3 prong oil pressure switch and electric fuel pump, is a fuel pump relay needed?

[sdowney717]

I am converting the boat to a holley red marine electric pump.

The safety switch I am going to use is a gm 3986857 3 prong oil pressure switch. Has a common, NO, NC prongs.

Can the electric pump use the switch without a relay OR is the electric load running threw the switch too much for it and it needs a relay?


The reason I ask is simply I want to replace the current low pressure idiot light switch with this 3 prong switch without a 'T' hookup.
the 3 prong switch will just be turning grounds on and off. I will put a relay running off the start wire to the starter and that relay will just ground the pump negative wire to make it run. If I do this, then all I need is the one pressure switch to run the idiot light and the pump. The relay will turn on and ground the negative pump wire when the starter is cranked. When the oil pressure builds up, the safety switch will close and ground the negative on the pump and it will run.

If I dont do this, I will have to put in a T and have 2 pressure switches. I currently have an oil gauge pressure switch, so really then I would have to have 3 oil switches on the block. See what I mean?

Currently there are 2 oil pressure switches on the block. One for the idiot light-buzzer, the other for the pressure gauge. I dont want to loose the buzzer idiot light.

 

[PBarrett]

Re: 3 prong oil pressure switch and electric fuel pump, is a fuel pump relay needed?

I made this conversion on my boat and I love it. I also added a momentary switch at the helm to fill the carb bowl before I tried to start the engine. It made a huge difference in how much the starter has to turn before the engine starts.

I used a relay and added a O/P switch on mine, but I can see what you are trying to do. As long as you use correctly sized wire, I don't really see why it won't work properly. I don't remember the draw, but I think it is about 2 AMP, so 14-16 gauge wire should suffice, depending on length.

Good luck,
PB

 

[Bt Doctur]

Re: 3 prong oil pressure switch and electric fuel pump, is a fuel pump relay needed?

test amps by hot wiring the pump thru an ammeter . the best way would be a slave to trip the fuel relay.

 

[04fxdwgi]

Re: 3 prong oil pressure switch and electric fuel pump, is a fuel pump relay needed?

The switch is not designed to handle the amps the fuel pump draws. It is designed as a control for the higher amperage relay. The thing that hurts the contacts of the pressure switch is the starting surge (higher amps) of the pump motor and if the contacts open when the motor is running, arcing the contacts. Sure, it'll work, for a while, but will have a very short life span and crap out when you likely need it most.

The relays are cheap and should be installed, plus I would carry an extra for a "spare just in case". If you have a spare, the origional will last for ever, it seems. Alot cheaper than a tow from your favorite remote beach area.

 

[sdowney717]

Re: 3 prong oil pressure switch and electric fuel pump, is a fuel pump relay needed?

ok, the relay has to be in there anyway for a momentary switch.
also switching hots is better than ground as someone said a fire could ground pump and make it run.

here is a modified schematic. GM did not use a pump relay. Tech support said switch can handle 6 amps.

here is a circuit where the switch is used to complete the negative side of pump. Advantage was I would not have needed to 'T' the new oil pressure switch.