Hey all... I just switched to an electric fuel pump (Airtex 8251) on my 1950 Mercury (bad vapor lock on really hot summer days). I am going to use one of the 3 prong oil pressure fuel pump shut off switches that have "IGN", "START" and "PUMP". I have an aftermarket wiring harness from Ron Francis. My harness comes with a 15 amp fused fuel pump wire already and at this point don't THINK I need to use a relay (correct me if wrong). The wire has a constant 12v with the key turned on... So my question is this: Looking at this picture that does use a relay, and assuming I don't need to use one, which terminal would the harness pump wire go to on the switch?
What is the current rating of your fuel pump? I doubt that it's anywhere near 15 amps my fuel pump pulls 4 amps. Since Ron Francis already furnished a 15 amp circuit I would say that you don't need any relay in which case you would hook the fuel pump directly to that P terminal on the oil pressure switch.
I sort of think of a relay as an upgrade that extends the service life of a switch. But I am clueless about the interworkings of those pressure switches as regards to arcing of the contacts, etc., so I defer to those who DO know.
those pressure switches last a long time without a relay...even with a pump like a Holley. I've been using them for decades. They will eventually leak, like most pressure switches. The original application was carburated GM cars with an in tank pump, such as the Riviera and Monza.
It is customary to start a thread for another topic, rather than simply adding an off-topic question, randomly to an existing thread. You will get much better visibility, as your potential audience will not be limited to those answering questions about adding a fuel pump to a Mercury with a vapor lock issue. And no. There is no backup light switch on your car.
The back up light switch should be on the transmission or somewhere on the shift linkage; no where else it could be.
I guess I thought I had this understood...but the more I look at it the more confused I get myself. With my harness having the constant 12V switched pump wire already... would that take the place of either the "Ignition" wire or the "start" wire on the switch? My understanding of that pressure switch is when the key is turned on, that supplies power to the pump to prime the carbs, but when the start button is pressed and oil pressure rises, that switches the connection so the pump only runs when oil pressure is present... so I'm confused as to how to use that harness pump wire as opposed to running two separate wires, one from the ignition switch and the other from the start button.
Need for a relay depends on the amperage draw of the pump. I have a Holley Red pump... no need for a relay. As far as the pressure switch goes, in the picture there are terminals at 9, 12, and 3 o’clock. They are closed (c), normally open (no), an normally closed (yep, nc). The wires in the same order are for fuel pump, 12 v power from the ignition switch, and starter solenoid. If I get your question right. Oh, and if you mix up the wires just right the starter runs when you pre lube the engine. That was a fun day.
The "ignition" wire, becaue that is the wire that provides power to the pressure switch when the ignition switch is in the "run" position.
The wire from the harness goes to the ignition post of the pressure switch but ONLY if it just has power in the ignition position (not accessory) of the key switch. Not sure if I'm miss reading your description or not but the feed from your harness only powers the fuel pump when there is oil pressure, the start wire powers the pump directly while cranking in effect bypassing the oil pressure switch.
I work at Ron Francis wiring, so feel free to give us a call and we will help you get sorted on your problem. If you can, please have your serial number from your panel handy and we will work from there. Our number is 1-800-292-1940 and we are here from 8:00 am to 4:30 pm eastern time.
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