Electric Fuel Pump Nightmares

tank1949

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I have 1994 Mercruiser Alpha 1 Gen II/ motors that have been replaced at least a couple of times. The latest motors, 5.7 Chevys both have electric fuel pumps and the pumps appear to be automotive with jury rigged wiring. I am trying to make a safe as possible. I have read other posts explaining how I should run a hot wire from ignition source (starter relay or alternator EXC) and a parallel wire to oil pressure switch so after initial start up, the 12 vts going to oil pressure switch would maintain power to the fuel pump. While running, if motor stalls and I lose oil pressure, the switch forces current to ground. However, there is a problem. I assume the oil pressure switches described in other posts are three spade configuration. My boat has audio alarms connected to temperature and oil pressure switches. My oil and temperature switches have one threaded post each. Can I get these oil pressure switches to work for both safety and send audio alarm????? I don't see how? It seems to me that parrel wiring up this on single post switch would be a potential fire hazzard or at least blow fuze if current goes to ground??????
 

alldodge

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Re: Electric Fuel Pump Nightmares

Your going to need to and a brass Tee to the rear oil pressure port. Tie them both in to the same port.
 

alldodge

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Re: Electric Fuel Pump Nightmares

While running, if motor stalls and I lose oil pressure, the switch forces current to ground. potential fire hazzard or at least blow fuze if current goes to ground??????

I need to add the issue about sending 12V to ground if the oil pressure drops. The 3 prong switches have a Common (C), Normally Open (NO) and Normally Closed (NC) connections. The C is connected to the pump, the NC side goes to the starter solenoid so when the key is turned to start 12V is applied through the switch to the pump. Once the engine is running the oil pressure moves the switch to the NO contact. The key switch is released and 12V comes from the key run side to the NO contact which keeps the pump running. If the engine looses oil pressure this opens the circuit and the pump stops. The link below is a good illustration

Google Image Result for http://waterdecontamination.tpub.com/TM-55-1930-209-14P-9-2/img/TM-55-1930-209-14P-9-2_247_1.jpg
 

Don S

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Re: Electric Fuel Pump Nightmares

Here is the wiring diagram of how Mercruiser wires them.
Ni wiring from the ignition switch back to the engine for the fuel pump.

Merc fuel pump wiring.jpg
 

tank1949

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Re: Electric Fuel Pump Nightmares

Don,


This wiring diagram shows no in line fuse to pump and implies a single pole oil pressure switch, which I have and it is connected to an alarm system. Yet, I don't see how it would ever work, since the current would go directly to ground and never get to fuel pump until oil pressure was built up. And, to me, and my simple mind, this set up would melt wire. Something is missing? The three pole offers a better solution, but if this switch fails, you may be dead in the water. Since my set up has audio alarms, it seem to me this is better set up. Interesting, Serra tech and I spoke yesterday and he indicated that I needed three pole switch, but that would negate the alarm function
 

Don S

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Re: Electric Fuel Pump Nightmares

You don't use the alarm switch, that one is for the alarm only. The fuel pump switch is for the fuel pump and choke and the OEM switch is designed to carry more amperage than the alarm switch. You need the correct switch.
The purple wire in the drawing is fused at the helm. No separate fuse for the fuel pump is needed.
 

tank1949

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Re: Electric Fuel Pump Nightmares

So, if I understand you correctly, I'd have two oil switches (one for alarm and one for fuel pump) and one oil sender switch?
 

Don S

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Re: Electric Fuel Pump Nightmares

So, if I understand you correctly, I'd have two oil switches (one for alarm and one for fuel pump) and one oil sender switch?

That's it.
 
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