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Two Relays on the same circuit? Relays in a series

5K views 9 replies 5 participants last post by  NTSQD 
#1 ·
Need some advice with fuel pump wiring....long read sorry

Car is a baja bug, (Rear engine, front fuel tank) with a Carter electric fuel pump in the front underneath a spun aluminum tank mounted under the bug's hood. This fuel pump draws less than 10A. I have remote battery terminals with heavy welding cable both at the engine in the rear and under the hood.

Complete rewire of the whole bug with a universal, street rod style kit from Kwik Wire in WI.

Issue: The Kwik Wire fuse block I'm using includes a fuel pump relay built in. The output for this wire is ignition hot. In order to use an oil pressure safety switch for the pump, I ran the fuel pump relay output wire (10ga) from the relay/fuse block back to the oil pressure safety switch on the engine. In order to complete the fuel pump wiring, I need to run a wire from the safety switch to the pump under the hood.

Since I've now doubled or tripled the length of the fuel pump wiring, should I add a second relay between the safety switch and the pump (at the under hood remote battery terminal)?

or just run heavier 10 or 8ga from the safety switch to the pump? Can two relays be run in series? I assume the first relay will just have nothing for a load on it
 
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#2 · (Edited)
I would not run the load thru the Oil Pressure switch, I doubt it's rated for 10 Amps. The wire that energizes the relay (coil or "trigger" wire) is what should be run thru the OP switch. You can't run 2 relay coils in series, they must be wired in parallel. But you're not really talking about putting them in series, just using the contacts of the first relay to energize the coil of the second relay. That would work, it's just an unnecessary complication. Low OP switches are often wired into the ignition circuit, instead of the fuel pump directly.
 
#5 ·
If I'm understanding your plan, the only load on the Kwik Wire FP relay would be from pulling in the coil of the second relay. The actual Fuel Pump Load would be carried across the contacts of that second relay.

From a generic relay diagram, IDK which one you're using:
FP Relay-->Safety Switch -->2nd Relay 86 contact --> 85 contact to Ground.
(The Safety Switch could alternately be placed between 85 and Ground )

That will energize the 2nd relay, which carries the FP load:

+12V Fused Battery feed --> 30 common contact --> 87 N.O contact --> Fuel Pump + --> Fuel Pump - to Ground

I think I got that right ?
 
#10 ·
With an electric fuel pump I prefer to put the low OP switch in the FP relay's control circuit because when I'm upside down and on fire I want the pump to stop feeding the fire.

If you use the Carter pressure switch (used to be an OE switch for Chevy Vegas, Summit) it has three terminals (Output, Start mode, Run mode) so you're not forced to wire switched to ground when switched hot is easier to do given the other wiring.
 
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