2006 Mercruiser 4.3 no power to Fuel Pump

[1tman1]

Hi Team
2006 Mercruiser 4.3. SN#OW302496
Doesn’t appear to be getting power to the fuel pump. Engine cranks. Fuel in the Fuel Filter. Engine fires when small amount of fuel put directly into Carb. Cannot hear fuel pump running.
Oil pressure alarm sounding when cranking. Not turning off after first 2-5 cranks.
Thought we had deduced that it was the Oil pressure sender switch that was faulty, as we were of the understanding that the Fuel Pump gets its power from the Sender switch when the switch opens (closes?!).
Just spoke with Mechanic who said that the Fuel Pump should run Automatically when the key is turned to start position. But have never heard it running before. He thought the Fuel Pump power comes from a connection to the Starter Motor??
Help Please?!
Thanks

 

[Bt Doctur]

Just spoke with Mechanic who said that the Fuel Pump should run Automatically when the key is turned to start position. But have never heard it running before. He thought the Fuel Pump power comes from a

Get another mechanic, elec pumps need a pressure switch to activate the fuel pump. Will only supply power IF it sees oil pressure . In start position the starters solenoid supplys power during cranking mode.
Locate the pressure switch by the oil filter ares, has a purple wire and purple /yellow wire. unplug these and join them together for testing. Trun key to "on" and pump should run. If yes, replace pressure switch . Most use a 40A cube relay to protect the switch from kigh current draw that burns the pressure switch internal contacts

 

[Bt Doctur]

 

[1tman1]

Thanks! Will try that and report back. Cheers

 

[1tman1]

So this is the pressure switch? Yes?

 

[1tman1]

And this is the plug that came off it. Look correct?

 

[1tman1]

And this is the pressure Sensor? One Lighter Blue wire coming out of the top of it with a push on plug

 

[Fun Times]

Yes the first photo in post number 5 is the pressure switch for the fuel pump as mentioned.

And this is the pressure Sensor? One Lighter Blue wire coming out of the top of it with a push on plug

This one's more called/considered more of an oil sender unit for the oil gauge at the helm... If you ground out the blue wire with the key on engine not running is fine and watch the oil gauge, the gauge should go to a maxed-out reading.

There's going to be another switch assembly up by the distributor area that sounds the warning horn when the oil psi is too low...Under 4 psi...The horn shouldn't sound once the engine is running.

See item number 8, 10, 34..., https://www.mercruiserparts.com/bam/subassembly/31491/11665/40
Here's your parts catalog for your serial number, https://www.mercruiserparts.com/4-3l-carb-alpha-bravo-0w300013-thru-0w

 

[QBhoy]

Hi. Is she definitely a carb and not an mpi ? If it’s an mpi your mechanic is correct. If it’s a carb, as above. If you aren’t familiar with the mpi…one might assume the throttle body is the carb. Looks like one at a glance.

 

[1tman1]

Yes the first photo in post number 5 is the pressure switch for the fuel pump as mentioned.

This one's more called/considered more of an oil sender unit for the oil gauge at the helm... If you ground out the blue wire with the key on engine not running is fine and watch the oil gauge, the gauge should go to a maxed-out reading.

There's going to be another switch assembly up by the distributor area that sounds the warning horn when the oil psi is too low...Under 4 psi...The horn shouldn't sound once the engine is running.

See item number 8, 10, 34..., https://www.mercruiserparts.com/bam/subassembly/31491/11665/40
Here's your parts catalog for your serial number, https://www.mercruiserparts.com/4-3l-carb-alpha-bravo-0w300013-thru-0w

Thanks… So just to clarify
#8 is the Oil Sender for the Gauge at the Helm (makes the Oil Gauge work on the Helm)
#10 is the Pressure Switch for the Fuel Pump (as in post #5) and sends signal to the Fuel Pump to turn on.
#34 is Switch for the Alarm for Oil Pressure (definitely working!!)
8 & 10 next to each other down under the motor near the Oil filter. 34 is up next to the distributor and screwed into the top of the block basically.

