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Electric Fuel Pump Inertia Shut Off Switch?

13716 Views 16 Replies 12 Participants Last post by  Brak
inertia switch?

I like the idea of an electric fuel pump but do any of them ...or any of the methods of plumbing them in... use an inertia switch to shut them off in case of an accident??
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even if they dont all you have to do is put 1 in the power feed for the pump and mount it to the body in the same plain as it came from
The most common method is an oil pressure switch. No oil pressure, no fuel. They switch on at about 5 psi. Then make sure the pump is wired properly via the ignition switch.

Many air bags use inertia switches. Volvo did it first, metal ball suspended between 2 springs. The problem is these are directional. Your car could get hit from any angle. I guess a spring loaded pendelum could work similarly. Hmmm... New invention???
Ford makes a shut-off switch (mid 80s cars anyway) in many cars it was in the trunk; worked from Gs from any direction, and heaven forbid, roll overs. Easilly resetable. It used the ball bearing and magnet setup. Works fine; fails safe.

Actually, I need one of those; why don't I have it in yet?

There's one of those in the Lady's 95 T-Bird. I don't know the mechanics of it, but I guess most of them are similar in nature. I know most, if not all, electric fuel pumps only work when there is oil pressure after the engine starts, and operates momentarily during starting. So if the engine quits, so does the fuel pump. Maybe something I can incorporate on my GT engine swap, 70 psi fuel spraying around is not a good thing.

I moved this to the Fuel System group.

Wasn't there a thread on this last year? The oil pressure sending unit idea came up, as did the inertia switch (I wonder how many Found On Road Deads are called that due to the inertial fuel pump switch? I rented a Mustang GT last summer, and it had a note right on the visor to tell you to re-set the inertia switch if it died).

I also recall that the alternator field coil terminal was another choice, and it was suggested to hook it up via a relay (so is that terminal D+ on the stock GT alternator?). But in either case, you need a bypass to the "start" wire to energize the pump while cranking, or it will be VERY hard to start!
keith i had a look for the thread you mentioned but could not see it so excuse a repeat
it may just be me but if you run the fuel pump of the oil sender then you have to crank the motor for several seconds to get pressure before you then get fuel which then has to build pressure
this to me would kill batterys at a fast rate as you are cranking more than nessesery
if you run it of the alternator you have to wait or the engine to spin up then get pressure in the fuel line this would be the same as above
the hole point of a cut off is to make the engine safe so if it goes from an ignition switched fuse to the cutoff then the pump you get fuel pressure first as you turn on the ignition then when you crank you will fire up quicker so saving the battery
i like the ford system as it is very sensitive
mind you we have less potholes than the states and the big C
and you can still have oil pressure if a car rolls over so still have fuel pumping(see pic of my ford http://www.opelgt.com/photos/showphoto.php/photo/2223/password//sort/1/cat/500/page/1 )
its no biggy to get out and set a trip once every now and then:D
O.K. guys, here's the deal. My monza has an electric fuel pump in the tank that works off the oil pressure transmitter. As long as I drive the car daily, no problem, the fuel remaining in the carb bowl allows a quick start next day. If I don't drive it for three days, it takes about 30 seconds of cranking for the fuel to get pumped up to the carb and it will start. Something I don't like to do. On the V-6 computer, there is a fuel request circuit that will energize the pump relay during the start cycle. If the fuel system is nice and tight, there should be residual pressure in the fuel rail and lines clear to the pump, so it will start almost immediately once the pressure builds up. Also there is a cold start cycle in the computer that dumps more fuel in the combustion chamber for assisting on the start. It's hard doing this much thinking so early in the morning, haven't finished my first cup of coffee yet.

Typically, the oil pressure transducer is bypassed when the ignition key is in the start position. Therefore, if the starter is on OR the engine has oil pressure, then the fuel pump can operate. There's no unnecessary engine cranking. Earlier cars did not have this feature.

One of the downsides of an electric fuel pump is low current available during engine cranking. The starter is sapping it, therefore, the pump can't get to the desired pressure. I guess you can use a primer button if this is a concern.
On my monza, I could put a switch in the wire that goes to the oil pressure transmitter to open that ground circuit, that way before I attempt to start the engine, the fuel pump circuit would not see a ground and allow the pump to run with the key in the on position prior to starting. But then I'd have to hide the switch, and remember to turn it off/on before I start the engine. If I forgot to cycle the switch it would bypass the safety reason it's there in the first place. Decisions, decisions.

too difficult all together

The Opel oil pressure switch only closes it's contacts when there is low oil pressure, which is bass-ackward for using it as a safety switch. A different one could be substituted, or a relay would have to be added to use the origional one.

As to a signal bypassing the oil pressure switch for start up, Opel took care of that already. It's the terminal most of you are no longer using on your starter if you switched to a pertronics that was used to supply hotter spark for starting only. Easy, and no switch to remember.

Used inertia safety switches can be had for less than $5 at most yards or on e-bay. Why not use both it and an oil pressure switch?

