Momentary to Constant - Negative Output?

Hoping I didn't miss something obvious during my searching. If so, flame on.

I'm trying to disable my fuel pump by interrupting the ground with a momentary to constant configuration. The idea is that when the key is in the ignition (ON position) and I press a momentary switch the fuel pump circuit closes and functions normally until the key is removed and which time the circuit opens again.

I've tried to adapt some of the circuits found in the relays section with no success; I suspect that this is because I have a negative output.

My 12+ input is the ACC circuit. The switching pulse can be negative or positive. I think it's configured as negative right now. Maybe it matters, maybe it doesen't, but I'd prefer if the pump circuit were isolated from the switch circuit. Thoughts?

Momentary Switch
One lead to ignition 12V+
Other lead to 86 on relay one

Relay One
30 to ignition 12V+
86 to momentary switch 12V+ output
86 to 87 on relay one and 86 on relay two
87 to 86 on relay one and two
87a not used

Relay two
86 to 86 on relay one and 87 on relay one
85 to ground
30 to one side of fuel pump ground wire
87 to other side of fuel pump ground wire
87a not used

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the12volt • Support the12volt.com

I knew I drew something like this before.
https://www.the12volt.com/relays/relaydiagram12.html



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the12volt • Support the12volt.com

That's so simple, too cool! Quick question.

the12volt wrote:

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Momentary Switch
One lead to ignition 12V+
Other lead to 86 on relay one

Relay One
30 to ignition 12V+
86 to momentary switch 12V+ output
86 to 87 on relay one and 86 on relay two
87 to 86 on relay one and two
87a not used

Relay two
86 to 86 on relay one and 87 on relay one
85 to ground
30 to one side of fuel pump ground wire
87 to other side of fuel pump ground wire
87a not used

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Is that supposed to be 85 to momentary switch 12V+ output? And that is after my switch, right?

Doh! No, I forgot to include terminal 85 on relay one.

It should read:

Momentary Switch
One lead to ignition 12V+
Other lead to 86 on relay one

Relay One
30 to ignition 12V+
85 to ground
86 to momentary switch 12V+ output
86 to 87 on relay one and 86 on relay two
87 to 86 on relay one and two
87a not used

Relay two
86 to 86 on relay one and 87 on relay one
85 to ground
30 to one side of fuel pump ground wire
87 to other side of fuel pump ground wire
87a not used

My first reply has the coil terminals reversed from my diagram in my second reply. Yes, the 12V+ to relay one terminal 86 is from (after) your momentary switch.

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the12volt • Support the12volt.com

The circuit almost works... seems that the coil on relay one momentarily de-energizes when the key is changed from ON to START. If I press the momentary again after the key returns to ON before the vehicle dies the fuel pump starts working. Can I install a capacitor or similar somewhere to maintain the charge for that time? Is so where?

You need an ignition source for the switch and relay, not an accessory source. Ignition will have 12V+ during start. Accessory generally drops 12V+ during start.

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the12volt • Support the12volt.com