Oil pressure switch wiring... To coil or not to coil?

[7MGFORCE]

Hi

 

I'm wiring a holley oil pressure switch to turn my fuel pump on / off as a safety measure.

 

The switch has 3 connection

1. To starter litle wire

2. Fuel pump

3. ????

 

I have read that I connect it to a 12v key on switch and NOT to connect it to possitive coil. BUT holley instruction said to connect it to possitive coil.

 

Which connection is better? I was going to connect it to one harness that I found near the radio that has 12 v key on. (the plug does not connect to anything)

[trwebb26]

Gotta think - it is just a switch... that is it. Nothing fancy.

 

connection 1) a ground reference

connection 2) voltage input

connection 3) when you reach the "trip point" for the switch (7psi for mine) - the switch will activate and whatever voltage input you put to connection #2 will go to connection #3

 

So as long as connection 2 is something that is on when the key is on and you expect the fuel pump to be running under "normal" circumstances - connection 3 should run to a relay to power your fuel pump. Make sense?

[7MGFORCE]

Gotta think - it is just a switch... that is it. Nothing fancy.

 

connection 1) a ground reference

connection 2) voltage input

connection 3) when you reach the "trip point" for the switch (7psi for mine) - the switch will activate and whatever voltage input you put to connection #2 will go to connection #3

 

So as long as connection 2 is something that is on when the key is on and you expect the fuel pump to be running under "normal" circumstances - connection 3 should run to a relay to power your fuel pump. Make sense?

 

Your switch must be different as it should not have a ground connection.

 

Is the relay needed this is the first time I have heard this. Even the instruction does not state adding a relay.

I know it's just a switch but I want to know for the constant key on 12v does it matter if it's comming of + coil or not

[Oddjob]

I'm in the process of installing a Holly fuel pump and the pressure switch. If it would help you, you can follow along here:

 

http://forums.hybridz.org/showthread.php?t=147928

 

and scroll down to post # 19 and # 21 (I'm working on 22 now).

 

To answer your question: first, don't ground any of those contacts!

 

The three contacts on the switch are marked "C" (common), "NO" (normally open), and "NC" (normally closed).

 

Common goes to the positive wire on the fuel pump.

Normally closed goes to the positive terminal on the starter solenoid. (The small terminal that goes positive when the key is held in "start" position, not the big one connected to the battery).

Normally open goes to any positive source which goes live with the key on. This one should have a 15A fuse inline. Don't use the coil, as it is often fed by a resistor wire, which will only apply 6V to your pump.

 

What happens with the above connections is this: When you crank the engine, the pump will run only while you are cranking. When the engine starts, and the oil pressure comes up, the pump will run. If the engine dies, the pump will stop. This switch is a safety, to keep your electric pump from squirting fuel onto a fire in a collision. It makes your electric pump act more like a mechanical pump in this regard.

[7MGFORCE]

I was mainly asking about the nc and since I baypass my resistor would that work?

[Oddjob]

I was mainly asking about the nc and since I baypass my resistor would that work?

 

The "NC" goes to the starter solenoid.

 

The "NO" could go to the coil, if the wire and fuse feeding the coil are big enough (they probably aren't), and you are certain you have 12-14V on the terminal. The wire you found under the dash is a much better candidate since it is probably fed by your fuse panel.

[7MGFORCE]

Okay connected it like stated above but why does it keeps blowing the 20amp fuse on the stock fuse box? When im using the holley switch it blows it almost instantly. However using the same feed 12v ign but wired directly to fuel pump it runs on 10amp fuse perfectly and not blow it. I'm using 10gauge wire When I wired it btw. Any ideas on why it would blow it?

[Oddjob]

Okay connected it like stated above but why does it keeps blowing the 20amp fuse on the stock fuse box? When im using the holley switch it blows it almost instantly. However using the same feed 12v ign but wired directly to fuel pump it runs on 10amp fuse perfectly and not blow it. I'm using 10gauge wire When I wired it btw. Any ideas on why it would blow it?

 

Yes, I think I do.

 

You are ahead of me in your installation. Mine is wired but not connected, as I haven't installed the fuel lines at the pump. Your post caused immediate anxiety, so I ran out and verified the circuit with a VOM. Thank you! I discovered on mine that the NO and NC connections were reversed. The markings are hard to read anyway, and half under the terminals. I'm not certain whether the switch is mis-marked, or my eyes are going. OK, I'm certain. The !@#^**!! switch is mis-marked.

 

If the COMMON is connected to anything but the pump, it will blow fuses. With a VOM on resistance (ohms), verify what the switch does. This will also eliminate the switch as defective. Disconnect all wires from the switch. The switch logic is this: engine off, C and NC should be connected and C and NO are open; engine running, C and NO connected and C and NC open. NO and NC should always be open.

 

After verifying the above, (and fixing the cross) I reconnected the switch and set the VOM to DC Volts, and checked the connection at the fuel pump. It did what was expected: engine off: zero volts; engine start: +12.4V; engine run: +14.7V and no blown fuses.

 

Here's one further test: disconnect the switch, and connect the C wire running to the pump to the NO wire running to your fuse panel. Start the car. If the fuse does not blow, and the pump runs, that eliminates most of your other wiring as a problem.

 

One more thing. The NC connection on the starter; you are on the small slide-on terminal (black/yellow stripe wire) and not on the battery connection? (You should be.) Being on the battery connection would blow fuses as well.