350 Mag fuel pump cuts off intermittently

[Mhill3784]

Makes sense to me. So the oil pressure sensor was only used to turn off the fuel pump on older model boats..?

 

[alldodge]

Makes sense to me. So the oil pressure sensor was only used to turn off the fuel pump on older model boats..?

Yes it had a direct feed thru with the carb motors. With the ECM it controls everything pretty much

 

[Mhill3784]

I am going to wire two jumper wires after the fuel pump connector and before the fuel pump itself connected to a volt meter and see if I am getting 12v to the pump when it cuts off. If the fuel pump cuts off and I still have 12 volts it could tell me that the pump itself is getting hit and shorting out? Is my thinking correct here.
of course if no 12volt to the pump when it cuts off then it’s somewhere in the wiring...

 

[alldodge]

Sort of, when the relay closes there will be 12V going to the FP. If the relay opens then the 12V goes away. If the relay remains closed but the pump stops, 12V can still be seen then either the pump broke connection or the ground was lost. If a ground is lost you can still see 12V going to the pump.

Checking for power is a good idea

 

[Mhill3784]

Thanks. Just pulled the connector at the fuel pump. It appears someone has put a volt meter probe into the connector and bent it. (I know the pump was replaced by previous owner)...so I will go to my Mercruiser dealer in morning and buy a new connector to wire in.

 

[Mhill3784]

If I run the voltmeter from fuel pump positive to fuel pump negative and show no voltage. Then if I run volt meter from fuel pump positive to negative on the battery post and still show positive then I will know it’s a broke ground. If voltmeter from positive at fuel pump to negative on battery shows no 12v then....

 

[alldodge]

If I run the voltmeter from fuel pump positive to fuel pump negative and show no voltage. Then if I run volt meter from fuel pump positive to negative on the battery post and still show positive then I will know it’s a broke ground. If voltmeter from positive at fuel pump to negative on battery shows no 12v then....

No, if I understand your comment correctly

With key ON (motor not running) there will be 12V on the FP harness connection for a couple seconds, then turn OFF. The relay applies 12V to FP when the relay closes.

To find out if its a bad FP ground, attach another wire to the ground side of the fuel pump and run it to another ground on the block. If the issue goes away then its inside the harness

 

[Mhill3784]

Since this is so intermittent....let’s say that when the condition does happen again and I have 12volts to the FP...BUT the FP ground is lost...can I run a seperate ground to the block or battery permanately?

 

[alldodge]

Yes, you can run an additional permanent ground to the FP and there will be no issues with running operations

 

[Mhill3784]

Your knowledge on this is incredible!!
Im running a seperate ground now. I will use the original 12v power lead with a split block and wire so I can get a volt meter reading instantly when it happens again. If 12volts is still there when pump isn’t running then we will know its the pump itself. If no 12volts then I will determine that the relay is closed. So then it’s between relay and pump. If relay has in fact opened and lost the 12v then....I don’t want it to be that lol!

 

[achris]

The ground side of circuits is a very much overlooked area. People focus on the power side, then can't understand why they have power at the plug, but it still doesn't work. Electrickery requires, not only a path TO, but also a path FROM.... Grounds are every bit as important as the positive side.....

Sidenote: Strictly speaking, DC doesn't have a 'ground', it has a positive and a negative. But since most systems in cars and boats ties the negative to the chassis/engine block, it's referred to as a 'ground'...

Chris.........

 

[Mhill3784]

That’s exactly why I’m going to go ahead and take the ground at the FP out of the question. If this was easier to duplicate then it would be a completely diff story. I’m secretly hoping that I don’t lose 12v to the pump so that it takes the ECM and Relay wiring out of the question
I do hope some others will chime in with their responses if they have any other ideas. I won’t be able to run it to see if I keep power to pump until Friday.

 

[achris]

If it was me (and BTW, my comment wasn't aimed at you, it was a general comment after years of observation.) I would run a small wire pair (light duty figure 8) up to the dash. I would connect a red LED (with the appropriate resistor) and use that as an 'instant diagnosis' light. Attach the negative lead directly to the battery negative (and rule out interference by bad 'grounds') and then connect the positive lead to any circuit under question.

Chris.......

 

[Mhill3784]

I will wire the 12v into a meter at the dash so I can see real time voltage. If the fuel pump (once started) runs off of the RPMs, would we next look at the ignition sensor in the distributor for shortly out and show no RPM to the ECM while under way?

99% of the time it primes the pump when the key is first turned which is powered shortly before it switches to turning the pump on when it reads RPM is actually there.

 

[Mhill3784]

Anyone know the part number for the Mercruiser fuel pump relay repair harness for 2006 350 mag

 

[alldodge]

Anyone know the part number for the Mercruiser fuel pump relay repair harness for 2006 350 mag

There isn't one, its either the power harness, or the ECM harness. If you fould the problem (sound like you have) try to bypass it.

Do you know if its the ground or the power leads?

 

[Mhill3784]

I actually have not found the problem. But since I have to pull the seats and everything to access anything I’m doing it all at one time. Hard wiring the fuel pump connector (someone stuck a volt meter probe in the connector) and also going to just rewire the relay harness because that’s the next weak link and about a year ago it wouldn’t engage..one time and only once.
i read a post on here where a guy had a Mercruiser part number for the relay connector but didn’t know it applied to my boat. (Mercruiser 879346A48 relay vase repair kit). Did a once over on all the wires. The one thing that I found loose was ignition sensor connector was able to be pushed in some and also there is a plastic block on top of the starter with orange and red wires...the nuts are tight but the bolt itself turns. I can’t figure out if it is supposed to or not... [h=1][/h]

 

[alldodge]

If the plastic block (pis are always best) looked like this,



Then its a 90 amp fuse used to portect the engine harness if the alternator shorts out. The fuse is inside the block

 

[Mhill3784]

Yep that’s it. Ok make me feel better, sorry I couldn’t get pics it started raining on me and had to throw the cover on. When I take it out Friday I will have a way to test 12v at the fuel pump. And going to wire in a new fuel pump relay with a way to test that the ECM is throwing a ground to close the relay if in fact the pump doesn’t have 12V. Also checked the anti siphon valve for the heck of it while I’ve got it torn down.

 

[Mhill3784]