Hi all!

Can someone explain this to me:

I have a fuel pump failure the other day.  I measure the voltage at the pump terminals: zero.

So I take two wires, attach them to the pump terminals and link them directly to the battery under the hood, start the car and drive it for about 50 feets.  Then the car stall again.  :'(

Since I was not to far from home I go there and take one of the 12 volts battery from my electric bicycle (actually 13.8 volts since it was fully charged) and use it to make the pump go.  It works perfectly and allows me to drive home.

Then i try the car battery again and: nothing!  The pump just refuse to work under 12 volts.  The pump turn but the fuel pressure seems to be too low.

So I use the 13.8 volts battery again to drive to the nearest repair shop.  The guy just replace the wire between the relay and the pump and it work!  The performances are totally sloppy but at least the engine doesn't stall.

So, the question is: Why does the pump work under 12 volts IF it pass thru the relay and not if it is applied directly from the battery?

As far as I know, a battery can only give what it have.  I've think of something related to the fuel pressure regulator or something but then how do you explain the operation of the pump with the auxiliary battery attached?

Laws of physics don't apply in a Zuki!

Totally puzzled 

You have a bad ground on the fuel pump, its back by the tail lamp.

Back by the tail lamp?  Do you mean a lamp is actually mounted in series with the negative terminal of the pump?  That sound really weird.

And, if the ground is the problem, why does the pump don't operate when it was grounded directly to the negative post of the battery with a wire?
 

Everything electrical on your truck connects to the chassis or frame as a ground or return path to the battery.

In the case of the electric fuel pump, the ground wire happens to go to the same connection point as one of the tail lamp assemblies (passenger side?).

Either the pump's wire or the way it is connected is presenting a big enough resistance (rust, corrosion, etc.) to cause the fuel pump to not receive it's share of the voltage.   A voltage drop across the resistance is high enough so that the pump no longer get the full 12+ volts (strangely enouigh, the actual voltage would be closer to 13.8 in a car too!).

Hope this helps! 

Thanks all for your effort, but the question is still unanswered.

Why does the pump work when it was connected with two wires (one positive, one negative, so the grounding was perfect) to an extra battery but not when it was connected to the own car battery?

Still puzzled...

What gauge wire did you use to connect to the car battery. If it was too thin then you would lose power along the wires. The pump should work OK at 12V and below. When my alternator went all the lights were very dim and the indicators and wipers stopped working. The car was still running but I had to pull over as I couldn't see where I was going.