4.3l mercruiser fuel pump voltage

[The_Only_Gert]

Hi guys first time posting here! I have a 92 mercruiser with the 4.3l carb model with an electric in line fuel pump....serial number is 0D814708..long story short I bought a fuel pump, worked for a few days on the muffs then quit on me, I took it off and hooked direct 12v to it via a battery and when I did I could hear it click but not pump, to I kept tapping on it till bam! It started working again and pumped fuel, however it didn’t sound healthy so I got a replacement...these are from summit racing, the $110 pumps that are 9psi...anyway the 2nd one did the same thing but no click this time just completely dead...so while the engine is turning over I check the voltage at the pump wires, my multimeter is reading around 8 volts when cranking...was unable to see what the voltage was when running...so my question is, what is the correct voltage supposed to be when cranking and when running? No fuel leaks so I know it’s not sucking air, had fuel all the way till the bottom of my fuel pump when disconnected...also the purple wire that is power for the fuel pump, according to the diagram it feeds off the starter stud as well as the oil pressure sensor thing...now on my 3 post starter I have the main red and orange feed coming to the big terminal, a yellow/red going to the right terminal and a purple on the left terminal...the terminal that has the purple wire is broken, I can wiggle it with my fingers but still has voltage no matter how I wiggle it, so is that my problem? Sorry for the lost post but I just ordered a expensive replacement pump and don’t want to fry that either.

 

[alldodge]

summit racing, the $110 pumps that are 9psi

Any pressure above 7 psi is going to blow pass the float valve in the carb and cause flooding. Will need to install a regulator to use.

multimeter is reading around 8 volts when cranking
the terminal that has the purple wire is broken

There should be full 12V when cranking or running. The 8V and loose terminal says its barely making connection and is unable to pass enough current to run the pump

Take the starter to a repair shop or get another starter

 

[Bondo]

.the terminal that has the purple wire is broken, I can wiggle it with my fingers but still has voltage no matter how I wiggle it, so is that my problem?

Ayuh,...... Welcome Aboard,...... That terminal supplies the fuel pump with battery voltage, when the starter is engaged,.....

So, Ya,.... that's probably atleast part of yer problems,.....

 

[Scott Danforth]

so what pump did you buy? because a marine electric fuel pump kit is about $80

clean all your connections everywhere in the boat. Start at the battery and dont stop until you have cleaned every connection all the way past the helm to the bow light.

any time your voltage drops below 10 volts, electronics/electrics start shutting off. below 9 volts, there isnt enough juice to fire anything - no gauges, no spark, no fuel pump, nothing. at 8 volts, you have either dead batteries or bad connections

 

[The_Only_Gert]

Here is the pump I bought https://www.summitracing.com/parts/crt-p61122

 

[alldodge]

Looked the part up at Carter and I'm not seeing max pressure only minimum pressure of 3 psi. Does list as 4.3 92-93 and 96

 

[The_Only_Gert]

Hey guys so took the boat out today on the new fuel setup and worked great! However I did notice one thing, I’m getting 14.2 at battery but on the purple terminal on the back of the alternator I’m only getting 12.5 to 13 volts, this is the voltage that my gauge also reads at the helm, I had it tested and it tested good, however my gauge fluctuated throughout the day between 12 and 13 volts, at low rpm or idle when I came into the dock the gauge dropped to around 11, this only happened When I switched to both battery’s to have the alternator charging, when on one battery the volt meter stayed around 12.5-13 for the most part, cleaned all terminals is this normal? Thanks for the previous help was a great feeling to get my first boat ever on the water!!

 

[alldodge]

Seeing lower then battery voltage on the boat gauge means there is corrosion somewhere, and could be inside the 10 pin engine connector. Its this way:

Battery to starter, to 50 amp breaker, to 10 pin connector, to key switch, then back to 10 pin connector, to motor circuits like the ALT, Its one long connection and the ground is the other side of the reading.