Circuit something like this, using any old relay:

-12v fused ignition to one side of the relay coil.
-Other side of relay coil to light sending unit. (relay picks up when ignition on and low oil pressure)
-Common side of relay contacts to POS of fuel pump
-NO side of the relay to the "coil start" terminal of the starter
-NC side of relay to +12v fused ignition for fuel pump.

-Neg side of fuel pump to one side of inertia switch.
-other side of inertia switch to ground.

In this set-up, if the ignition switch is in the "on" position, and there is no oil pressure, the fuel pump will not run unless you are actually trying to start. Once oil pressure builds up, the pump will run normally. Basicly if the light is on on the dash, the fuel pump will not be running unless the motor is cranking. Added to this, though is that any time if the inertia switch is tripped the fuel pump will shut off and not run until the switch is reset.
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fuel cutoff

this $49.95 part reduces the risk of post crash fires by instantly shutting off the fuel pump. Resets with the push of a button.

wire it in series as expalined in the instructions, i was at this vendor today and the part is in stock. Unless there is a more simple idea, i think i will buy one.

part # 10-739 from Victoria British LTD, 800 255 0088 order catalogues at www.victoriabritish.com


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E-pump safety devices

As oldopelguy said, you need both safety devices (inertia and OP cutoff switches) for complete EFP safety. The OP cutoff switch needs to be the SPDT (single pole, double throw) type with 3 contacts (NAPA OP6610) and wired as follows: common - FP+, nc - ignition switch start contact, no - switched 12V.

The best and quietest low pressure/high volume FP I've found, and which I use on my 2.15 SSD GT, is the Carter P60504 gerotor pump. The attached show the OP switch and fuel pump.


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I knew my old fuel pump in the Gt was going out but didn't realize how bad it was. I put in a new electric fuel pump wired to the fuel pressure switch along with a MG-b inertia switch (ebay- $5 plus shipping ) just for good luck and fire the old girl up. OMG- what new power I have especially at the higher RPM's. Down side is that I put the new pump on the engine side of the old pump without by passing the old one- so I literally have two pumps working at once- Is this a bad thing?? :D
If your looking for a cheaper way to get it, my friend has a 93 crown vic, and the reset switch is in the trunk, we had to reset it after he was rear ended before, and its just a push of a button, and i imagine it wouldn't be too expensive since they're relatively common, just a thought
Tom, in my 454 cu. in. motorhome, there are electric pumps at the two fuel tanks, main and reserve, plus the engine mounted mechanical fuel pump. I would think two things, if you aren't losing oil at the engine mounted pump and haven't ruptured the diaphragm in it, pumping gas into the oil pan, you should be o.k. HTH.
As oldopelguy said, you need both safety devices (inertia and OP cutoff switches) for complete EFP safety. The OP cutoff switch needs to be the SPDT (single pole, double throw) type with 3 contacts (NAPA OP6610) and wired as follows: common - FP+, nc - ignition switch start contact, no - switched 12V.
I know this is an old thread, but in searching i found some useful information that supplements the info provided by Tekenaar...

http://www.studebaker-info.org/text3/fpsafetysw.html said:
(June 2007)
GM oil pressure switch..... 1/8" NPT screw-in switch with three flat-blade terminals. These switches areavailable from most local auto parts stores. Three part numbers: NAPA (Echlin brand) p/n OP-6610 or Standard brand p/n PS-133 or Delco PS-9. While at the parts store, also get a standard sealed-beam headlight 3-prong socket connector. This connector fits the new oil pressure safety switch. You may also need a couple of 1/8" pipe tees and short nipples if you also have an oil pressure gauge.

The switch has three prongs marked "I", "S" and "P." When there is NO oil pressure, the "S" and "P" are connected. When there is oil pressure, the "I" and "P" are connected.

The "I" wire is connected to 12 volt ignition feed ( 12V side of the Ignition dropping resistor if you have one). The "P" wire goes up to the pump relay. The "S" wire goes to the starter solenoid "S" terminal to energize the pump during cranking.

If you want a pump priming switch, for occasions where the vehicle has been idle for a long period and the carburetor bowls are dry, a momentary bypass switch is wired between the "I" and "P" contacts. With the ignition on and the switch pressed, the pump will run to prime the carburetors, before turning the key to the start position.

the S wire from the pick-up terminal of the starter solenoid is energized only when the key is turned to start the motor. The current goes through the normally closed (at rest) contacts of the oil pressure switch to the fuel pump relay (between contact "S" and contact "P"). When the engine starts and the oil pressure rises, the switch transfers, causing current flow from the ignition switch (contact "I") through the transferred switch contacts to the fuel pump relay (contact "P"). As long as oil pressure is maintained and the ignition is on, the pump keeps running. If oil pressure is lost from an oil pump failure, the engine automatically shuts down.

With this setup, you can use any fused relay, like the ones with a mounting ear, and any suitable inertia switch.
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