No power at fuel pump-help

D

Dean Lampe

• May 11, 2012

• #1

I am using the grey wire in the GM harness marked fuel pump to turn off/on the low pressure pump but the blue wire is not hot! Whats up with that? The instructions say that it is controlled by the ECU but I would think it would need to be hot before you start the car. The big green 30 amp fuse is good and there is power to the fuse. There is also power to the relay, although I don't know which terminals on the relay should be hot and which ones shouldn't.

Any ideas? or can some one walk me through the testing process?

 

Last edited: May 11, 2012

Ken Roberts

Supporter

• May 11, 2012

• #2

The fuel pump fuse (battery power) should attach to the #30 on relay.

Then the power travels thru to #87 in the relay once the coil is enegized which then goes to the fuel pump.

The ECM signal wire should attach to #85.

The ECM signal wire then travels thu the coil in the relay to #86 where it goes to ground after leaving the relay.

 

W

Wayne

• May 11, 2012

• #3

Have you turned ignition on to energize relay?

 

D

Dean Lampe

• May 11, 2012

• #4

85 = ground
30 = main power big orange wire hot when ignition on
86 = green/white stripe hot when ignition on
87 = gray wire going to pump

It seems like it should be working. The power is coming into the rely, the relay ground is good, the ecu power wire is hot to trip the relay but no power at the end of the gray wire.

What's the deal? :furious:

 

Ken Roberts

Supporter

• May 11, 2012

• #5

Bad relay? Switch it out with another one near by that isn't being used at the moment. The fuel pump is generally powered by battery power and not ignition power. Perhaps the current is dropping out when the starter engages somewhat like when the radio goes dead as the engine is starting. The fuel pump relay might temporarily not be seeing that ignition voltage as the engine is turning over. The starter might be robbing it.

 

Last edited: May 11, 2012

Ken Roberts

Supporter

• May 11, 2012

• #6

Make a short jumper wire. Remove the relay and jump #30 and #87. That should be sending power directly to the fuel pump. Are you sure you have a good ground at the pump?

 

J

jay cozz

• May 11, 2012

• #7

in relation to the fuel pump question i was wondering since the gray GM wire puts out 30 amps and the each of the 2 fuel pumps take about 12.5 amps each is it possible to just splice the gray fuel pump wire to power both pumps?

 

D

Dean Lampe

• May 12, 2012

• #8

After an hour on the phone last night with Wayne, we figured it out. It was the factory ground terminal in the fuse box. The little tang on the relay was not making good contact with it when it was plugged in. You could test it every way possible and it all checked out. It was something that simple.

Side note.

Wayne found this little piece of information. When you turn on your ignition the fuel pump wire is only hot for about 2 seconds, then it goes off. It comes back on when you start to crank the engine and stays on. Has anyone else noticed this?

 

Allan

• May 12, 2012

• #9

My power to the fuel pump does the same thing and so have my previous LS engines.

 

A

Al Wohlstrom

• May 12, 2012

• #10

Dlampe said:

After an hour on the phone last night with Wayne, we figured it out. It was the factory ground terminal in the fuse box. The little tang on the relay was not making good contact with it when it was plugged in. You could test it every way possible and it all checked out. It was something that simple.

Side note.

Wayne found this little piece of information. When you turn on your ignition the fuel pump wire is only hot for about 2 seconds, then it goes off. It comes back on when you start to crank the engine and stays on. Has anyone else noticed this?

Click to expand...

My LS7 does the same thing, the pump comes on for about 2 seconds when the ignition is turned on and comes back on during startup and while running. When I started it the first time I ran a jumper from the pump to the battery until it built up the initial fuel pressure and then stared it.

 

Ken Roberts

Supporter

• May 12, 2012

• #11

The fuel pump is designed to turn on/run only when it sees pulses from the crank sensor. The ECM first runs the fuel pump for a couple of seconds then shuts it off to charge the fuel rail. This will allow the engine to fire sooner and provide the needed rotations for the crank sensor to send pulses to the ECM.

If you listen closely you can even hear the throttle blade open and close quickly before starting. This is a self test for the throttle body.