Wiring question for 4.3l Merc

[soulhawaii]

Aloha,

I have a 2005 4.3l Merc engine, yesterday randomly the fuel pump stopped getting voltage. I energized the pump and it works. I bridged the oil pressure sensor, that's not it. I traced it back to the starter wire that is purple and yellow. Now i know from before when i turn the key, before i start the engine the fuel pump starts going, now it does not do that. Looking at the wiring diagram, how does the fuel pump get energized before starting to crank the engine? Where does the power come from, and trace back to? I'm at a loss, because when cranking or just key turned, no power to the pump, and I can't figure out where the issue is.

 

[alldodge]

Need a serial number
From what your explaining I might guess it has a TKS carb, but do verify

 

[Bondo]

Looking at the wiring diagram, how does the fuel pump get energized before starting to crank the engine? Where does the power come from, and trace back to? I'm at a loss,

Ayuh,.... When, 'n only when the key is turned to start, the fuel pump is powered by the purple / yellow wire which pulls power off from the starter's solenoid,......
When the motor starts, ya left off the key, 'n the fuel pump is powered from the oil pressure switch,.....
Which draws power from the ignition circuit off the key switch,.....

From yer story,..... I suggest you remove, 'n clean all wiring connections in both circuits,...... Starting at the battery cables ends,....
Both ends,.....

Corrosion is evil, but common on boats,....

 

[soulhawaii]

Thank you both for the replies. I will try to look up the serial number, but i do think it's the TKS carb.

Bondo, before this happened, my fuel pump would make noise when key is in the on position before i crank.. now nothing when crank or otherwise. So today, i went to clean all connections, tested power at the purple yellow wire on the starter, when cranking i get 10v. But still no power to pump. I jumped it on the starter from the other 12v source, and the pump runs and boat starts.. Kinda sounds like its a starter issue?

Also the choke on the carb, should it be getting power before the boat is cranking? I get nothing when i just switch the key to ignition.

 

[alldodge]

TKS wiring block diagram
As Bondo mentioned pump only gets power while cranking and it comes from the starter. After motor starts it comes from the OP switch

 

[soulhawaii]

Thank you for the diagram.. So according to this if i have my oil pressure switch bypassed, i should be getting power to the pump at ignition, before i even crank? But I'm not, where should i be looking for loss of power?

 

[alldodge]

Check for power at the oil pressure switch with key ON. If power is not there trace back to key. Should have 12V on Red/Purple and with key ON should have it on purple wire

 

[nola mike]

Thank you for the diagram.. So according to this if i have my oil pressure switch bypassed, i should be getting power to the pump at ignition, before i even crank? But I'm not, where should i be looking for loss of power?

Yes, with ignition on. Check the purple wire at the OP switch. Should have 12v. If not, check coil (+). Should have 12v off purple wire there.

 

[soulhawaii]

Okay here are my findings. No power at coil, no power at fuel pump and no power at oil pressure switch, power at purple wire at alternator and power at purple wire at ignition key.

 

[nola mike]

Should be a common take off point for the purple somewhere, not sure where on the TKS. You'll have to trace the alt purple back...

 

[alldodge]

Some motors have an additional fuse on the motor, but if it's not there then there is a broken connection. Pin 5 on the engine plug is what powers the motor,

 

[nola mike]

If he's got power to the alt, the problem is after the plug...

 

[soulhawaii]

Okay so after looking at that and doing some testing, somehow there's no power at the plug on the engine (cannon plug), once I applied power there the pump runs. But somehow there's power at the alternator? Not sure what's happening. Anyways, power at the key, but not at the plug, i checked all the fuses, gauges work, everything else works. Going to try and trace the wire through the side of the boat, but so far don't see any issues.

 

[alldodge]

One side of power to the ALT comes from the 50 amp breaker and before the motor plug. The excite voltage (purple wire) comes from the key and thru the motor plug

 

[soulhawaii]

Okay great to know. Somewhere after the igniton and before the plug i'm losing power. I ran a new wire from after the igintion, spliced into the purple, and back into purple before the plug and now everything works fine. I'll try to keep looking for where the issue is tomorrow.