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Jun 22, 2008 12:47:12 GMT
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Planning the wiring of my electric fuel pump - do I need to use a relay? I used one on my electric fan set-up as it draws a lot of juice, but do I need one for the fuel pump? It's a Lucas 4FP if that makes any difference. Is there a certain point at which you should use a relay (i.e.: above a certain amount of amps)? Also, do I have to use a specific fuel pump relay, or can I get away with a regular 4-pin job? Thanks  |
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Jun 22, 2008 12:50:19 GMT
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i was thinkin this too, i need to add a kenlowe to my bready so.........  ;D |
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Jun 22, 2008 13:11:59 GMT
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I was thinkin this too, I need to add a kenlowe to my bready so......... ;D With a leccy fan set-up, you can use a regular 4-pin relay. I built mine out of scrapyard parts (can't afford Kenlowe!) and it works perfectly! ;D Well, I could maybe do with a switch with a lower cut-in temp but I'm re-doing it all so I'll resolve that issue and tidy things up. If you want pics / diagrams of how I did mine, I'd be happy to help. |
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Jun 22, 2008 13:15:26 GMT
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if in doubt, use a relay... it cant hurt |
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Your car is not 'epic', this thread is not 'epic'....the OCEAN is epic, the UNIVERSE is epic.... please stop misusing this word!! It would appear Hotrods are the new VWs - aint fashion funny! '69 BUICK LESABRE 350 |
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Jun 22, 2008 14:03:25 GMT
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a relay only really need to be used if the current required by the ancillary is greater than the rating on the switch.
If your pump switch is rated at max 5A and the pump needs 10A then you gotta use a relay. But most pumps are pretty low-consumption with regards current
And, I would have thought a standard 4 pin would be fine - a relay is a relay
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Your car is not 'epic', this thread is not 'epic'....the OCEAN is epic, the UNIVERSE is epic.... please stop misusing this word!! It would appear Hotrods are the new VWs - aint fashion funny! '69 BUICK LESABRE 350 |
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Jun 22, 2008 14:22:00 GMT
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Basically, as has been said - If you component you are fitting draws more current than the switch is rated for, you must use a relay.
If in doubt, use one. It will save wear on switches further up the line, and if wired in correctly can provide a cleaner supply to whatever component, although this isn't particularly important with fuel pumps and fans, as these will run off just about anything vaguely resembling DC, although a separate feed to the fuel pump or fan will help reduce interference on other components.
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Jun 22, 2008 14:31:06 GMT
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Relays just allow you to switch currents greater than the existing switch and wiring is rated for.
However I think relays on electric fuel pumps are a good idea anyway, regardless of exceeding rated currents etc.
I have the supply to my pump wired through a 10A relay.
The control signal to the relay goes through an oil pressure switch, and will kill the fuel pump if the engine stalls or has catastrophic oil pressure loss.
A feed from the 12V start signal overides this interlock during starting.
So it could save the engine and more importantly it could save me.
Electric fuel pumps emptying the entire fuel tank through a broken hose (possible in a RTA) ain't a good thing IMHO.
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Jun 22, 2008 14:40:01 GMT
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Thanks for the responses - a relay will feature in my plans then. I just was confused as to why OEM fuel pump relays are so expensive - I wondered if there was something special about them. The pump will be running constantly as it's feeding a carb, so it'll be a fairly simple set-up really. So it could save the engine and more importantly it could save me. Electric fuel pumps emptying the entire fuel tank through a broken hose (possible in a RTA) ain't a good thing IMHO. Very good point. My problem is I don't have an oil pressure light, just a gauge, so I can't wire it through that. It's definitely a consideration though. |
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Last Edit: Jun 22, 2008 14:42:22 GMT by BenzBoy
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Jun 22, 2008 15:10:12 GMT
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You could use an inertia switch off something like a newish saxo/106 - They are just 2 wire things that operate like a switch, and can handle the 15A of the standard pump, so using them to switch a relay would be fine.
Easy enough to find at a scrappy, and very simple to mount anywhere
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dubzi
Part of things
Posts: 711
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Jun 22, 2008 15:54:28 GMT
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Relays just allow you to switch currents greater than the existing switch and wiring is rated for. However I think relays on electric fuel pumps are a good idea anyway, regardless of exceeding rated currents etc. I have the supply to my pump wired through a 10A relay. The control signal to the relay goes through an oil pressure switch, and will kill the fuel pump if the engine stalls or has catastrophic oil pressure loss. A feed from the 12V start signal overides this interlock during starting. So it could save the engine and more importantly it could save me. Electric fuel pumps emptying the entire fuel tank through a broken hose (possible in a RTA) ain't a good thing IMHO. Good idea, didn't think of that when I put a leccy pump on the Bedford. How's it actually wire up though? My pump has to run for a few secs to fill the float chamber up, if I switch on from cold there'll be no oil pressure and it won't start will it? I also have no relay just a 7.5A inline fuse at the moment, it runs from an ignition dependent spare terminal I found under the dash. |
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Happiness ain't at the end of the road. Happiness is the road.
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Jun 22, 2008 17:27:41 GMT
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The pressure switch I'm using is a single pole double throw type. So it has 3 terminals - normally open (NO), normally closed (NC), and common (COM). With oil pressure present (engine running) the COM and the NO are connected together. With no oil pressure (engine stopped) COM and NC are connected. The COM is connected to the + side of the relay coil that feeds the fuel pump. The NC is fed from the 12V start - i.e only +12V when starting. The NO is fed from a +12V ign - i.e. only +12V when ignition on. With the engine stopped and the ignition on, the fuel pump relay is not energised. When you crank the engine, 12V is fed through the NC and COMM and energizes the fuel pump relay. The relay will continue to be energized as long as you are cranking. When/if the engine starts, the pressure switch will change over (due to rising oil pressure) so now the NO and COMM are connected together - so the fuel pump relay will be energised as long as the ignition is on and there is oil pressure. It is a bit delayed in operation to be a true safety interlock and an inertia switch is better as Coggers said. Then again an inertia switch won't save your engine if you suddenly lost all oil pressure |
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