4.3 Merc wiring to fuel pump question

[robbmcc]

I have a 2004 Mercruiser 4.3 with 2bbl and a no start problem and have read all the manual has to say and can't quite understand the circuit from +12 to the fuel pump.

There's no voltage at the fuel pump with key in "on" or crank position. There's voltage (although a puzzling voltage like 8 or there abouts) at the blue terminal by itself on the end of the oil pressure cutoff switch (the round thing - I might be confusing the oil pressure gauge sensor with cutoff switch - manual is not good on explaining this). But there's no voltage coming out of the oil pressure sensor in the purple with yellow strip wire that goes to the fuel pump. The other wire coming out of oil pressure sensor goes off to the pressure gauge in the dash.

So I'm concluding the sensor and or cutoff switch are bad. Should there be voltage in the purple with yellow with key in on position? Which should I replace first? I did a continuity check of the switch (per the manual which I just got am still digesting - I'm new at this stuff) without removing it and it tested to have no continuity which it said meant is was bad. Am I on the right track here, I'm a total newbie with boats? I did jump the pump to the battery and it runs fine. What about the 8 v? I know my volt meter is accurate. Is that the real problem? I tried to trace back that blue wire which seems to be supplying power and lost it at the big engine connector where it must change color. Should there be 12 v there?
TIA... Robb McC

 

[Don S]

Re: 4.3 Merc wiring to fuel pump question

The blue wire is your for your gauge sender. The oil pressure switch is located on the port/aft side of the block, just above the oil filter boss.
The purple yellow wire comes from the starter solenoid which bypasses the switch when cranking.
Sounds to me like the wiring on the switch is all rusty and corroded.

 

[robbmcc]

Re: 4.3 Merc wiring to fuel pump question

Thanks Don,

All of these connections are sparlking clean - after all this is a 2005 boat. So the blue wire comes off the end of the oil pressure sender and goes off to the gauge in the dash, right? OK so that sender is the round thing, now I'm getting educated. I wonder why it has 8 volts in it with key in on position.

So the 2 wires coming oil pressure cutoff switch confuse me. I am almost positive the purple yellow stripe goes directly to the fuel pump. The other wire on the switch goes to the dash area - can't say exactly where yet. But none of these wires at the switch or at the pump have power with key on. Which one should have power? Am I wrong about the purple/yellow? Does it go to starter and then to fuel pump?
Thanks,
Robb McC

 

[Don S]

Re: 4.3 Merc wiring to fuel pump question

With the engine off, and the key in the run position, you DO NOT have power to the fuel pump. It won't run.
You will have power to the fuel pump while the engine is being cranked over. Power from the starter solenoid (Terminal D) energizes the purple/yellow wire.

Once the engine starts, the oil pressure builds and closes the contacts in the oil pressure switch that allows power to the pump while running.

 

[robbmcc]

Re: 4.3 Merc wiring to fuel pump question

thanks Don,
Still confused but getting less so with each post. So the purple w/yellow wire on the fuel pump does not go directly to the cutoff switch? Which wire goes to the D terminal on the starter solinoid? Does the purple wire on the other side of the cutoff switch go to +12 v somewhere in the dash area thru the ignition switch as I suspect? I'm just starting to get it here - first time I ever worked on a boat...
Robb McC

 

[95yj]

Re: 4.3 Merc wiring to fuel pump question

Look at the wiring diagram that Don posted for you. Purple wire is +12vdc on your engine harness and is connected to a bunch of things there. It also provides power to the oil pressure switch so that if there is oil pressure, the pur/yel wire receives +12vdc. The pur/yel is also connected to the starter circuit and to the positive of the fuel pump. Therefore if there is oil pressure or else the key is in the start position, the fuel pump will receive +12vdc. Note that on the diagram, your electric choke is also powered by this same circuit.

To diagnose, you want to be sure the pur/yel has +12vdc when cranking and then also once the engine is started and has oil pressure.

 

[robbmcc]

Re: 4.3 Merc wiring to fuel pump question

Thank You 95yj,
I am in complete understanding of the circuit now (even though I didn't see the wiring diagram from Don thanks to my IT folks who must be blocking photobucket) and can't wait to get over to the boat. What I was missing was the fact the purple with yellow wire goes from the fuel pump to the d+ term on the starter solonoid in addition to going to the switch I thought it only went to the switch and of course was confused... Consider this thread solved...Thanks All.