Inertia Switch Wires HOT!

[D-Dubya]

I just finished my build a couple weeks ago and while not yet legal, I can't help but get out in the car a few times a week for short drives. Don't worry, I have temporary tags so I am not "illegally" driving around. I have noticed that the two wires that plug into the bottom of the inertia switch get quite hot to the touch after just a few minutes of driving. Has anyone else experienced this? Any suggestions from the group on what might be causing it and how to fix it?

As always, thanks in advance for your advice.

[DanQ]

I honestly have never reached under the dash to touch the inertia switch, but now I will need to check that out. It should not get hot, as heat equals resistance and that is merely an on or off switch, which should have little to no resistance. If there is resistance at that switch, it may be supplying your fuel pump with less than proper voltage/current. You could always unplug it and measure the resistance across the switch, and it should be very close to zero ohms. Just thinking, you do have it mounted with the reset button up, correct? Any other position can result in false tripping.

[narly1]

If the inertia switch is in series with the fuel pump its current draw demands might be too large for the switch and wiring.....thus requiring a relay.

Earl

[phileas_fogg]

As I recall, the inertia switch is rated to 30A (but I'm going from admittedly poor memory, so if you can find the part number on the switch, look it up!).


John

[narly1]

Note that for 30A @ 12Vdc the wire should be 6 ga.

Earl

[narly1]

If the inertia switch is in series with the fuel pump its current draw demands might be too large for the switch and wiring.....thus requiring a relay.

Earl

Or heavier wire if the switch is sufficiently rated.


Earl

[D-Dubya]

Note that for 30A @ 12Vdc the wire should be 6 ga.

Earl
Thanks. I have not checked the resistance through the switch. As I recall, I ran the negative from the pump to the switch and then grounded the switch to the frame. Isn't the inertia switch just a closed switch that opens if tripped, causing a break in the circuit?

Regardless, the wires coming from the plug supplied with the switch are probably 14 gauge. Pretty small for 30A @ 12V.

Does everyone use that inertia switch in their builds? I know it is a safety component, but do all builds have it?

[OB6]

I ran mine in series with the fuel pump relay ground to avoid the switch carrying the load of the pump.

[D-Dubya]

Thanks Terry, Do you happen to have a diagram on how you did that? I am not real saavy when it comes to relays.

[narly1]

Thanks. I have not checked the resistance through the switch. As I recall, I ran the negative from the pump to the switch and then grounded the switch to the frame. Isn't the inertia switch just a closed switch that opens if tripped, causing a break in the circuit?

Regardless, the wires coming from the plug supplied with the switch are probably 14 gauge. Pretty small for 30A @ 12V.

Does everyone use that inertia switch in their builds? I know it is a safety component, but do all builds have it?

Before you start worrying about the switch resistance I'd be finding out what the current draw for your fuel pump is first and then make sure that the current rating of the wire and switch are adequate for that. The way you have the switch wired all of the current drawn by the pump motor has to flow through the inertia switch and its wiring.

14 ga wire is only good to about 5-10 amperes @ 12Vdc depending on the length involved.

Do all builds have an inertia switch? NO. Should they? IMO YES.

As yourself if you want your fuel pump to still be running and possibly be squirting out gas in the event of an accident?

Earl

[OB6]

Thanks Terry, Do you happen to have a diagram on how you did that? I am not real saavy when it comes to relays.

I'm not using the RF harness, so it may be best for someone else to chime in. Maybe the harness is already setup to use the fuel pump relay ground? I just don't know. I'd hate to steer you wrong.

[AndrewIdaho]

I'm not using the RF harness, so it may be best for someone else to chime in. Maybe the harness is already setup to use the fuel pump relay ground? I just don't know. I'd hate to steer you wrong.

In looking at the Rev W of the RF Manual it shows the Inertia Switch Wired to the coil of the Fuel Pump Relay:

204443

Andrew

[OB6]

In looking at the Rev W of the RF Manual it shows the Inertia Switch Wired to the coil of the Fuel Pump Relay:

204443

Andrew

Yup, that's essentially how I have mine wired (using an LS harness from PSI).

