Electrical fuel pump

[digitech]

Right Bernie! The proper way is to have the pump run for about 30 sec at ignition switch-on and then default to only running when the engine does (like with oil press.). I'll try to come up with a schematic and post it if no one else beats me to it.

[donm]

I think the impact switch is great idea and it would be simpler to install; just mount it firmly to the body and in series with the 12v pump supply. Of coarse it would have to operate in any direction of hit and even better, upside down.

I'm sure you can buy a switch like this online but how would you decide on the proper G level? I imagine you would want a rating simalar to an airbag switch. Any other thoughts on this idea?

Don

[nibbes123]

Hi guys the inertia switch can be purchased at any Ford dealer part #XF32-9341-AA this is used in most of their cars and trucks ,the cost is very little to buy.
Nick

[nibbes123]

Hey digitech
If possible can you post a schematic like you were saying on the last post.
Nick

[Fiat88]

Interesting alternative. Hope someone came make a installation diagram and list what parts will be needing.

[baltobernie]

Hi Digitech,
Maybe a CMOS 555 timer, in keeping with the "old school" theme of the car? They're pretty cheap and available everywhere. Enough output to drive the relay coil. Why doncha make 'em and sell them?

[digitech]

As usual, I tend to speak before I think and I apologize! Thinking the situation over a bit further, it is less important ( or even not at all! ) for a carb'ed engine to run the fuel pump for a few seconds ( and I'm thinking 15 sec. or so would be better ) in the "on" position of the ignition switch. This is more a requirement for fuel injection, to ensure the lines are pressurized; with a carb, you have the float bowl already filled with fuel, if it didn't drain back! Of course, 80's series Hondas with the feedback carb used such a system and it worked well so it isn't exactly wrong to do so.......

That said, the simplest and safest way would be to use an inertia switch and a relay, if the inertia switch can be bought inexpensively. It provides the most protection in an accident and uses the least number of parts unless you also use the oil idiot light output which would add another relay.

Or you could use two relays, one being the aforementioned timer relay which may be a bit expensive.

The other way is a bit more complex for some people. Use a 555 timer, set a 15 sec. interval with a cap./resistor, drive a 2n2222 transistor to switch in a double pole/double throw relay that switches in 12v through one set of contacts to the fuel pump. Run the output of the oil pressure sender ( idiot light!), which is a ground if I remember correctly, through the other set of contacts to drive another relay that supplies 12v to the pump. Add a pair of steering diodes to guard against reverse current and your're good to go. The whole thing, relays included, would fit on a 2"x3" piece of ckt. board and cost about $7 - $10. But, there is no rollover protection with this circuit - if the engine is running, there will be fuel flow. Of course, if you are upside down in a Spider without roll bars, that may be the least of your concerns.

Yes, I could build 'em but I don't know about the liability!

[DUCeditor]

use a relay, switched by the oil pressure "idiot light" sending unit. when the motor has oil pressure the fuel pump runs, when the pressure goes below 5psi it shuts off. good for 2 reasons. if you loose oil pressure the fuel supply will quit, keeping you from ruining your motor. secondly if in a bad accident the motor stalls, the fuel pump will also shut off.

How would that effect start up after off-season storage. No fuel delivery no oil pressure/no oil pressure no fuel delivery. Isn't that a catch 22?


Addendum: I posted this before reading the entire thread. Obviously I'm not the only one to have realized this possible contradiction. But, being a contrarian, I suppose that is to be expected. And on a contrary note: I just can't quite get into the safety, safety, safety thing. If I did I couldn't drive a Spider or (obviously!) ride my Ducatis. Mind you, roasting is not a good way to die, but what is?

"Add life to your years, not years to your life."

-don

[donm]

Do we even need a relay for the inertia switch? I bought the switch and just planned to hook it up in line with my electric pump. Would that be OK?

Don

[digitech]

I doubt the inertia switch wil support the current draw for the fuel pump but you can check the specs; it's always better to use relays for sourcing power.

[majicwrench]

Ford fuel pumps current runs right thru inertia switch. Yes, switch can handle the current draw of a fuel pump. Needs no relay.

[digitech]

Well, there you go then - depending on the cost of the switch, that's going to be the best, easiest way to go.

[Zippy]

Here is a picture of my suggestion for the wiring.
I would use a source from the ignition to power a relay. It can come from the ignition switch or another relay that is turned on by the ignition switch. Then I would get a new source of power from the fuse block or battery to power the fuel pump. If your choice of power is the battery you would run the circuit through a fuse or circuit breaker to kill the power in event of a short circuit. Then through the inertia switch, then through the normally open (NO) contacts on the relay, then to the fuel pump. This way the only time the pump will run is when the ignition is on. The relay will do the switching of the fuel pump motor load so it doesn't add any additional current to the ignition switch and the inertia switch will provide the safety shut off in case of an accident.
Disclaimer, This is a suggestion only and you should consult with a qualified person before proceeding with this installation.

[baltobernie]

If the only desired addition to the car is an inertia switch, it can simply be wired in-line to the OEM feed to the pump; no other circuitry is required. Just mount the switch firmly to a sturdy part of the trunk, in a vertical position, and you're good to go. In the event of a collision, the purely mechanical switch trips, cutting power to the pump. I don't know what the spec is of the switch, but it is undoubtedly many more G's than even a stiffly-sprung Spider can transmit.

The OP wanted suggestions for a circuit to supply voltage to the pump only if the oil pressure warning light was Off, except for a brief period during startup. That requires a delay circuit, reset by cycling the ignition to Off. Early OE electric fuel pump circuits did a "cheap and dirty" version of Zippy's diagram by simply wiring the pump to both the ignition terminal and the start terminal of the ignition switch. The pump ran during cranking, and also during running. Some designs may have wired an interrupt using the oil idiot light in the "ignition" feed, which would sorta kinda deliver what the OP wanted, but in overnight cold start, the pump would run for the first time only while cranking, and that would take some time to replace evaporated fuel in a carburetor (or build fuel pressure in an FI).

[Zippy]

If the only desired addition to the car is an inertia switch, it can simply be wired in-line to the OEM feed to the pump; no other circuitry is required.
Snip snip
The OP wanted suggestions for a circuit to supply voltage to the pump only if the oil pressure warning light was Off, except for a brief period during startup. .

Well I looked at the kind of car the OP has and it's a 78, same as mine and I have never seen an electric fuel pump circuit in the wiring diagram, but yes if you want to install an inertia switch in a car with factory wiring for an electric fuel pump just put it in series with the pump wiring. Also, I don't see where OP asked about the oil pressure switch, I have a problem with that idea because there are 2 conditions when that light is out, 1 when there is no power to the switch and 2 when there is no oil pressure. This is a problem because you would need to add additional wiring and relays to provide power to the pump only when the ignition is on and there is oil pressure. One of those relays would have to use the normally closed contacts and that means they would also be closed on failure, not good in my mind. I have heard that some GM cars used an oil pressure switch but it could be an entirely different unit than ours. Anyway, since the inertia switch is relatively inexpensive I decide to go that way. Being about safety, and the OP's comment about the inertia switch, I tried to provide him with a wiring diagram that would be the simplest and safest to install. Sorry if I wasn't clear in my original post.
Al