Electric fuel pump interrupt

NHGuy

Captain
Joined
May 21, 2009
Messages
3,631
Hi, I am about to install a replacement engine in my boat without provision for mechanical fuel pump. I have a new marine pump and have been doing the reading to get up to speed,
I can understand how to run the pump during starter cranking. But I am foggy on how to make the oil pressure switch cut the electric supply if the engine stalls while the key is on..
My boat is wired as a 1997 350 4 barrrel with (full) Thunderbolt V.
I have a nice wiring diagram that shows how to use an oil pressure sender as a shut off. But I wonder if I can take the beeper circuit feed and use it for the same purpose?
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
42,308
But I wonder if I can take the beeper circuit feed and use it for the same purpose?

The beeper circuit is hot so long as the key is on, the beeper goes off when a ground is applied (outdrive, overheat, loss of oil pressure). I would suggest using any of your purple wire connections to feed the pump (ALT, coil, etc)
 

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,318
most take a lead off the + dist and use a 3 pole switch.NO from the dist, NC from the solenoid and the switch plumbed into the oil pressure gallery.
the feed for cranking only comes off the starter solenoids outboard lug.
Your also better off using a cube relay in the circuit to supply the 12v to the pump. Less chance of overloadfing the contacts in the pressure switch.
 
Last edited:

Rick Stephens

Admiral
Joined
Aug 13, 2013
Messages
6,118
It's pain, but you have to plumb in a pipe tee where your current oil pressure sender is. Add a pressure switch. Anytime there is oil pressure, there is power to the pump. This is the only safe way to make sure you don't blow yourself up if you get something as simple as a stuck float.

Rick
 

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,318
Fuel20Pump20Relay_zpszzmuczql.gif
 

stonyloam

Vice Admiral
Joined
Mar 13, 2009
Messages
5,827
Agree with the dr 100%. Use a relay to power the pump. The pressure switch contacts may not be able to handle the current draw of the pump.
 

NHGuy

Captain
Joined
May 21, 2009
Messages
3,631
Alldodge answered it right out. And no doubt I will be using a relay, that was already in the works.
So I can use the beep circuit to signal the relay, which will run my 10 amp circuit to my pump, which will receive it's power from the purple ignition-on circuit.
Voila!
Any arguments?
No big booms,right?
Rick, tear me a new one if I deserve it.
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
42,308
Yes..... and ..... No
Yes you can use the beeper circuit but you have to be on the positive side of the beeper/horn, basically your attaching to the purple wire. If you connect to the other side power goes thru the beeper before it gets to the pump.
fuel pump switch.jpg
Fuel Pump Wiring.jpg
 
Last edited:

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,318
Diagram is good, but you would have to run a seperate wire from the dash to power the relay, just as easy to pick it from the purple wire at the alt or the coil or the choke.
UntitledAW_zpstfewifz6.jpg
 
Last edited:

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,318
DSCN3508_zpsfzmstbgh.jpg
removed from a Chevy Blazer. unit was located under the hood in the center of the firewall.actually took about 8 of them from the junk yard
 
Last edited:

NHGuy

Captain
Joined
May 21, 2009
Messages
3,631
UntitledAW_zpstfewifz6.jpg

This screenshot is also in the 1998 manual #24.

How does the 98, which has an electric pump, get the oil pressure switch into the pump circuit. I'd love it if I could just do the purple/yellow wire and be done.
Interestingly that wire starts at the starter extra lead if I am reading the diagram right.

All Dodge, is your diagram going to remove voltage to the pump circuit when the oil pressure drops? I'm not seeing it like that. Wouldn't that take some kind of diode or such. When the beeper items are tripped they have low resistance, effectively completing a circuit. Right?

Perhaps the above system uses a separate oil switch as Rick was saying.

Boy, can I not figure out electricity sometimes!
 

Rick Stephens

Admiral
Joined
Aug 13, 2013
Messages
6,118
AllDodge put the oil pressure switch marked in blue. That is the main source for on/off.
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
42,308
All Dodge, is your diagram going to remove voltage to the pump circuit when the oil pressure drops? Wouldn't that take some kind of diode or such.

Yes, the oil pressure switch closes only when oil pressure has built up. If pressure is lost, switch opens and power is removed. I'm not seeing it like that. No diode is needed.

When the beeper items are tripped they have low resistance, effectively completing a circuit. Right?

The beeper has resistance, so if the beeper goes off there would still be a full 12V on the purple wire side. Only way this would change is if there was a short, and if a short happened the motor would die do to it.

The purple/Yellow wire is only charged (12V) when the motor is trying to start (cranking). Once started the purple wire transfers 12V power to the pur/yel to the pump

If you remove the relay and the fuse it is the same circuit. Once the oil pressure switch closes it supplies 12V from the purple wire on the motor.

As mentioned previously, it would be best to get 12V from the ALT or choke wire instead from the beeper circuit at the helm

Edit to clarify pur/yel connection
 
Last edited:

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,318
once the oil pressure goes away the switch opens cutting off power to the relay the purple/yellow from the solenoid can go here
Fuel20Pump20Relay_zpsdvgfnuky.gif
 
Last edited:

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
42,308
Lets try this; during cranking the motor, power comes from the starter connection to the pump. After the motor starts, key is released and power is removed from the pur/yel from the starter, engine is running off fuel left in the carb.
Slide2.JPG
When oil pressure builds up, oil pressure switch is closed and 12V power now comes from the purple wire coming from main power (ALT, and others).

Slide1.JPG
 

NHGuy

Captain
Joined
May 21, 2009
Messages
3,631
OK, on my last post I didn't even see the BRIGHT BLUE item marked oil switch in post #8. Told ya I can be dense.

And yes I'd be better off pulling my electric power inside the engine compartment.

So to backpedal a bit. How does the 1998 system from manual 24 work?

Does it have a sensor for the gauge, an oil switch for the beeper, and another oil switch for the fuel pump. Looks like it was that all along!

Sorry to be doing all this "bench racing." I wish it would warm up so I could install this stuff.
 

NHGuy

Captain
Joined
May 21, 2009
Messages
3,631
Yup, that's what Mercruiser does. 2 separate switches, one for the beeper, one for the fuel pump, and a sensor for the oil pressure gauge! Thanks for holding my hand.
 
Last edited:

NHGuy

Captain
Joined
May 21, 2009
Messages
3,631
So,reviving this for a question. Where to position the oil pressure switch? I am not feeling like it ought to be teed. Where is there a port?
 
Top