Creating a Negative Control Switch with a Relay

[Papa]

I'd like to create a negative control switch and want to know if this will work for that purpose:

https://farm5.staticflickr.com/4529/38110424955_14d65819d1_b.jpg (https://flic.kr/p/214FTAk)

[NAZ]

Yes if you add (positive) current to pin 86 you will have continuity thru pin 87 & 30. While pins 85 & 86 are not polarity sensitive DIN standards dictate 85 as negative and 86 positive just the way you drew them. And pin 30 is usually hooked to your power source but will work fine as you've designed the circuits.

[BB767]

Hi Papa. I am not sure the fuel pump will work as you expect it to with the system you have drawn. Cutting the tan jumper wire will remove power from the fuse panel relay that actually turns on the power to the fuel pump, thus, it will not run. If you simply tap into the tan wire your ground will work as drawn BUT, you will not be controlling the pump with the EFI computer! The pump will simply run all the time via the FFR harness if it is wired as directed. The green ground wire from the EFI computer controls the pump by supplying the necessary ground to the relay to make it run. Simply grounding the green wire will do nothing for the pump or the computer. Unless of course I totally do not understand the system!

[Papa]

Yes if you add (positive) current to pin 86 you will have continuity thru pin 87 & 30. While pins 85 & 86 are not polarity sensitive DIN standards dictate 85 as negative and 86 positive just the way you drew them. And pin 30 is usually hooked to your power source but will work fine as you've designed the circuits.

Thanks, NAZ & BB767. As I understand (not much at all) what the FAST EFI system wants to do, it appears that the FAST ECM will apply a negative signal on the fuel pump control wire, which I want to turn into a positive signal to power my fuel pump relay.

"Fuel Pump Relay Green wire Negative control side of a relay for powering a fuel pump. Do NOT wire directly to fuel pump."

Could I also just connect the green wire to the existing fuel pump relay ground to accomplish the desired result?

[BB767]

That’s what I did, it works perfect. Curious to hear what the experts have to say.

[Papa]

That’s what I did, it works perfect. Curious to hear what the experts have to say.

I was trying to explain your recommendation to the guys from FFR and BluePrint and I think I just got myself more confused. I found this discussion on another forum:

http://www.cpgnation.com/forum/threads/ez-efi-green-wire-to-existing-fuel-pump-relay.25663/

It got me thinking of how to create the switch I drew to essentially close the circuit on the existing fuel pump relay using the signal from the ECM.

[Itchief]

Papa

You could also pick up the ground wire from the inertia switch and run it to the efi lead

Rick

[BB767]

That is exactly what I did as that IS the only ground wire for the fuel pump relay.

[BB767]

I was trying to explain your recommendation to the guys from FFR and BluePrint and I think I just got myself more confused. I found this discussion on another forum:

http://www.cpgnation.com/forum/threads/ez-efi-green-wire-to-existing-fuel-pump-relay.25663/

It got me thinking of how to create the switch I drew to essentially close the circuit on the existing fuel pump relay using the signal from the ECM.

I know what you are saying and it sounds like a good idea on paper, just a little more complicated than I want to do.

[Itchief]

You also get to keep the inertia switch in the circuit which is a good thing

[BB767]

Yep. Only drawback that I can see is if there is a problem with the circuit it could be either the inertia switch or the computer but that should not be to hard to narrow down.

[Papa]

Papa

You could also pick up the ground wire from the inertia switch and run it to the efi lead

Rick


That is exactly what I did as that IS the only ground wire for the fuel pump relay.

Looking at the schematic for the main harness, that ground is also where all the other grounds in the main harness tie into. So, would I need to cut the ground wire side of the inertia switch and connect that to the EFI control and terminate the other end that runs back to the ring terminal in the foot box?

[BB767]

Not to hijack the post but since it was mentioned, my inertia switch is mounted with the red reset facing down. Has anyone had any problems with it mounted this way? It is easier to access and reset in this position but I could turn it over if necessary.

