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Fuel pump relays


enginerd
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I want to field opinions about fuel pump relays. For functional wiring of a race car, it's very easy to put the fuel pump on a switch so you toggle on before firing the engine. However, when rewiring our fuel system years back I made sure to run the 12v to FP through the stock fuel pump relay so that the fuel pump energizes ONLY when the ECU is telling it to (which I think is linked in some way to the crank sensor). My reasoning is that if I were to crash I want that pump to stop pushing fuel as soon as the engine stops rather than waiting for me to kill the switch (which I may not be aware enough or able to do).

 

What about you guys?

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The e21 pictured to the left was built with no fuses or relays (by an unnamed 17 year old).  Yeah, that worked about as well as you'd expect.  Fortunately when the brake light circuit burned up it didn't take out anything absolutely essential and we still finished the race.

 

For my new build, the E46 will be using the OE fuse/relay panel for most circuits (including the fuel pump).  As indicated above my electrical engineering skills are not better than the German folks.

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Many racing bodies require that the fuel pump automatically disables when the engine is stopped.  For the exact reason you listed.  If you crash your primary focus may be escape.  If a line is compromised, and there is 255 LPH of fuel spraying on a hot engine, bad things can and will result.

 

Most ECU cars will do this from the factory.

 

 

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On Running Bulls I rewired the car but put in a new fuse socket with stock 15amp fuse so that ECU powered it as normal. We also tee-ed off downstream of the fuel pump relay so that we could do a pump out if necessary with a second source and switch.

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  • Technical Advisory Committee

Factory wiring using the ECU will provide a safety factor as described by shutting off the pump when the engine is not running. The AECM (Airbag Electronic Control Module), or ORC (Occupant Restraint Controller) performs the same function in a crash/accident event. 

Similar function can be obtained with do-it-yourself wiring by placing an oil pressure sensing switch in the circuit powering the fuel pump. When the engine dies, power to the fuel pump will be interrupted. But - You would lose the capability of the AECM/ORC killing the engine (ignition & fuel circuits) in an accident event. The possibility would still exist that the engine and fuel pump could remain running in the DIY wiring situation.

It's hard to beat the safety factors built into the OE electrical system.

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I cheated.

 

I have a Painless system with ignition box That ties into the ECU (In the background. A9L unit with a tune.). I did not have to jerry rig anything. Instead of trying to make a 30 something year old wiring system work this replacement system is very simple and safe. And it was relatively inexpensive. 

2C00F1F0-711C-4B1F-BD12-EBC2D2BD3A85.jpeg

Edited by Cam Benty
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  • Technical Advisory Committee

I did not want the single-point-of-failure of the OEM fuel pump relay.  I have always run one pump controlled by the ECU and one pump controlled on a separate switch.  both are appropriately fused and relayed.

 

When I added a surge, my old HP pumps now became my lift pumps.  The (new) surge HP pumps are on their own switches and relays and fuses.  My ECU only has one Fuel pump relay control wired up.  I didn't want to have to cut into my harness to relocate that to the surge pump, so, we are living with having a manual fuel pump control for now.

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Fortunately megasquirt (at least the version we run) has a fuel pump relay signal output.  If it didn't I would use the oil pressure switch method as I have not interest in having a fuel pump running if the engine isn't running.

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In my haste going back and forth between the Megasquirt and the factory ecu/fuel pump ecu I have my pumps wired to be switched only.  It scares me to have it this way, so before we race again I plan to wire it correctly using the fuel pump output from megasquirt.  It's really a safety item that I should not have had for even one race, so yes the right way is to have the ecu control it.

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2 hours ago, tneker said:

Fortunately megasquirt (at least the version we run) has a fuel pump relay signal output.  If it didn't I would use the oil pressure switch method as I have not interest in having a fuel pump running if the engine isn't running.

 

You better figure out how to rig that thing to run without the engine. We plan to protest fuel capacity next road america race....making it all of 1:20 minutes is fishy! Class B trophy’s are big time!

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