Fuel pump issues

[woodguy81]

So I’m helping someone with their new “free” boat. It’s a late 80’s model with a merc 350 with the thunderbolt v ignition and gen one alpha one drive. I can’t read the serial number anymore. Problem is that it started pretty readily after a tuneup and fluid change. Let it run a bit and it died. No fuel pump. Getting 9 volts at the pump harness. 12.7 everywhere else. Anyone got any sort of diagram or anything for this? Like any old boat there seems to be a lot of extra loose wires hanging around. Also it’s got an annoying sound with the key on, but if I move the wires around it stops. Tach also reads variable rpms when it’s not running. She’s a mess to say the least. Lol.

 

[Rick Stephens]

There should be no resistance between pump and power. Disconnect the fuel pump and recheck the power. It may be the electrical windings in the pump are breaking down and drawing too much power.

Here's the diagram:

 

[woodguy81]

There should be no resistance between pump and power. Disconnect the fuel pump and recheck the power. It may be the electrical windings in the pump are breaking down and drawing too much power.

Here's the diagram:

View attachment 344848

I tested the power at the harness with the pump disconnected. Key on 9 volts. Before it shut down the pump pressurized with the key on. Now nothing. Thanks for the diagram. I’ll trace the wiring back.

 

[Scott Danforth]

Free boats are the most expensive boats to own

 

[woodguy81]

Free boats are the most expensive boats to own

Agreed. And he was warned. I’m not a volunteer though. I’m getting paid for my time. I’ve basically rebuilt my entire boat so I’m familiar with a lot of the systems. Mine is a good bit newer though.

 

[flashback]

Free boats are the most expensive boats to own

no truer words spoken.

 

[MichaelBC]

With the key on there should be no power. Power is supplied when the engine gets started by the starter selenoid and kept alive by the oil pressure switch.

When you say the pump resurized with key on then the wiring does not match the plan.

 

[H20Rat]

I tested the power at the harness with the pump disconnected. Key on 9 volts. Before it shut down the pump pressurized with the key on. Now nothing. Thanks for the diagram. I’ll trace the wiring back.

LIke MichaelBC mentioned above, that is bad! It should absolutely not pressurize with the key on.

Take a look at your oil pressure sensor on the diagram above, very likely that is probably corroded internally and was allowing a small amount of voltage to leak through without oil pressure. (and probably not allowing enough current to flow to actually run the pump) You can TEMPORARILY bypass the oil pressure sensor with a chunk of wire, but only for testing.

(the sensor is located low on the aft/port side of the block, on a T with the sensor used for the gauge)

 

[woodguy81]

LIke MichaelBC mentioned above, that is bad! It should absolutely not pressurize with the key on.

Take a look at your oil pressure sensor on the diagram above, very likely that is probably corroded internally and was allowing a small amount of voltage to leak through without oil pressure. (and probably not allowing enough current to flow to actually run the pump) You can TEMPORARILY bypass the oil pressure sensor with a chunk of wire, but only for testing.

(the sensor is located low on the aft/port side of the block, on a T with the sensor used for the gauge)

Ok. So if I’m understanding this corrrctly I should read no voltage with the key on not running at the pump. I get 7 volts as I’m cranking it over so it drops. I gotta remove the exhaust from the port side to check out the oil pressure switch. Before I go that far is there anything else I should look at?

 

[Rick Stephens]

Most typical installation put the oil pressure switch for the pump at the port side of the distributor just behind the intake mani.

 

[woodguy81]

This forum is great. Found the switch. It was previously bypassed and had recently corroded off from inside the connector. I stripped them and connected together and it started right up. So went to the marina to get the switch to put it back together correctly. Was told just to connect the wires and it’ll work fine. Lol. I ordered the switch online and will fix it properly when it arrives. Nice to know the mechanic at the marina cares so much for safety.