a voltage source so I control all of my accessories by the hot wire. Just me being a dork I guess.