[JJK]

Doug,
My inertia switch wiring harness plug would not physically stay locked to the switch, so I had to purchase a new inertia switch. If your wiring plug is also loose, this can introduce resistance to the switch which can make things warm. For a quick test make sure the wiring harness plug is seated and cannot be pulled out easily.

[JJK]

I believe the Walbro 255 in tank pumps run about 10 amps, so a 30 amp switch is fine and no excessively large wires should be needed.

[edwardb]

If you are using the Ron Francis harness (always helpful to state that when asking electrical questions) the standard inertia switch wiring is providing the ground wire for the fuel pump relay in the RF panel. Typically pin 85. No high current, including the fuel pump itself, is going through the inertia switch. So all the discussion about wire sizes, fuel pump current, etc. while interesting really shouldn't play into why the wires to your inertia switch are getting hot. In order for the pump to run, the inertia switch is closed completing the ground circuit for the RF relay. The current draw is very low. Quick Google search says 100 to 200 milliamps. The high current for the fuel pump is on the power side switched by the relay. Pins 87 and 30. Something isn't wiring correctly. Is this an installation where you broke into the RF fuel pump circuit, e.g. Coyote or aftermarket EFI?

[D-Dubya]

Well, not sure how I could have screwed that up, but I did. I just dropped the fuse block and took a picture of the back of the Inertia Relay. The tan wire in the 6 o'clock position goes to the Fuel fuse. The other tan wire goes to the fuel pump. I assume the red wire is power, so that leaves the black wire, which according to the manual should go to one side of the inertia switch with the other side going to ground. Am I on the right path?

[D-Dubya]

I obviously don't have it wired correctly. I have a BPE 347 with a Holley Sniper and Holley in-tank fuel pump. It has been a while since I installed all that and as my "not very good" memory serves, I think I just ran the ground wire from the pump directly to the inertia switch and grounded the other side to my ground block. I should have run the black wire from the inertia switch relay to one side of the inertia switch and the other to ground and while also grounding the pump as well.

[weendoggy]

I obviously don't have it wired correctly. I have a BPE 347 with a Holley Sniper and Holley in-tank fuel pump. It has been a while since I installed all that and as my "not very good" memory serves, I think I just ran the ground wire from the pump directly to the inertia switch and grounded the other side to my ground block. I should have run the black wire from the inertia switch relay to one side of the inertia switch and the other to ground and while also grounding the pump as well.

Ford wired the pump two ways, one is via ignition exciter wire to power the relay, and one direct to power the pump through the inertia switch. I like using a trigger (exciter) wire to engage relays. This lets the relay handle the load. If you have it wired like you say, the entire load is using the inertia as it's bridge to run. Not a good plan for me.

With the Sniper, it has a relay built in, and with a little extra wiring to have ignition power go through the inertia switch and then to power the relay, it's a done deal. Diagram with Sniper and CD Box but all you need to do is follow the fuel relay portion.

204454

[dbo_texas]

I obviously don't have it wired correctly. I have a BPE 347 with a Holley Sniper and Holley in-tank fuel pump. It has been a while since I installed all that and as my "not very good" memory serves, I think I just ran the ground wire from the pump directly to the inertia switch and grounded the other side to my ground block. I should have run the black wire from the inertia switch relay to one side of the inertia switch and the other to ground and while also grounding the pump as well.

This is the correct answer - the inertia switch should just close the ground side of the fuel pump relay coil (i.e. as edwardb mentioned, is the low current side of the relay trigger pin 85 wired in series with the inertia switch). The actual fuel pump runs of the triggered side (i.e. power side) of the relay (i.e. pins 87 & 30) and should not be wired to the inertia switch.

[rich grsc]

https://thefactoryfiveforum.com/attachment.php?attachmentid=204454&d=1632668012&thumb=1

Weendoggy shows one of the best ways to wire the switch with a relay. These switches where designed for a 95LPH pump, not the high volume pumps being used

[RoadRacer]

So cool though that you caught this. So easy to lose a car to miswiring. Hot wires are pretty terrifying.