Fuel pressure safety switch merc 4.3

[xlav8r]

I am converting over to an electric fuel pump on a 4.3 mercruiser and the switch supplied by parts man looks to me to be a oil pressure sender(one electrical connection terminal). can someone show me what the oil pressure switch should look like. How many electrical connection terminals should it have?

thank-you
Brent

 

[lime4x4]

Re: Fuel pressure safety switch merc 4.3

there should be 3 connections

http://cgi.ebay.com/ebaymotors/Holl...ptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories

 

[rodbolt]

Re: Fuel pressure safety switch merc 4.3

depends on the switch and who marinized it.
if its a single terminal switch its should be normally closed and is used for the pump ground path.
once oil pressure is above about 3 PSI the switch is closed, drop pressure the switch opens the ground path and the pump stops.

 

[lime4x4]

Re: Fuel pressure safety switch merc 4.3

I like the holley's one cause it closes at 15 psi

 

[xlav8r]

Re: Fuel pressure safety switch merc 4.3

Thanks fellas, rodbolt's explanation would makes sense then,to me as per the switch they supplied. Rodbolt, I must then assume that I will hook up a hot wire (only in the run position from ignition) to the + side of the pump and so when the engine cranks over to start, the oil pump develops enough pressure to close the switch, providing ground and allowing the pump to continue to work until you lose oil pressure (turn off ignition) Is that correct?

Brent

 

[achris]

Re: Fuel pressure safety switch merc 4.3

Thanks fellas, rodbolt's explanation would makes sense then,to me as per the switch they supplied. Rodbolt, I must then assume that I will hook up a hot wire (only in the run position from ignition) to the + side of the pump and so when the engine cranks over to start, the oil pump develops enough pressure to close the switch, providing ground and allowing the pump to continue to work until you lose oil pressure (turn off ignition) Is that correct?

Brent

No. The oil pressure switch is a safety feature to stop the pump in the case of an engine stall with the key still ON.

The pump wiring should be such that the pump is supplied with power ONLY if either of theses conditions are met.
1. Starter motor is running, or
2. oil pressure is present and the key is in the ON position.

The oil pressure switch needs to have 2 terminals. It is in the main power supply wire between the ignition key and the pump itself. There should also be a wire from the starter motor to the pump. This provides power to the pump so the engine doesn't need to have oil pressure to get fuel up to the carb during cranking....

Chris.......

 

[achris]

Re: Fuel pressure safety switch merc 4.3

I like the holley's one cause it closes at 15 psi

And these engines have an idle oil pressure of around 10 psi (4 psi is minimum spec), so that would work well....

 

[Don S]

Re: Fuel pressure safety switch merc 4.3

The switch only has 2 terminals.
Look at this wiring diagram, it will be obvious.

 

[xlav8r]

Re: Fuel pressure safety switch merc 4.3

the switch they sent me only has one terminal and the package that it came in noted it was for electric fuel pump. They must have given me the wrong switch.

the electrical schematic that I have down loaded for a 92 4.3 mercruiser shows a purple wire from alt. going to the pressure switch which then connects to a purple/yellow wire that on one end attaches to the + side of the pump and on the other end connects to the starter coil.

I am a bit confused

Brent

 

[Don S]

Re: Fuel pressure safety switch merc 4.3

A one terminal switch will work, but it's kind of a low rent way of getting the job done.
You end up having to hook up your ground wire from the pump to the switch. when oil pressure builds the circuit is completed and the pump will run.
Of course there is no way of running the pump to fill the carb during starting. Not good if your fuel leaks/evaporates out of the bowl after setting for a couple of weeks.
Then you just wear your starter motor out trying to get it started.
CHEAP and EASY is only good for so much, then it costs a lot more in the long run.

 

[xlav8r]

Re: Fuel pressure safety switch merc 4.3

would you know of a part # or ? that I should be looking for. It makes sense to wire as you said but I am not sure as to what I need to ask for at the parts store. When I told them what I was doing , they sold me the single terminal switch with the fuel pump, is it two terminals on the switch that I am to look for with a 6 psi min activation?

B

 

[achris]

Re: Fuel pressure safety switch merc 4.3

The Merc part # for the right switch is 87-864252A01. Costs $28.38 from www.mercruiserparts.com. I know it sounds a bit pricey, but it is the right switch (2 terminals), and marine rated....

Chris..........

 

[Don S]

Re: Fuel pressure safety switch merc 4.3

You need a 3 to 5 psi Normally Open oil pressure switch with 2 terminals. Many of generic 3 terminal switches are both NO (Normally Open) and NC (Normally Closed) in the same switch you just need to know which terminals to use.
or make it simple and go to your local Mercruiser dealer and tell them you want the oil pressure switch kit part number 87-864252A01. With the new numbering system, just drop the 87- in the part number.

 

[xlav8r]

Re: Fuel pressure safety switch merc 4.3

thank-you fellas, I will get this switch

B

 

[xlav8r]

Re: Fuel pressure safety switch merc 4.3

which terminal on the starter

Brent

 

[Don S]

Re: Fuel pressure safety switch merc 4.3

which terminal on the starter

Brent

For what?

 

[Bt Doctur]

Re: Fuel pressure safety switch merc 4.3

Already had the merc one fail, I think because the contacts are for "relay operation" vs direct hookup.I use PS127 or PS126 Standard Automotive

 

[xlav8r]

Re: Fuel pressure safety switch merc 4.3

which terminal on the starter does the other lead from the oil pressure switch connect?

 

[Don S]

Re: Fuel pressure safety switch merc 4.3

The purple/yellow wire goes from terminal D of the starter solenoid in the picture below to the oil pressure switch.

 

[xlav8r]

Re: Fuel pressure safety switch merc 4.3

thank-you Don S, I don't see a fuse in the electrical diagram, there should be one , correct?