Fuel problems and Im at a loss

[swtch7]

I recently had a post on here but wanted to start a new one to clear up any confusion.
Mercruiser 4.3l v6 1996
electric fuel pump
maxum 1900sr2

A little back ground, the end of last season my boat would get up on plane and then cut out. I suspected the oil pressure switch so I jumped it. At this point i could turn the key (not turn over the motor) and prime the motor before I turned it over. Then the fuel pump seemed to start acting up. I would turn the key and not hear it kick in, checked the voltage (9.5v) hook it back up and it would start working again. It was very intermittent.


So here I am in the dead of winter trying to trouble shoot this. Currently the oil pressure switch is bypassed. I suspected the fuel pump was the issue so I ordered a new one only to find out the new one does the same thing. When I hook up a 12v source to either fuel pump they dont kick in, but if I hook it up to my 9.5v battery charger they hum and sound like they are working.

Others have suggested wiring. I took off the ground points around the motor and cleaned them thoroughly.

I get the following results
9.5v at the fuel pump
10.5 at the oil pressure sending unit (jumped)
12.8 at the battery.

but the fuel pump doesn't engage. Looking at a wiring diagram I should initially be receiving voltage from the starter, then oil pressure switch once the motor is running for 5 seconds, but i never get it started to get that far.

Any help would be very appreciated. I just dont understand how im getting voltage at the plug but the fuel pump wont kick in. Wiring? Grounds? another bad fuel pump? please help.

thanks

 

[Bondo]

Re: Fuel problems and Im at a loss

I get the following results
9.5v at the fuel pump
10.5 at the oil pressure sending unit (jumped)
12.8 at the battery.

Ayuh,... I gotta believe there's corrosion or something goin' on, as the voltage should be the Same at all those points...

 

[wire2]

Re: Fuel problems and Im at a loss

There will be a voltage drop at various points of the wiring due to Ohm's law, since current is drawn. But 9.5 at the pump seems low, as if the pump is blocked downstream and drawing excessive current. Undo a connection near the engine, put a piece of hose on it, into a container, and try the pump.

The charger will make it hum because the output is 120 pulses per second. (full wave rectifier from a 60 Hz source)

 

[swtch7]

Re: Fuel problems and Im at a loss

thanks for the info so far. Before I swapped the new pump with the old I tried unhooking just the top first, no luck. Also, neither the new pump nor the old pump not bolted in the boat but hooked up to the boat harness.

Should i just try jumping straight from the starter (purple/yellow) wire to eliminate all wiring on the boat?

 

[Bondo]

Re: Fuel problems and Im at a loss

Should i just try jumping straight from the starter (purple/yellow) wire to eliminate all wiring on the boat?

Ayuh,... Jumper from the Battery to the fuelpump,...

It works, or it Don't.....

If it Does,... Find the fault in the harness...

 

[swtch7]

Re: Fuel problems and Im at a loss

Ayuh,... Jumper from the Battery to the fuelpump,...

It works, or it Don't.....

If it Does,... Find the fault in the harness...

Neither fuel pump works straight from the battery. The only time that I can get either fuel pump (old or new) to work is hooking it up to my battery charger.