Reading a little different will be ok, but more then a volt is to much. That said it could also be how the hand held and boat gauge gets the ground for its reading. The Bat switch or even if one Bat is weak can cause issues

 

[The_Only_Gert]

Could you please help me find the wiring diagram? I have a few that have slight differences and want to make sure I’m following this right

 

[alldodge]

From manual 7 and shows 8 plugs and note also shows for 6

 

[The_Only_Gert]

Is that different from mine because my engine has a electric fuel pump

 

[alldodge]

Your serial number shows an electric fuel pump but my manual list electric fuel pumps came in as of 1993 and serial number 0F

In any case here is the one with electric

 

[The_Only_Gert]

If I follow your path, at the starter there should be two wires that have battery voltage, the red wire to the circuit breaker and the orange wire that goes to the alternator. So the other two terminals that are purple and red/purple should not have voltage yet correct?

Back to the circuit breaker side. The red wire to the circuit breaker turns into a red/purple wire that combines with other red/purple wires before heading to pin number 6 on the 10 pin connector. so any red/purple wire that is before the 10 pin connector that is connected to the red/purple wire from the circuit break should have battery voltage, such as the top terminal on the starter slave solenoid, and the red/purple terminal on the back of the alternator correct?

pin 6 (red/purple) then goes toward the switch, but first there is a connection, then a 20A FUSE then the switch. so after I turn the switch to on which should give voltage to the purple wires at the gauge cluster correct? which then goes to pin 5 at the connector and towards the motor. At this point the purple terminal on the alternator should have batt voltage, which splices off and goes to the oil presser switch, when the sensor closes it should send the rest of the battery voltage to the fuel pump, electric choke and the the starter. so my question is what does the purple/ yellow wire of the starter do? The electric pump should already be getting voltage from the oil pressure switch, also what does the purple terminal do on the alternator? I think one is labeled excite (turn the alternator on) and I cannot remember what the other one was for. So basically there should be little to no resistance in the system correct? if batt is a 14.5 charging the wires should be at lets say around 14 volts also. Not like 12.5 or 12 volts. Thanks for all the help you guys are awesome.

 

[alldodge]

so my question is what does the purple/ yellow wire of the starter do?

When the starter is cranking and before oil pressure builds it applies 12V to the fuel pump. Once motor starts power is removed from the purple/yellow and the pump may stop for a brief second before oil pressure builds and the oil switch supplies power

what does the purple terminal do on the alternator?

This turns the ALT ON by applying power to the field winding (excite) and the other smaller wire applies power to the regulator so it tells the ALT how much power it needs to produce to keep battery changed

So basically there should be little to no resistance in the system correct?

This is correct

if batt is a 14.5 charging the wires should be at lets say around 14 volts also. Not like 12.5 or 12 volts.

Also correct

 

[The_Only_Gert]

Okay so testing voltage following this, at my key (this is all non running) my voltage is 12.5, same as batt...if I unplug the purple wire at the key and test the key for volt drop I still get12.5 so the key is good. When I plug in the purple wire and turn the switch go on again and test the voltage drops to 12, so I have a half a volt drop. Where do I go from here?

 

[alldodge]

Back toward the motor on the Red/purple wire because there is resistance somewhere
Now if the battery is also 12 and not 12.5 then the battery or something is pulling some current

 

[The_Only_Gert]

Well when I turn the key my dash lights turn on, they are wired with the key, I did notice that with the battery charger on my battery when I turn the blower on I loose a whole volt, could that potentially be my problem? It’s getting dark here so I’ll do some more resistance checking in the morning

 

[alldodge]

Lights on with ignition is a bad setup and many of the cheaper boat builders did this because it was easier and cheaper. If all that is tied to the red/purple, it would be understandable why the voltage drops.

 

[The_Only_Gert]

the boat also has a glass fuse panel under the dash, if I remember correctly it has power all the time, unless I turn the master battery switch off.