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TRS Tech Library Side Menu
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One
of the least appreciated components in your fuel
injection system is the fuel pump inertia switch.
This little black box is designed to function
flawlessly forever (or close to it), so it's easy to
forget it's there. Its purpose is to shut off the
fuel pump(s) in case of an accident, thus preventing
fuel from being pumped all over the vehicle if there
is a leak in the system. This obviously is a
worthwhile thing to do, and one less thing to worry
about if you are hanging from your seatbelt with
your wheels in the air!
But
what happens if this switch goes bad on you? How
will you know, and how can you figure it out?
In
my case, the switch went bad slowly over a period of
months. It was exceedingly frustrating, and not just
a little bit expensive, too! I got to meet a lot of
nice wrecker drivers.
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I
was glad (for once) that the company had hung a cell
phone on me as I sat at the side of an Iowa freeway
watching a beautiful sunset over the snow-covered
fields, with a strong wind rocking my Bronco II at
-20° F! I had been driving along, listening to
tunes on the radio, enjoying life when the engine
died. No warning. Dead. Just like I had turned the
key off. The only sound was 4 Goodyear GSAs whining
down as I coasted to the side. We replaced the high
pressure fuel pump that time, and the next time,
when it died in a parking lot, we replaced the
filter. The next time, as I was pulling into a
parking space 20 miles from home, it did the same
thing. As I sat waiting for my wife to come get me,
I heard a "click" that I couldn't place. I
tried the key, it fired right up, and I drove home
with no problems. I heard that "click" one
other time, and it ran after, too.
When
it would run, it ran fine. When it wouldn't, there
was nothing. Eventually, one day in the spring when
it did it again in the parking lot at work, I did
some thinking. The only things needed for the engine
to run are compression, fuel, and gas. I can't think
of a self-healing condition related to compression
that could give me the symptoms I was having. I had
spark, as shown by the use of a spark tester. That
left fuel. I cranked, and it wouldn't fire, so I
popped the hood and checked system fuel pressure.
There is a little test port on the fuel rail that
resembles the inflation stem of an inner tube for a
bicycle tire. If you take off the cap, and push on
the stem in the middle of the tube, you should get a
squirt of gas if there is pressure. I got nothing,
proving that the pump was not working. There was
plenty of gas in the tank, but the pump wasn't
getting it to the engine.
I
went under the back end to see I could confirm my
diagnosis. While my wife cranked, I tried to feel
any pump vibration or noise. Nothing! The '88 Bronco
II has a low pressure pump in the tank, and a high
pressure pump on the frame rail. Both of them were
dead. This is good. If one was dead, I'd have to
replace a pump, but if they're both dead, it means
there is an electrical problem upstream from them.
I
spent some time with the EVTM (Ford's Electrical and
Vacuum Troubleshooting Manual. If you don't have one
for your model and year, get one! It is well worth
it to have every circuit in the truck diagrammed and
explained in considerable detail.) Looking at the
fuel system, there was a pump relay, the inertia
switch, and a few connectors. It didn't take much
time to prove that the relay was good 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, just at
the front edge of the carpet. I pushed the RESET
button on top of it, and the motor cranked right up
and ran like it was supposed to. I drove straight to
my local McParts store, and was told that the FPIS
is a "dealer only" part. When I went back
out, it wouldn't start. Finally I held the RESET
button down while my wife cranked, and it started.
At home, it wouldn't start again, no way, no how, so
I jumpered it out by unhooking the connector and
sticking a piece of wire in it across the two
terminals. This is good enough to get to the dealer
for a new one, or to limp home in an emergency, but
it's not a good idea to drive without the protection
the FPIS gives you in an accident. Since I replaced
the switch, I have had no more problems with this
truck. The new switch was $40-.
So
how does this switch work? Because I'm an engineer,
and curious by nature, I did an autopsy, and took a
few pictures of the workings of this part. It's a
neat piece of engineering. Somebody put a lot of
thought into it. Hopefully you won't have the same
type of problem I had with it, but just in case
you've been wondering about it, here's how it works.
