Submitted by Craig Evers

One the the least appreciated materials in her fuel injection mechanism is the fuel pump inertia switch.

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This tiny black crate is design to duty flawlessly forever (or close to it), for this reason it’s basic to forget it’s there. Its purpose is to shut off the fuel pump(s) in instance of one accident, thus avoiding fuel from being pumped all over the automobile if there is a leak in the system. This clear is a worthwhile thing to do, and one less thing come worry around if you room hanging from your seatbelt through your wheels in the air!

But what wake up if this move goes poor on you? how will friend know, and also how have the right to you figure it out?

In my case, the move went bad slowly over a duration of months. It to be exceedingly frustrating, and also not just a small bit expensive, too! I gained to fulfill a many nice wrecker drivers.

I to be glad (for once) the the agency had hung a cell phone on me as I sat at the side of one Iowa freeway city hall a beautiful sunset over the snow-covered fields, through a strong wind rocking mine Bronco II at -20° F! I had been control along, hear to melody on the radio, enjoy it life as soon as the engine died. No warning. Dead. Similar to I had turned the an essential off. The just sound to be 4 Goodyear GSAs whining under as i coasted to the side. We changed the high push fuel pump that time, and the next time, as soon as it died in a parking lot, we changed the filter. The following time, as I was pulling into a parking space 20 miles from home, that did the exact same thing. Together I sat waiting because that my wife to come gain me, i heard a “click” that i couldn’t place. Ns tried the key, it fired best up, and also I drove residence with no problems. I heard that “click” one various other time, and also it ran after, too.

When it would run, that ran fine. As soon as it wouldn’t, there was nothing. Eventually, at some point in the spring once it did that again in the parking lot in ~ work, ns did some thinking. The just things needed for the engine come run space compression, fuel, and gas. I can’t think of a self-healing condition related come compression that can give me the symptoms i was having. I had actually spark, as displayed by the use of a spark tester. The left fuel. Ns cranked, and it wouldn’t fire, so ns popped the hood and also checked system fuel pressure. Over there is a tiny test port on the fuel rail the resembles the inflation stem of an inner tube for a bicycle tire. If you take off the cap, and also push top top the stem in the middle of the tube, you should get a squirt that gas if there is pressure. I gained nothing, proving that the pump was not working. There was plenty of gas in the tank, however the pump wasn’t getting it to the engine.

I go under the earlier end to see I could confirm my diagnosis. While my mam cranked, ns tried come feel any kind of pump vibration or noise. Nothing! The ’88 Bronco II has actually a low pressure pump in the tank, and a high pressure pump ~ above the framework rail. Both of them were dead. This is good. If one was dead, I’d need to replace a pump, however if they’re both dead, it way there is one electrical difficulty upstream from them.

I spent some time v the EVTM (Ford’s Electrical and Vacuum Troubleshooting Manual. If girlfriend don’t have one for your model and also year, obtain one! it is fine worth that to have every circuit in the van diagrammed and also explained in substantial detail.) Looking at the fuel system, there to be a pump relay, the inertia switch, and also a couple of connectors. It didn’t take lot time to prove the the relay was great and the connectors were hooked up. That just left the FPIS (fuel pressure inertia switch), which is located under the dash in the passenger foot area, simply at the front edge of the carpet. I propelled the RESET button on peak of it, and the motor cranked appropriate up and ran like it was claimed to. Ns drove straight to my neighborhood McParts store, and was told the the FPIS is a “dealer only” part. Once I went earlier out, it wouldn’t start. Ultimately I held the RESET button down if my mam cranked, and also it started. At home, it wouldn’t start again, no way, no how, so ns jumpered it the end by unhooking the connector and sticking a item of wire in it across the two terminals. This is great enough to acquire to the dealer because that a brand-new one, or come limp residence in an emergency, however it’s not a an excellent idea to journey without the protection the FPIS offers you in an accident. Since I replaced the switch, i have had no much more problems v this truck. The brand-new switch was $40-.

So how does this switch work? since I’m an engineer, and also curious by nature, i did an autopsy, and took a couple of pictures the the functions of this part. It’s a neat item of engineering. Somebody put a many thought into it. Hope you won’t have the same form of trouble I had actually with it, however just in instance you’ve been wondering around it, here’s just how it works.

Switch Requirements:

The move is designed to open when a sudden acceleration is skilled by the vehicle. This could be the an outcome of hitting one object, such as one more vehicle, a tree, or a wall. It might be the an outcome of the car rolling over, as right into a ditch or turn off a roadway shoulder (or worse!) It must stay closed in ~ all other times, also when bouncing follow me an ugly logging roadway or over potholes in a paved road. If it activates, and also the vehicle is quiet drivable, it need to be quickly resetable so the you deserve to drive away from there. It should never open otherwise (this is wherein mine let me down!)

Switch Design:


Internally, the switch is very similar to a relay. It has a couple of contacts that hook come the connector in the automobile wiring. One is for power coming in, the other is for strength going the end to the pumps. In this picture, you deserve to see that the switch is closed. The circuit is complete, and also the pumps will run.

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Instead the the electric solenoid coil or heat expansion device that a relay uses to break the circuit, this switch offers a mechanical element consisting that a steel sphere in a funnel. The ball is organized in ar by a magnet in ~ the bottom that the funnel. When subjected come the design shock forces, the sphere will break away from the magnet and also roll up the next of the funnel, hitting the eight of the switch mechanism.