Mercrusier 4.3 electrical problem; ignition fuse & fuel pump

[chris paddrik]

1996 stingray, mercrusier alpha one 4.3L. The problem is 2 fold, likely related;
1. ignition fuse is blowing intermittently. its not the starter, it was installed less than a year ago and working fine. typically the fuse will blow after i start the engine, moving the the key from 'start' position and release it back to 'on'. Or it doesn't blow at all. I read about this problem and followed every wire i could find, looking for a short, couldn't find any worn wires anywhere. I disconnected, cleaned the connections, and reconnected every electrical connection, including all grounds. then the fuse quit blowing every time, but still will blow randomly.

2. the fuel pump is not getting power, so the engine never fires on. I've followed all the instructions I could to identify the problem. I jumped the fuel pump directly to the battery, and it works fine. I've checked the voltage all along the power supply to the fuel pump, and the oil pressure switch was the culprit. but when i jumped the 2 wires that go in to/come out of the switch, the fuel pump still wont come on. Next i checked the voltage at the fuel pump connection with the oil pressure switch still jumped, it reads 11-ish with the key in the "on" position, and when the key goes to start, the starter fires on, the voltage at the fuel pump drops to 8.4-ish.

So i know the oil pressure switch is bad, but that still doesn't explain why when i jump the switch the pump wont come on. And I'm pretty sure that a short is responsible for the ignition fuse blowing, but i can not locate it. So....are they related? and what do i do?

 

[alldodge]

Re: Mercrusier 4.3 electrical problem; ignition fuse & fuel pump

I take it your talking about the 50amp push button breaker and not a actual fuse, right? First I wouldn't say your oil pressure switch is bad. You can jumper it and the pump will not run so if that was part of the problem the jumper would have fixed it. You have a bad connection somewhere and may also what is causing the power drain causing the blowen breaker. Check the starter slave solenoid fist, its connections and enything which happens to be around it first. This is item no. 8 in the pic, also check and see if the fuel pump current draw. The fuel pump works when you go stright to the battery but if you use a small 16awg wire does it get warm?

 

[chris paddrik]

Re: Mercrusier 4.3 electrical problem; ignition fuse & fuel pump

no this is the 10 amp 'ignition' fuse under the steering wheel, next to the fuses for the radio/nav lights/blower ect. when that fuse blows all the gauges die down and there is no power to anything.
I'l check the the solinoid. but i do think the oil pressure switch is bad, b/c when I check the voltage, there are 11 volts going into the switch, and when i leave it connected, and check the voltage at the power going into the pump, there isnt anything.
thanks

 

[Don S]

Re: Mercrusier 4.3 electrical problem; ignition fuse & fuel pump

no this is the 10 amp 'ignition' fuse under the steering wheel,

Might be because it should be a 20A fuse. #9 in the drawing from the service manual.

 

[alldodge]

Re: Mercrusier 4.3 electrical problem; ignition fuse & fuel pump

no this is the 10 amp 'ignition' fuse under the steering wheel, next to the fuses for the radio/nav lights/blower ect. when that fuse blows all the gauges die down and there is no power to anything.
I'l check the the solinoid. but i do think the oil pressure switch is bad, b/c when I check the voltage, there are 11 volts going into the switch, and when i leave it connected, and check the voltage at the power going into the pump, there isnt anything.
thanks

I'm sure you know this but just in case. If your checking it at the pump connector it may also be the connector has an problem. 12V on PUR wire, 12V on PUR/YEL during cranking only, once started oil pressure switch closes and 12V passes to pump

 

[chris paddrik]

Re: Mercrusier 4.3 electrical problem; ignition fuse & fuel pump

wow i feel stupid. ive been staring at the manual for 3 days now and never figured that out. the fuse is supposed to be 20, there was a ten in there to begin with, and that is was i was replacing it with. so the fuse issue seems to be solved. thanks.

Now, on to this fuel pump. i dont understand what you're saying in the prior post "12V on PUR wire, 12V on PUR/YEL during cranking only, once started oil pressure switch closes and 12V passes to pump"

 

[thumpar]

Re: Mercrusier 4.3 electrical problem; ignition fuse & fuel pump

The pump gets switched on by 2 sources. One is when the starter is cranking and the other is the oil pressure switch.

 

[Don S]

Re: Mercrusier 4.3 electrical problem; ignition fuse & fuel pump

Now, on to this fuel pump. i dont understand what you're saying in the prior post "12V on PUR wire, 12V on PUR/YEL during cranking only, once started oil pressure switch closes and 12V passes to pump"

When the starter is cranking the engine, power from the starter solenoid is powering the fuel pump. When it starts, the power to the fuel pump shuts off.
Since the engine is running, the oil pressure switch now has pressure and closes allowing voltage from the ignition (purple) wire to power the fuel pump.
This is done to keep the fuel pump from running when the engine isn't. It's a safety issue.