just fitted fuel pump and rewired it direct to battery as the pump was prev only getting 10v with lights/heaters etc on. i read that i should be getting altenator voltage at pump on idle but i am getting 12.8 alternator is at 13.7. lights on etc i am still getting 12.8 at the pump which may be a good thing but im concerned when driving it my not be good. i have used 27amp wire with 20amp fuse inline and a 12v 40 amp relay. i assume these bits correct to use? the relay is an old one i had lying about so is there a possibility the relay can be restrictive? or is the voltage ok? car is s1 rst if it matters. any help appreciated ta

 

defo seems a voltage drop there, yes relays can cause it if there's a resistance over the coil shouldn't be hard to find using a muti-meter all tho the pump will prob run fine with that voltage

 

Unless there is a substantial cable from alternator to battery, wire the power source directly to the alternator.

Ensure both power wire is suitably sized, and also the pump has a proper ground.

27A wire means nothing, what size/gauge is the wire ?

 

not sure about gauge but it says on the label 12v 2.5mm. got the earth for the pump and the new relay direct to battery

 

Run the earth to a good clean point on the chassis local to the pump. It will be better than a long small diameter cable running all the way to the battery. You can double it up with the cable if you want.

2.5mm^2 isnt huge by any means but should be adequate.

I like to oversize pump wiring. 4mm would be better, even if it is overkill.

 

What stevie says.

Don't underestimate how critical a good earth is.

 

Can do volt drop testing with car running, along the cables to see if you're losing power or ground.

You need to see as close to 0v as possible when going from the battery to the pump on the power cable, then same along the ground.

If your meter shows voltage when testing that will be where it's losing voltage (volt drop)

If for arguments sake you test the pump and its getting 12v when running, and you have 14v across the battery, you're losing 2v somewhere, if you volt drop test the ground and it shows 2 volts..hey presto..

More on volt drop testing here. http://www.fluke.com/fluke/uses/comu...sevoltdrop.htm

 

I have a digital Voltmeter in my dash to show pump voltage so i can monitor it under load. Easy to wire in and cost less than 2 quid on ebay.

 

Pics Jim please

 

that doesn't sound to bad to be fair, with engine revving a little better the amperage will increase from alternator and the voltage drop will be less. under load and some revs gives much better performance than idle/no load.

 

+2............