Today I tried my hand at installing a fuel pump relay and oil pressure safety switch, but didn't quite get the result I'm looking for. The fuel pump isn't supposed to run until the oil pressure sensor hits 5 psi, but right now the pump turns the second I hook the ground cable up on the battery, so obviously I've got my constant wrong, but not sure how to make it all work together. This is how I have it right now:

From the relay:

30 - to battery +
85 - to ground
87 - to fuel pump
86 - to "C" on oil pressure switch

From the oil pressure switch:

NC - to starter solenoid
NO - to ignition wire

Any ideas would be great. Thanks.

hi

might want to re think your wiring .


get a fuse holder put it as close to battery positive as you can get it .

use no 12 wire only and run to oil psi switch , 15 ib n.o.

then to fuel pump , then to solid ground ! simple effective !

no relay needed !

Definitely use a relay or even 2 of them depending on how many amps your fuel pump draws. I would also wire in a bypass switch so you can prime the fuel pump when you want to with the motor off. You can hide it too for anti-theft deterrence. I have two of them so I can turn off the pump with the motor running also.

The trigger 12v power wire is the only wire that runs through switch and fuel pump relay. You run the "on" main 12v supply wire through your relay to the pump with larger gauge wire.

Use similar sized ground wire for the fuel pump. You can use the chassis as ground, but it is better to use quality, properly sized wire directly to you battery.

A quick search on Google will give you numerous wiring diagrams to choose from. I always use Marine grade switches, fusing and wire as the quality is much higher than most readily available automotive stuff. Make sure to use quality properly sized fusing or circuit breakers to protect your new wiring.

I run a light power wire (14 ga. if I recall correctly) with a 3 amp fuse from a power supply to a toggle switch in the dash, then to the oil pressure switch, then to the trigger post on the relay. Run at least a 12 or better 10 ga. wire (that is fused) from your power supply to the relay and then the same 10 or 12 wire to your fuel pump. Good ground also.

According to what you say the NC to the starter solenoid and the NO to the ignition wire, you are feeding power from both sides of the switch. One side of the switch needs to go to the relay post that triggers the relay on. The other side needs to go to a power supply.

Wiring is not my specialty. Not at all. So forgive me if I made any errors in terminology, etc.

Suggested diagram

Ok so I did some resistance testing, results were as follows:

NC to NO = 1
C to NC = 000
C to NO = 1
Switch housing to frame = 001

I also did some other testing:

With ignition wire (86)(yellow) disconnected from C on the oil switch, there is no power to the purple (87) or yellow (86) wires at the relay. Not even when cranking. Nor is the fuel pump kicking on.

When I attach 86 to C the pump turns on, and 87 and 86 get power at the relay.

When 86 is disconnected from the oil switch NC and C have power when the key is off.

When the key is on, all three terminals on the switch have power, C, NC, and NO.

All this said, it didn't lead me to any conclusions, but I'm no electrician either. Does this lead anyone to a conclusions?