[BB767]

Looking at the schematic for the main harness, that ground is also where all the other grounds in the main harness tie into. So, would I need to cut the ground wire side of the inertia switch and connect that to the EFI control and terminate the other end that runs back to the ring terminal in the foot box?

You are right. I just cut the ground as it comes out of the inertia switch and tied it to the green efi wire leaving all the other grounds in tact since they all tie in downstream of the inertia switch.

[Itchief]

I would just look at the plug for the inertia switch and determine which wire comes from the relay and then cut the other wire that goes to ground you could ground the unused end if you want to or just seal it with heat shrink tape

Do a lineman splice with soldier and heat shrink tape

I’m not sure about the mounting of the inertia switch

Rick

[Papa]

You are right. I just cut the ground as it comes out of the inertia switch and tied it to the green efi wire leaving all the other grounds in tact since they all tie in downstream of the inertia switch.


I would just look at the plug for the inertia switch and determine which wire comes from the relay and then cut the other wire that goes to ground you could ground the unused end if you want to or just seal it with heat shrink tape

Do a lineman splice with soldier and heat shrink tape

I’m not sure about the mounting of the inertia switch

Rick

Okay guys. It's starting to make sense now. I'll just need to determine which side of the switch ties to the ground in the harness; simple continuity check on the inertia switch plug.

Thanks,
Dave

[Papa]

Not to hijack the post but since it was mentioned, my inertia switch is mounted with the red reset facing down. Has anyone had any problems with it mounted this way? It is easier to access and reset in this position but I could turn it over if necessary.

I would flip the switch over as vibration and gravity could cause it to drop (trip).

[Itchief]

You are cooking with gas now

Good luck

[BB767]

I would just look at the plug for the inertia switch and determine which wire comes from the relay and then cut the other wire that goes to ground

Do a lineman splice with soldier and heat shrink tape

I’m not sure about the mounting of the inertia switch

Rick

From the reasearch I have done you should not use solder joints on automotive wiring unless you are REALLY good at it. I am not. The same reasearch tells me a well executed crimp connection is a very acceptable second choice. Never had a problem with these in 40 years. Not trying to be contrarian, just stating my opinion.

[edwardb]

Not to hijack the post but since it was mentioned, my inertia switch is mounted with the red reset facing down. Has anyone had any problems with it mounted this way? It is easier to access and reset in this position but I could turn it over if necessary.

They're designed to be mounted red button up.

[BB767]

I would flip the switch over as vibration and gravity could cause it to drop (trip).

Yes, that is what common sense tells me too. Just wondering if there is something in the design that I do not know which makes this untrue. Would prefer it in the current position if possible.

[Itchief]

No problem old habits

I soldier and heat shrink connectors and spade terminals also

[BB767]

That sounds like a definitive answer. I’ll get it turned over. Thanks.

[BB767]

I wish I could make a decent solder joint. Just not patient enough I guess.

[Itchief]

A pencil soldier iron will work for most connectors except for the ones with large wires

Use a rosin core soldier; tin the soldiering iron tip with soldier and always heat the connector

I tin the wire with soldier before I put it into the connector and then crimp, soldier and heat shrink tape

[BB767]

A pencil soldier iron will work for most connectors except for the ones with large wires

Use a rosin core soldier; tin the soldiering iron tip with soldier and always heat the connector

I tin the wire with soldier before I put it into the connector and then crimp, soldier and heat shrink tape

I just squeeze the pliers until they click!

Sorry, couldn’t resist. Thanks for the advice, I will certainly try your suggestions. I have tried practicing on my own many times and I just could not get a satisfactory result.

[skidd]

Just for reference.. this site has a pretty good selection of standard relay wiring that can accomplish a lot of common circuits.
https://www.the12volt.com/relays/relaydiagrams.asp

The OPs diagrams seems to be a perfect match for this one
https://www.the12volt.com/relays/relaydiagram9.html
https://www.the12volt.com/12voltimages/pos2neg.gif