The
switch is designed to open when a sudden
acceleration is experienced by the vehicle. This
could be the result of hitting an object, such as
another vehicle, a tree, or a wall. It could be the
result of the vehicle rolling over, as into a ditch
or off a road shoulder (or worse!) It must stay
closed at all other times, even when bouncing along
an ugly logging road or over potholes in a paved
road. If it activates, and the vehicle is still
drivable, it must be easily resetable so that you
can drive away from there. It should never open
otherwise (this is where mine let me down!)
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Internally,
the switch is very similar to a relay. It has a couple
of contacts that hook to the connector in the vehicle
wiring. One is for power coming in, the other is for
power going out to the pumps. In this picture, you can
see that the switch is closed. The circuit is
complete, and the pumps will run.
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Instead
of the electric solenoid coil or thermal expansion
device that a relay uses to break the circuit, this
switch uses a mechanical element consisting of a steel
ball in a funnel. The ball is held in place by a
magnet at the bottom of the funnel. When subjected to
the design shock forces, the ball will break away from
the magnet and roll up the side of the funnel, hitting
the arm of the switch mechanism.
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In
this picture, the funnel is lying on its side, but the
ball is held in by the magnet. When I examined the
ball closely, I could see marks on its surface where
it had been rattling around loose for some time.
Evidently, my magnet wasn't as strong as it had once
been. This is a rare failure, but that doesn't make it
any less frustrating when you're driving home in a
wrecker!
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Here
is the switch circuitry in the closed (operating)
position. The long arm that reaches off to the left is
connected by a spring to a short arm that pushes
against the right contact. In addition to being part
of the circuit path, the left contact provides a
fulcrum to lever the long and short arms. The two
holes in the housing are for the mounting screws that
hold it to the vehicle body.
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When
the ball pushes against the bottom of the long arm, it
moves it up, and the linkage moves the short arm,
opening the circuit and cutting off power to the
pumps. The circuit then stays in the open position
until reset. The linkage is designed to be stable in
either the open or the closed positions. In my switch,
I found some signs of arcing on the faces of the
contact surfaces between the right contact and the
short arm, similar to the arcing on a set of ignition
points in a distributor. This indicates that there had
been a marginal connection, with vibration causing the
two surfaces to move apart slightly, for some time.
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The
white arm is the reset arm. It presses against the
spring to move both the long and short arms back into
position.
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Here
the reset arm has pushed the linkage back into
operating position. Note that the notched end of the
reset arm is lower in this position.
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This
is the other side of the switch housing, showing how
the reset button engages the notch in the reset arm.
When the button is pushed down, the top part of the
reset arm pushes the spring to the right (because this
is the other side of the housing, that would be to the
left in all the previous pictures.)
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So
there it is. The ball sits in the cup, held in place by the
magnet, until a shock hits it. The ball breaks free, rolls
out of the funnel, and pushes the long arm up, then rolls
back to the magnet and waits for the next time. The linkage
of the long arm, spring, and short arm causes the short arm
to move away from the contact, breaking the circuit and
turning off the fuel pumps. Pushing on the reset button
(which is white on this switch and red on the replacement)
causes the linkage to reset itself.
SO
If you are ever in an accident, or come hard off a jump, or
do something else that puts a shock into the switch, your
truck will probably die and it won't start afterwards. Push
the RESET button on your inertia switch, and you'll probably
be ready to roll again. For hard off-road use, you may want
to replace this switch with a conveniently placed toggle
switch that performs the same function, but won't open when
it takes a solid hit. (It could also be an anti-theft
device.) For normal street use, the factory design is pretty
good. I'm keeping it, even though it caused me so much
hassle. Now that I know about this, and I have a new one
installed, I'm running my little Bronco II like I always
wanted to.
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