Fuel pump/OPS wiring

[cygnus x1]

I was comparing my fuel pump/OPS wiring to what is in the Atomic 4 parts manual and found differences. My OPS has three terminals. One connected to the coil +, one to the pump and one to the coil -. The manual shows two terminals, one to the coil +, and one common to the pump and solenoid R. So what is the difference? Electrically what is the solenoid R connection +, - , other? I also wanted to add a low pressure warning buzzer. The only way I can figure out how to do it with my OPS, would be a relay, operated by the pump connection (12V ?) that would trip the relay coil and supply 12 V to the buzzer, when the voltage to the pump is cut off?

[sastanley]

Jeff, I am not an expert on this, but I don't think the coil (-) is a ground...it should have nothing except the points or EI (-) connected. The ground for the OPSS should be to a real ground source, OR what I've more commonly seen is the NPT base itself that screws into the block is ground.

The coil (+) is hot anytime the key is on, as its 12v supply is fed by the purple wire from the ignition switch.

Sometimes, there is a "hot only during cranking" terminal on the starter to power the OPSS/fuel pump while cranking. I don't know what that is labeled. Most people seem to think that a few seconds of cranking should generate enough oil pressure to trip the OPSS closed and send 12v to the fuel pump.

Man, do I love my mechanical fuel pump..if it's been sitting a while, a couple pumps on the bale right after I open the intake and my fuel pressure gauge shows 2.5 PSI and off I go. Otherwise, it instantly begins pumping with the first revolution of the flywheel.

[JOHN COOKSON]

Normal Open & Normal Close

Your OPSS probably has a normal and a normal close terminal. Normal (open or close) tells if the switch is conducting or not when a condition is present - in this case oil pressure.

So the electric fuel pump will want to be connected to the normal open terminal and the buzzer to the normal close terminal. The normal open terminal will be open (not conducting) when there is no oil pressure. When the oil pressure rises this part of the switch will close and the fuel pump will operate. Before the oil pressure rises the normal close part of the switch, which is connected to the buzzer, will be conducting and the buzzer will sound. Thus when the oil pressure rises the buzzer will become quiet and the fuel pump will start operating. You will need a cutoff switch before the buzzer so it doesn't drive you nutso when you are trouble shooting. That said the buzzer is a good reminder that the key or switch is on.

If you need to figure out if you have a normal open\close switch, if it's working, which terminal is which here's away: Install the switch and run the engine with a temporary bypass hot wire to the fuel pump and then get on the switch with an ohm meter. Or pressurize the switch in some other way.

I always fill the cavity of the OPSS with oil before I install it so I'm sure there is no trapped air bubble.

TRUE GRIT

Edit: It is not customary to run the coil through the OPSS. You may have two normal open terminals. Testing will tell.

[ndutton]

My OPS has three terminals. One connected to the coil +, one to the pump and one to the coil -.

NOO!! (I'm not as polite as Shawn)
There is no way there should ever be any wire other than the black one from the distributor on the coil '-' post (that's a white lie). The wire you have from the OPSS to the coil '-' is not correct. To explain the white lie, the ONLY other wire allowable on the '-' post is a tachometer sense wire. Some have it, some don't.

For this reason I prefer the two terminal OPSS. Connecting it is easier too because the terminals are interchangeable. This also requires a different oil pressure switch for the alarm system if installed. I prefer keeping the two systems separate. A failure of one does not affect the other.

The Moyer engine panorama on the home page is a great example of a properly wired electric fuel pump and OPSS.

[ILikeRust]

For this reason I prefer the two terminal OPSS. Connecting it is easier too because the terminals are interchangeable. This also requires a different oil pressure switch for the alarm system if installed. I prefer keeping the two systems separate. A failure of one does not affect the other.

Hear him! Hear him!

I bought the Moyer oil pressure/water temperature alarm kit, which came with a new, separate oil pressure sensor. It occured to me that I now have two (actually three) different oil pressure sensors on the engine, but after giving it a moment of thought, I liked the fact that they are separate, and I know what each one does. One is the OPSS for the fuel pump. The second is a sensor for my electric OP gauge. The third is for the alarm. As Neil says, a failure of one does not affect the others, and it should make it easier to determine what failed if one does fail.

As Neil also points out, the two-terminal OPSS just makes it a lot easier to hook up, because it's just a simple switch - either open or closed - so the terminals are interchangeable. It's kinda hard to get it badly wrong.

[cygnus x1]

I figured out what the OPS third terminal connection to the coil - was. It was used for turning off the ignition system. This was an automotive OPS. What I plan to do now is use it to supply a ground the the warning buzzer. The buzzer will have the hot lead to the OPS and this supplied ground. It will only go off when the OPS is below 6 psi and that connection is grounded. My coil - will only have the wire from the points and the tach sensor.
Thanks again for everyone help!

[ndutton]

Jeff, you lost me. A typical oil pressure safety switch doesn't make any terminal to ground as you're wanting for the alarm system. For yours to work the way you say tells me it isn't the typical variety.

My curiosity is really up now. Can you post a diagram of the OPSS wiring, both existing and proposed?