What relay to use for 12v fuel pump mod.

Fitted an Aeromotive 340 lph fuel pump to the car, car starts fine but after only a few seconds the fuel pressure wavers then slowly drops to zero (I have a fuel pressure gauge installed) and the car cuts out. It will start again immediately but the same thing keeps happening.

If I bridge FP & B+ in the diagnostic port in the engine bay then start the car and remove the bridge straight away the fuel pressure holds and the car will tickover seemingly indefinitely but give it a couple of hard revs or turn the car off and start it again and the problem returns.

I've just bypassed the fuel ECU and the car seems to be running fine, so it does appear that the pump needs 12volts to function properly, but i understand the drawback to this mod if a relay isn't used so I'd like to do it properly.

But if I'm using the 9v-12v signal wire on the coil side of the relay (as per the bloody huge picture below, sorry) which type of relay should I use, a 9v or a 12v or does it matter?

Thanks for your time & any advice you can impart.

A 4 pin 30amp relay will be fine.

A 4 pin 30amp relay will be fine.

But what voltage mate? If the feed to the coil varies between 9 and 12 volts what voltage relay would be best?

Can a 9v supply enegise the coil in a 12v relay?

Can a 12v supply damage a 9v relay? (If there is even such a thing be had)

Yeas 12v. All it is is a switch. The voltage from the fuel pump ECU doesn't have to be 12v

Legend Hodge.

Cheers mate.

yey my totally awsome fuel ecu bypass wiring diagram has helped someone out

yeah sorry did i not put in the thread that a 12v 30amp 4 pin alavailable at your local maplins/motor factor would do the job

looks like you had the exact same problem i had with the walbro lph 400, i done this mod some months ago now and its still going well

yey my totally awsome fuel ecu bypass wiring diagram has helped someone out

 

yeah sorry did i not put in the thread that a 12v 30amp 4 pin alavailable at your local maplins/motor factor would do the job

 

looks like you had the exact same problem i had with the walbro lph 400, i done this mod some months ago now and its still going well

It seems to be the case alright mate, wish I could be absolutely certain though, I have a track day tomorrow!

I should have a suitable 12v relay secreted in the garage somewhere, I can do the install at lunchtime and leave the car running for a little while to see how it goes.

And your diagram was/is a definite help, plus it's not as big and cumbersome as it appeared to be in the post preview so ignore the comment about it being unwieldy!

mate the install is so easy you just need a meter to see what wire to the fuel ecu has the 12v and 9v and to make sure you the 9v through the coil and 12v through the switch,a soildering iron and so electrical tape

you dont have too use new earths you could just use the original wiring but a nice local one is better

There are probably several none critical relays under the bonnet to "borrow" if you get stuck, or just bridge it out with a piece of wire.

Bringing up an old thread here and I'm useless when it comes to relays. Looking into doing this myself but need to clear few things up

Regarding what wires go to what number on the relay Going off this:

Would they be this:

pump positive = 86

12v power = 30

9 v = 86

ground = 85

Now going of this article that uses the on relay version:

http://mkiv.com/techarticles/12v_mod/12v_mod.htm

Would it be correct in saying:

12v wire is blue with orange stripe (to go on relay number 30)

Fuel pump positive is blue with red stripe (to go on relay No 86)

And after typing all that i came across the blow image from here:

http://www.supraforums.com.au/forum/topic/27751-new-and-improved-12v-fuel-pump-mod/

And how I'm super lost as the relay numbers cant match up 85 is meant to be ground... and 30 T bar from 12V?

Or is that because a 5 pin relay is different?

Just follow the bottom image in your post above.

Ok chis. Even though on a relay is usually number 85 is ground?

85 or 86 can be ground or energizing input, it doesn't matter which you choose as grounded.

Ok thanks chris :-)

Ok all fitted and car car starts and runs. Looks like I wired it in ok

can you describe what wire you put on what, as i can see there is a blue and red blue??

Cut the blue/red wire supplying power from the fuel pump ECU to the fuel pump.

Connect the ECU side of that wire to one side of the relay coil. (pin85)

Connect the fuel pump side of the blue/red wire to the big spade connector on the relay (pin30).

Connect the other side of the relay coil (pin86) to the body of the car with a short piece of suitable wire to create a ground.

Using another piece of suitable wire, take a 12v feed from the blue wire supplying power to the fuel ECU, connect that to the other big spade connector on the relay (pin87 on a five pin relay).

thats alot for the explaination, as i gonna mount a relay in mine to eliminate the 9v to the pump..