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PostPosted: October 2, 2017, 8:00 am 
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Joined: May 22, 2017, 7:36 am
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So I want the fuel pump of my build to only run when the engine is running. I don't want to use a oil pressure safety switch. Is there an alternative way of doing it, for example energizing a relay with a current from the alternator? Or from a tach signal from a negative lead on the coil? If so, how would that be wired? Ultimately I could just hard wire a switch from the battery, but I feel like a safety option may be good. I don't have any way of mounting or using a hall type sensor, its an old Mazda rotary. Thanks guys.


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PostPosted: October 2, 2017, 1:57 pm 
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Joined: May 27, 2006, 9:46 pm
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Location: BC, Canada. eh?
A buddy of mine's using an oil pressure sensor switch to provide power to the fuel pump. The idea is that fuel won't flow until the oil pressure comes up, and that in a crash, if the oil pressure stops, so does the fuel.

Personally, mine's going to run off the normal switched ignition circuit, with an inertia safety switch which kills the pump in the event of a collision. Fords are a great source for these at wrecking yards - they're a stand-alone switch, usually easily accessible (in later Mustangs, they're in the trunk on the left side), can be reset with the push of a button, and can be tested right at the car it's on. Cheap that way, too - I picked up a pair (one for my buddy), along with a handful of other bits & pieces, all for the princely sum of $5.

These switches are easy to install - two bolts, mounted anywhere on the car on the frame itself, with the power wire for the pump going through the switch.

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PostPosted: October 2, 2017, 9:51 pm 
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Joined: December 7, 2012, 8:28 am
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Mine is wired to the ignition circuit (used to turn off the engine). Surely you have a chicken and the egg situation as the engine will not run until the fuel pump it on and providing fuel pressure.

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PostPosted: October 2, 2017, 10:11 pm 
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The easiest is an oil pressure switch, and a relay tied to the starter circuit that causes the pump to also kick on when cranking. That should get fuel to your carb and eliminates the chicken/egg issue.

A pressure switch is a good idea on a Wankel anyways, along with a reliable temperature gauge.

Happy to see another one of these gems used in a car!

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PostPosted: October 3, 2017, 12:44 am 
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Joined: December 24, 2007, 5:11 am
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I suppose that cranking speed is enough to key the oil pressure sensor otherwise it is the chicken and the egg situation. Mine is ignition on. Hope that doesn't wind up burning my ass.

As for the Ford trunk switch, i had one in a Probe. When it works (and there wasn't a major rear ender) you are puzzled why the damn thing won't start.

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PostPosted: October 3, 2017, 4:23 am 
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Location: BC, Canada. eh?
The oil pressure switch system does work - even cranking for a couple of seconds is enough to raise the oil pressure to the point where the fuel pump will kick in.

Personally, I think it's a bit overkill, thus my inertia switch plan. If you're looking for one at a wrecker, you can easily test the switch simply by thumping it with the handle of a screwdriver. The ones I picked needed quite a sharp rap to trip, so there's no way normal vehicle operations will trip it. Nonetheless, I'm mounting mine beside my (hidden) gas filler, in the event that I need to reset it. In a way, it can also be used as a heck of an anti-theft device - if you trip it before you leave the car somewhere, there's no way a thief is going to be able to start the car!

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PostPosted: October 3, 2017, 3:25 pm 
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I'm not sure if you are going the megasquirt route, but the fuel pump output from MS is dependent on receiving a tach signal. Once the engine stops spinning, the pump turns off.

There are numerous aftermarket devices that do the same thing. From Googling, perhaps even an 70's VW fuel pump relay could be wired in.

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PostPosted: October 4, 2017, 5:51 pm 
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Joined: March 30, 2011, 7:18 am
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Location: central Arkansas
benny_toe wrote:
I suppose that cranking speed is enough to key the oil pressure sensor otherwise it is the chicken and the egg situation.


Early GM ECMs turned the fuel pump on when the engine was cranked. It would keep the pump running, then look for oil pressure - they used a two-contact oil pressure switch. If there was no oil pressure the ECM turned the pump off. Otherwise, the ECM went into normal 'run' mode... and never looked at the switch again. The engine would run just fine without oil or upside-down, at least until the fuel pump sucked wind.

I spliced a Ford inertia switch into the ECM power and mounted the switch on the console by the shifter in case I ever needed to reset it.


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PostPosted: October 4, 2017, 5:51 pm 
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Argh. Double-tap again. The forum doesn't like exiting a thread with the back button any more...


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