Merc 4.3 Electrical Question

dg512055 · Mar 8, 2010

I have compared the wiring diagram for my power package to the actual on the boat. I have found that the purp/yel wire that connects to the extra starter solenoid terminal has been jumpered directly to the purple wire on the coil. I looks like this wire (purp/yel) powers up the fuel pump while the starter is engaged, and then the oil pressure switch powers the pump after the engine is running. The purp/yel wire has been disconnected from the starter terminal, and an extra wire is attached to it near the starter and runs to the coil (purple wire). This make the fuel pump run as soon as the key is switched on. The question is, this is how the boat was wired when I got it. Is this something that the boat builder would do for some reason, or has someone else changed it? I can see where this would be good if the boat has set for a while and carb was empty. It could be bad for some other situations though. Any ideas?

Sorry guys, I meant to put this in the engine section. Maybe it will get moved soon.

DG

HT32BSX115 · Mar 9, 2010

Re: Merc 4.3 Electrical Question

Howdy,

Go to http://forums.iboats.com/showthread.php?t=342393

And you'll find a service manual that you can download. There's schematic diagrams there.

Also, NO the fuel pump should not be wired so that it runs all the time the key is on (with the engine NOT running)

It sounds like someone may have bypassed the oil pressure safety switch.

Cheers,

Rick

achris · Mar 9, 2010

Re: Merc 4.3 Electrical Question

It's often the practice to bypass the oil pressure switch as they are vulnerable to getting wet and corroding. Then they stop working. This is a really dangerous thing and should never be done! The correct way to fix the problem is to replace the pressure switch and get the machine back to working the way the manufacturer designed it....

With the pressure switch bypassed and the fuel pump running all the time... There is the potential for an engine to suffer a problem where it has stalled, the carb needle and seat doesn't close (for whatever reason) and the engine fills with fuel... When you turn the key you'll find out why there was a pressure switch there in the first place...

[size=+3]KA-BOOM!!![/SIZE]

Chris........

180shabah · Mar 9, 2010

Re: Merc 4.3 Electrical Question

Switches are pretty cheap too, no real reason to bypass other than for testing or to make it back to the trailer.

Silvertip · Mar 9, 2010

Re: Merc 4.3 Electrical Question

The other reason for the oil pressure switch also involves stalling, but in the event there is a fuel system leak and heaven forbid a fire breaks out, the pump will continue to pump fuel onto the fire.

180shabah · Mar 9, 2010

Re: Merc 4.3 Electrical Question

Actually thast is THE reason for the switch. The pump should only run when the engine is running. Not running = no oil pressure = no power to the pump.

FWIW - this should probably be in the I/O section where the OP could get a copy of the Merc manual for his engine....

HT32BSX115 · Mar 9, 2010