In a real pickle with fuel pump issues. Advice appreciated!!

[George B]

I am thinking you might have a damaged wire or poor connection. You may be able to read voltage with a meter but the circuit doesn’t seem to be able to carry the current needed to run the pump.

 

[rockola1971]

Sounds like fuel pump wiring harness/plug (Vehicle side NOT fuel fump side) needs replaced. Its a fairly normal procedure to need to replace the vehicle plug side for fuel pump since it is passing power every second the vehicle is running....for years and years. They tend to overheat and get melted. Especially if the fuel pump shorts and melts and draws an excessive amount of current until it is dead.

It is not at all uncommon to read voltage at a terminal of a connector but it wont pass any power because of a very high voltage dropped caused by burnt/carboned up terminals.

 

[jj3393]

We replaced that harness (female). Think the ground is bad and trying to determine where to re run a ground for it

 

[Fless]

One other thing you could check is the PCM connectors (under the hood, driver side near the front). They can get water in them and it can hamper the connections. Pull both connectors and check for water migration, corrosion, and a good connection, especially in the lower part of the connectors.

If you can connect a couple of bulbs (for a good amp load) to the pump 12v connector feed, you could see if they light brightly or peter out when the ignition is on.

 

[Coveman]

Do these vehicles have the low-oil pressure fuel cutoff switches? That would interrupt the power supply.

 

[SnowDrifter]

Do these vehicles have the low-oil pressure fuel cutoff switches? That would interrupt the power supply.

nope, just a float in the pan for oil level

To confirm things with OP:

Fuel pump has power and ground, however there's a voltage drop when the fuel pump turns on, and the unit fails to function?

 

[swathdiver]

ECM?

 

[V327839]

Yes they do have an oil pressure cutoff. I had a similar issue with my 01 Yukon then with 04 sub after engine swap. If the oil pressure sender at the back of the block behind the intake reads fully open, it will interrupt fuel. Crank all you want, no start and no codes. Try replacing that switch if possible.

 

[Fless]

Hopefully @jj3393 will come back and let us know if there was a solution.

 

[rockola1971]

Yes they do have an oil pressure cutoff. I had a similar issue with my 01 Yukon then with 04 sub after engine swap. If the oil pressure sender at the back of the block behind the intake reads fully open, it will interrupt fuel. Crank all you want, no start and no codes. Try replacing that switch if possible.

Thats very odd. Using a 2002 schematic there is no oil pressure switch in series with the fuel pump or the fuel pump relay. I havent seen a GM vehicle use that configuration for around 30yrs.

 

[Fless]

Thats very odd. Using a 2002 schematic there is no oil pressure switch in series with the fuel pump or the fuel pump relay. I havent seen a GM vehicle use that configuration for around 30yrs.

I'd agree. The oil pressure switch can be disconnected but the truck will run.

 

[rockola1971]

I'd agree. The oil pressure switch can be disconnected but the truck will run.

At one point back in the early 90's IIRC GM had used a oil pressure switch in series with the fuel pump relay coil (IIRC) so if there was not enough oil pressure it would prevent the engine from running by cutting the fuel off. Now the oil pressure unit at the back of the block is actually a oil pressure sensor, not a switch. It reads actual PSI pressure and thats it. It does not have any kind of switch on/off ability nor is rated for any meaningful current.

 

[exp500]

When you spliced in the new connector harness for fuel pump, are you sure the wireing is correct? Most color codes do not match pins on aftermarket connectors.

 

[rockola1971]

When you spliced in the new connector harness for fuel pump, are you sure the wireing is correct? Most color codes do not match pins on aftermarket connectors.

Good point. When you wire up a DC motor (fuel pump) backwards it causes the DC motor to run backwards and the impeller of the pump has fins that face a certain direction to create the pumping action. Just like a water pump works with its impeller. If wired backwards the fuel pump would not create any meaningful pressure.

 

[iamdub]

When you spliced in the new connector harness for fuel pump, are you sure the wireing is correct? Most color codes do not match pins on aftermarket connectors.

You might be on to something!