Is it likely that both 8 & 10 have failed?
Because the alternative I’m guessing is that if they haven’t failed, then it’s most likely the Fuel Pump itself?

There isn’t any Fuse anywhere in that system that could have failed??

Sorry for all the questions and clarification, New to this… Thanks Again

 

[1tman1]

Hi. Is she definitely a carb and not an mpi ? If it’s an mpi your mechanic is correct. If it’s a carb, as above. If you aren’t familiar with the mpi…one might assume the throttle body is the carb. Looks like one at a glance.

Hey there. Based on the Serial number, we’re pretty sure it’s a Carb.

 

[QBhoy]

Hey there. Based on the Serial number, we’re pretty sure it’s a Carb. View attachment 374131

Yeah. You’re spot on. Thought I’d mention just in case. Fair play.

 

[Bt Doctur]

take a paperclip and insert it into the plug, with key on, pump should run

 

[1tman1]

take a paperclip and insert it into the plug, with key on, pump should run

Into the Pressure Switch Plug? And if it doesn’t run?

 

[Scott Danforth]

Into the Pressure Switch Plug? And if it doesn’t run?

More trouble shooting

 

[1tman1]

Update…
Have grounded the Blue wire from the Oil Sender & Oil Gauge maxed out.
Removed the Oil Sender, cleaned it up (was a bit messy looking around the base)
Cranked the motor with the Oil Sender removed, Oil pumped out. Reinstalled the Oil Sender, connected wire, Cranked the Motor and Oil Gauge came up to operating pressure (progress! Because it wasn’t before!)
Couldn’t get Fuel Pump to Run by inserting paper clip across the Sensor Plug. However, traced the Purple/Purple Yellow wire back to Fuel Pump Plug, and got a reading from the Sensor when key turned on.
So Fuel Pump “seems” to be getting power to it, but is not running

 

[nola mike]

Update…

Couldn’t get Fuel Pump to Run by inserting paper clip across the Sensor Plug. However, traced the Purple/Purple Yellow wire back to Fuel Pump Plug, and got a reading from the Sensor when key turned on.
So Fuel Pump “seems” to be getting power to it, but is not running

Not sure what you're saying here. Is the "sensor" the oil pressure switch? If so, the purple wire should be hot with the ignition on. The purple/yellow wire to the switch and pump shouldn't have 12v unless the engine is running, cranking, or the op switch is jumped and ignition on. Note that #8 and #34 from the parts diagram have nothing to do with the fuel pump or operation of the engine.

 

[1tman1]

Update… And thanks to all for suggestions and help!
Having established that we had power going to the pump, but the pump not working. We removed the Fuel Pump and Lines. We then powered the pump without any lines attached and it ran!! It would seem that somewhere in the process of removing, we either removed a vacuum in the fuel pump or something inside it became unstuck! Success! Reassembled the Fuel Pump & Lines, Cranked the engine and we had Fuel to the Carb!! After a little more persuasion (she’d been sitting a little while) she fired and is now running!!
Thanks Again for all the help!
Most appreciated.
Cheers

 

[Scott06]

Update… And thanks to all for suggestions and help!
Having established that we had power going to the pump, but the pump not working. We removed the Fuel Pump and Lines. We then powered the pump without any lines attached and it ran!! It would seem that somewhere in the process of removing, we either removed a vacuum in the fuel pump or something inside it became unstuck! Success! Reassembled the Fuel Pump & Lines, Cranked the engine and we had Fuel to the Carb!! After a little more persuasion (she’d been sitting a little while) she fired and is now running!!
Thanks Again for all the help!
Most appreciated.
Cheers

Did you check your antisiphon valve ?

 

[Silvertip]

Something that doesn't work and then mysteriously works is cause for concern as that mystery repair may come back to bite you when you leaste expect it -- like miles from the ramp!I suggest you really look into this further.