Picture of a fuel pump inertia switch

Inertia Switch

We just posted details on how to wire the fuel pump to our Infinitybox POWERCELL.  That was a very simple part of the wiring process in this 1967 Mustang.  We received a follow up question to this post about wiring a fuel inertia switch.  We’ll give details in this post.

A lot of manufacturers, Ford especially, use an inertia switch to interrupt the power going to the electric fuel pump in case of an accident.  These switches use an internal magnet and a ball bearing to close the circuit providing power to the fuel pump.  If there is a large impact to the car from a collision, the force of the impact knocks the ball out of its location.  This opens a set of contacts in the switch, which interrupts power going to the fuel pump.  These switches can be sensitive.  They may shut down the fuel pump if you get a flat tire or if you hit a large pot hole.  A lot of us here remember the prank of pounding on the driver’s rear fender in a mid-1990’s Thunderbird or Mustang to kill the fuel pump and leave a guy stranded.

Picture of a fuel pump inertia switch

Picture of a fuel pump inertia switch

If there was an accident, this switch opens and cuts off power to the fuel pump.  There is a reset button on the top of the switch.  If the red plunger is up, that means that the switch tripped.  If you press down on the plunger, it will reset the switch, letting the electric fuel pump operate again.

In some cars, these switches are in the rear near the fuel pump.  In other cars, they are up front behind the dash.  Wiring an inertia switch to your fuel pump powered by our 20-Circuit Kit is really easy.

You can purchase these switches from any on-line source.  This link will take to you the popular Ford switch sold on Amazon.  You can find the switches but you can’t always find any data or specifications on them.

Switches are only designed to carry a maximum amount of current.  You always have to consider that when you choose a switch.  That is true when you pick your inertia switch.  These switches were designed to interrupt the feed going directly to the fuel pump.  This means that the switch was designed to carry the current for that pump.  In the case of this 1967 Mustang, we are installing a higher-performance pump that draws much more current that the original OEM pump.

To be safe, use the inertia switch to interrupt the MASTERCELL input wire for the fuel pump.  Each of our inputs only require a very small amount of current to operate.  The contacts in the inertia switch can easily handle the small amount of current from the MASTERCELL.

Under normal operation, the MASTERCELL input will be connected to a switch that turns on the input for the fuel pump.  This could be a signal coming from an ECU, it can be from a separate fuel pump switch or it can be tied to the ignition switch.  The inertia switch would be wired in series.  If there were to be an accident, the input from the MASTERCELL would be interrupted by the open contacts in the inertia switch.  This would cause the MASTERCELL to send the command to the POWERCELL to turn off the fuel pump output.  Wiring the switch in with the MASTERCELL input ensures that you will never overload the contacts on the inertia switch.

Click this link to contact our team with questions or comments on this post.


Picture of the Littelfuse Minifuse

Fuse Sizes

Here’s the answer to another popular question that we get from customers when they’re wiring their cars with our Infinitybox wiring system.

What size fuse should they use for the different outputs on the POWERCELL?

Remember why the fuse is there…  it is there to protect the wire not the end load.  There is no need to protect the headlamp with a fuse but you do need to protect the wire going to the headlamp.  A fuse is a thermally operating device that is intended to be the weakest point in an electrical system.  If the system is drawing too much current, you want the fuse to open intentionally, protecting the rest of the system.  You never want your wire to be the weakest point in your electrical system.

Fuses are designed to protect against two kinds of faults.  The most common is a short circuit.  Let’s say that the wire to your headlights gets pinched against the chassis and breaks the insulation.  You now have a very low resistance path to ground which will carry a lot of current.  You want the fuse to open before the insulation on the wire gets hot enough to cause damage.  The other scenario is a low-overload.  These are usually resistive connections to ground that will still carry enough current to damage the wire.

To properly select the size of the fuse that you need to consider two things.

First, what is the minimum gauge of wire that the fuse needs to protect?  All of the output harnesses on the Infinitybox POWERCELLs use 14-AWG wire.  But, if you spice our output wire to a wire with a smaller gauge, you need to size the fuse to protect the smaller wire.

Second, you must understand the current draw for your load connected to the output wire.  The fuse must be large enough to handle the steady-state current of the load plus any inrush that the load may have.  Anything inductive (motors, coils, solenoids, etc) have inductive inrushes that could be up to 10 times the steady-state current draw.  Incandescent bulbs also have inrushes.  Remember that the resistance of the filament in the bulb is a function of its temperature.  When the bulb is cold, the resistance is low so it can carry more current.  As the filament heats up, the resistance increases as does the current flowing through the bulb.  The inrush through a light bulb can be 5 to 10 times its steady state current.

For those who want to learn more, this link will take you to a good document from Littelfuse that talks about how to properly size a fuse for a specific application.

LED lights draw far less current than their incandescent brothers and have practically no inrush.  You can use a much smaller fuse.

Here is a simple chart to use as a reference for sizing your fuses.

Headlights- 20-amp
Starter solenoid- 20-amp
Ignition- 25-amp
Fuel pump- 25-amp
High-Beams- 20-amp
Turn-signals- 15-amp
Cooling fan- 25-amp
Power window regulator- 20-amp
Lock actuator- 15-amp
Parking lights- 15-amp
Horn- 10-amp

Use this chart as a guideline to pick your fuse sizes.  You may need to adjust depending on the gauge of wire connected to your load and the specifics of your load.  Remember, these are guidelines.  If you change the gauge of wire that is included in the kit, you have to reevaluate the size of fuse required.

Contact our technical support group at (847) 232-1991 for more details.