1996 mercruiser 4.3LX fuel pump question

[natergade]

Hello. I am in the process of diagnosing what could be wrong with my boat. I am not getting fuel to the carb.

Things I have completed/diagnosed:
-Bad starter. Replaced. Works flawless.
- Replaced slave solenoid
-checked for 12V from purple/yellow wire from starter solenoid while cranking engine over (since there is no power to fuel pump when key is in ON position. Carbed engine.) Yes. There is power.
-direct power to fuel pump and it turns on and pumps fuel to carb

My question. How does the starter solenoid keep feeding power to the oil pressure sensor once the key turns back from START to ON? I have a voltage of 10-12V while cranking, but drops to zero after engine cranks over and it ends up stalling due to no more fuel. (I put fuel in carb to get it running for a few seconds) I feel like I am missing something here. I can’t even test to see if the oil pressure switch is working correctly due to no power reaching it once key goes back to ON…?

Thanks for the help in advance!

 

[natergade]

Update: I jumped the oil pressure switch and the fuel pump turns on and fuel does pump into carb with the key in the ON position. What does this tell me?

 

[Rick Stephens]

The purple wire which goes to ignition and choke also goes to the oil pressure switch from the ignition switch.

 

[Rick Stephens]

Update: I jumped the oil pressure switch and the fuel pump turns on and fuel does pump into carb with the key in the ON position. What does this tell me?

Sounds like a bad connection on the afore mentioned purple wire.
.

 

[nola mike]

Sounds like a bad connection on the afore mentioned purple wire.
.

Or bad oil pressure switch.

 

[natergade]

Update: I jumped the oil pressure switch and the fuel pump turns on and fuel does pump into carb with the key in the ON position. What does this tell me?

Hello. I am in the process of diagnosing what could be wrong with my boat. I am not getting fuel to the carb.

Things I have completed/diagnosed:
-Bad starter. Replaced. Works flawless.
- Replaced slave solenoid
-checked for 12V from purple/yellow wire from starter solenoid while cranking engine over (since there is no power to fuel pump when key is in ON position. Carbed engine.) Yes. There is power.
-direct power to fuel pump and it turns on and pumps fuel to carb

My question. How does the starter solenoid keep feeding power to the oil pressure sensor once the key turns back from START to ON? I have a voltage of 10-12V while cranking, but drops to zero after engine cranks over and it ends up stalling due to no more fuel. (I put fuel in carb to get it running for a few seconds) I feel like I am missing something here. I can’t even test to see if the oil pressure switch is working correctly due to no power reaching it once key goes back to ON…?

Thanks for the help in advance!

I realized I just answered my own question. The purple wire that gives 12V while the key is ON feeds power to fuel pump only when the oil pressure builds up to certain pressure. Everything is indicating that I have a bad oil pressure switch In my opinion. Can anyone confirm my diagnosis?

 

[natergade]

The purple wire which goes to ignition and choke also goes to the oil pressure switch from the ignition switch.

View attachment 366206

Right. Thank you. I double checked. I have it wired correctly! So the oil pressure switch still needs to close when starting the engine correct? Since it won’t get fuel while trying to crank over, nor get fuel while running (after putting fuel in carb) it still leads me to believe that it is a bad oil pressure switch. I believe we have figured the issue out. I will order one. Do you by chance know a good resource to order one from? Can’t seem to find one that matches the one on mine. Not a great picture but this is what mine looks like. Also here are the serial numbers if that may help. Thank you!!

 

[natergade]

The purple wire which goes to ignition and choke also goes to the oil pressure switch from the ignition switch.

View attachment 366206

Do you have the correct wiring diagram for the oil pressure switch. As in which terminal the purple/yellow and purple wires are suppose to connect to? Maybe they were put on backwards?

 

[achris]

Do you have the correct wiring diagram for the oil pressure switch. As in which terminal the purple/yellow and purple wires are suppose to connect to? Maybe they were put on backwards?

It's a switch. Doesn't matter....

 

[achris]

Update: I jumped the oil pressure switch and the fuel pump turns on and fuel does pump into carb with the key in the ON position. What does this tell me?

That the oil pressure switch is either faulty, or you have corroded connectors (the usual 'failure mode' for these switches)..

By jumpering out the switch, you just proved everything else in the system is working correctly.

Chris.....