fuse size? again, and now some elec problems???

[Jiffer]

this time for a relay to an electric fuel pump, what would be the correct size?

cheers

 

I have got my fuel pump wired and runnig, but it keeps blowing fueses?

checked all wirering , all wired corectly and all good with no shorts,

I have found what seems to be a spare live feed from the back of my ignition barrel (on a mk2 cortina)which I used to feed the relay, then an isulated fuse carrier with a 5 amp fuse in from the battery, the pump runs and the car starts a treat but when I switch off the ignition it blows the fuse?

any ideas

cheers

[Jiffer]

anyone

[over_head_cam]

5 amp sounds a bit low for a relay. I'd try a 10 or maybe even a 15, but not any more than that. What pump is it? and does it work at all or does the fuse pop straight away? Could be wired wrong maybe?

[samurai]

Google "car relays" to see what amps come up

 

[Jiffer]

its a facet posi-flow (I was told this is best suited to 40s, but he proberly just wanted to sell me it)

the pump works fine and car starts but when I turn off the ignition the fuse blows,

the fuse is from the battery feed into the relay and then to fuel pump

Its got on me nerves now so left it till tommorow.

[over_head_cam]

5 amp is not enough for a relay. You need to know the rating for the relay, not the pump. I'd try a 25 or even a 30 amp. If it blows that as well, there is something wrong somewhere

[samurai]

"The relay should be a 30-amp relay and a 30-amp in-line fuse is necessary. The fuel pump can be grounded to the chassis. The switch for the interior does not need to have a high amp rating since a relay is being used."

 

Thats what i found

[Jiffer]

Thanks for the help, it is a 30amp rated reley so I will pick up some larger fueses and try when I get back later

[Vista]

I have to disagree with some of the above advice as the relay is just an electrical switch, the fuse is there to protect against overload of the switch and the wiring that may be caused by the service that's being fed drawing too much current. The fuse therefore should be sized to be adequate for the pump not the relay but should be before the relay in the main supply circuit (as opposed to the trigger circuit) as that way it will protect both the circuit and the relay.

 

As to why it's blowing when you switch off, just a guess but I suspect it's that the moment the engine cuts out there is more current draw from the pump as it's trying to pump against a dead head (as there's no longer any fuel being drawn out of the carbs) Momentarily, the current draw is probably higher than your f amp fuse can handle. I'd establish the running current of the pump and then size your fuse to be slightly larger to allow for start current loads.

[V8ian]

Vista is right about the Fusing, its there to stop damage to wireing or components,

I thinking the spade off the back or your key switch may be live in a different sequence, so when you switch off the engine, you are possibly feeding something else that draws a lot of amps,

How would I wire it???

I would run a perm live from the battery thru the load side of the relay via a 10 amp fuse, then from the other side of the relay to the pump,

the switching side of the relay needs to come off a known switched live, it dosent need to be a heavy duty wire as you will probaly not draw any more load than 1 amp, you could put a hidden switch in the wire too as a extra security item, wont stop the car being driven a short distance, but the fuel in the float bowls wont last long,

Vista, the dead head thing possibly wont work as the pump will be blowing fuses if the engine stalls,

[Mad Mex]

I'd agree with Vista and Ian on this...i used to be an auto electricain on HGV's and what they say is correct...the fuse size is not the issue and you were lucky you used a small fuse as a large fuse may of allowed the current to pass and blow the pump...

 

my suspiscion is uve taken your feed from the cranking circuit which often "pulses" when you swithch the igntion off...this is more then likely what keeps blowing your fuses..

 

find a new perm feed like ian said,( closer to the battery the better but ideally not straight from the battery) wire it all up with no more then 15 amp fuse...does not matter what the relay is rated at as this is its capacity not what it actually needs...it needs a very small current to switch the relay which then obviously completes the circuit

 

Electrics are a funny thing to give advice on over the net as really need to be there....

 

Hope this helps

[Jiffer]

bigs thanks you lot

going to go through it all again tonite