4.3L electric fuel pump

[templeguru]

Greetings...my first post as a newbie so your patience is appreciated. I'm trying to help a friend get his boat going and the problem seems to be fuel delivery. He replaced the fuel pump last season and got about 3hrs of boating before it quit. Based on the tests I've done, the fuel pump isn't getting power. the ignition has power, the boat will even run if I pour fuel down her throat and the pump works with direct power so?.. Is the fuel pump power circuit routed through the ECM? I don't want to misguide him towards a $200 part that is undamaged.
1999 Mercruiser 4.3L - S/N 0L620402
2BBL carb
ECM # 861251-1

The key "ON" position has a continuous tone rather than beeps. Also, perhaps unrelated but in the course of diagnosing the problem, it seems like the trim function has quit working as well.

Thanks for reading.

 

[alldodge]

Howdy

When key is turned to ON, power is supplied to the purple wires. Note in the pic that the purple goes to item 5 which is the oil pressure switch. If key is just turned on and the motor is not running, the switch will be open. So no power is being applied to the pump (item 6).

When key is turned to Start, power is applied to the purple/Yellow striped wire coming from the starter. This applies power to the pump while the motor is cranking. Once the motor starts, key is released and power is removed from the purple/Yellow wire, UNTIL oil pressure builds up and switch 5 closes, then 12V power is restored.

This circuit has nothing to do with the ignition system. Check the Black wire from the pump to ground, make sure it has continuity

 

[templeguru]

Thanks AllDodge, I'll give it a try and let you know what happens. It seems possible that the issue he's experiencing could be oil pressure related even solenoid/starter? are there any clues to rule out those possibilities? I'm not a professional mechanic but I have some backyard experience with electrical testing equipment.

 

[alldodge]

If you manually jumper the fuel pump so it runs, will the motor continue to run, or does it also die or not start?

If it does continue to run then. on the port side down low is the oil pressure switch, along with the oil pressure sender and they are Teed together.
http://www.mercruiserparts.com/bam/subassembly/31511/2694/60

The switch will look like this and have two wires going to it.
http://www.cpperformance.com/p-61088-kit-oil-sensor-mercruiser-87-864252a01.aspx

Remove the connector off the switch, then jumper the two contacts together. Now turn the key to ON and the pump should run. If it does then the switch is bad, if it doesn't there is another problem.

 

[achris]

Very common for the pressure switch terminals to get a bit of corrosion on them and become resistive enough to stop the pump running. To test, just pull the terminals off the pressure switch and join them together. This is for testing only. The pump will then run continuously. If that happens you know it's likely the switch terminals need cleaning or the switch replacing.

Chris........

 

[templeguru]

Indeed, once I bridged the terminal connectors the pump ran continuously. I was just wondering if that definitively indicates its the switch or if the issue could stem from the pressure sending unit. As AllDodge pointed out, they're Teed together. Also, is that switch a marine specific component or same as automotive application units? Sourcing parts in my area has proven to be a bit difficult.
Thanks for all the information, my friend is extremely pleased to have his boat ready for the season.

 

[Bondo]

Indeed, once I bridged the terminal connectors the pump ran continuously. I was just wondering if that definitively indicates its the switch or if the issue could stem from the pressure sending unit.

Ayuh,.... Welcome Aboard,...... Did ya unhook, 'n rehook the terminals on the switch,..??
As Chris says, that's sometimes enough to disturb the corrosion, 'n make the connection,....
Donno the automotive app., but iboats probably has 'em, haven't looked yet,....

The sender is only for the gauge,....
The switch is for the fuel pump,....

 

[Bt Doctur]

You will find the fuel pump is drawing a bit more current and burned the contacts. Best to use a cube relay to power the pump and use the pressure switch to power the cube relay coil.

 

[alldodge]

Indeed, once I bridged the terminal connectors the pump ran continuously. I was just wondering if that definitively indicates its the switch or if the issue could stem from the pressure sending unit. As AllDodge pointed out, they're Teed together. Also, is that switch a marine specific component or same as automotive application units? Sourcing parts in my area has proven to be a bit difficult.
Thanks for all the information, my friend is extremely pleased to have his boat ready for the season.

Your pump runs non stop when jumped, which is good. My question is the same, does the motor run without issue if the switch is jumped?

If it does that but will not start, then there could be two issues.

If it will run when jumped, and if there is no corrosion on the terminals, then you need a new switch. You can get the new switch from the local auto store and there is no marine equivalent.

If it runs when jumped, but will not start unless the relay is jumped, then there is an issue with the wiring from the starter to the pump, or the contact with the starter.


Let us know how things happened

 

[templeguru]

Thanks to everyone for all the good info. achris & Bondo, you clearly know your stuff. Despite the clean appearance of the motor and the oil pressure switch terminals, it would seem that corrosion affected the terminal contact. It only took a little bit of scraping (basically just repeatedly plug in/plug out) to get the system working without any new parts whatsoever. Unfortunately, the trim function has quit working and I'm not at all familiar with that system. I recognize the hydraulic pump and hoses but the problem appears to be another electrical gremlin...I will research the existing forums for answers.
Thanks again...see you in the Q & A forums