Fuel pump getting power, but NOT running

chevrolet
caprice
capriceclassic

#1

Hello all, I created an account just for the sake of getting help with this tricky problem…

I have a '95 Chevy Caprice Classic 4.3L Sedan, and it cranks but does not start.
This started happening after about a year of the car just sitting in the driveway. Before she sat for that year, she ran just fine.

I am under the impression that the problem is the fuel pump, it is NOT running when the ignition key is turned to the “on” position,
this is very confusing because a volt meter placed on the connectors going directly into the pump motor shows 12+ Volts when the key is turned, for about 2 seconds, as it should, however the pump motor does not run. Also, I have already replaced the pump relay AND the entire pump assembly (though the pump I used as a replacement was also pulled from a junkyard vehicle).

The MOST confusing part is, when either of the fuel pump motors that I have are plugged directly into the car battery, they run just fine. So I have no idea what the problem could be at this point!

Please help! Thank you for reading.


#2

The fuel pumps may work when connected to the battery because they’re not doing any work. But if you then ask them to start pumping a liquid they may not work.

I’ld install a new fuel pump just to take it out of the equation.

Tester


#3

My first suggestion is going to be to work the plug for the pump. They’re probably spade or pin style (though I couldn’t say for certain), and the female ends tend to lose their springiness over time. They get bent out a little and don’t make good contact. I don’t know what you’re doing with the meter, but I’m often jabbing the pointy ends of the leads in to test stuff, so that doesn’t rely on those ends being tight. So first I would separate the leads from the plastic housing of the plug (I can’t say specifically how to do it on this plug, but there are usually little plastic release tabs that let you slide the leads out). Then clean them up really well and use some gentle plier pressure to squash them a bit - make it so the male ends have to push in tight. If that didn’t help, I might then go on to finding a new connector end. These can often be found at auto parts stores, come with a pig tail, and you cut off the old and solder in the new.

Given what you’ve found in testing - that’s all I can think of.


#4

The pump will not stay energized if there is no crank signal . . .


#5

So you have power (12 volts) at the pump. What about ground?


#6

because a volt meter placed on the connectors going directly into the pump motor shows 12+ Volts when the key is turned

Just to be clear, you’re backprobing the connector that directly feeds the pump with both meter leads? Not one on the chassis and one on the connector, right? The pump is connected when you’re taking readings?


#7

Some vehicles control the pump operation on the return side of the circuit. From what you say though it sounds like there is a problem at the connector to the pump motor. To verify a good ground connection place the ground reference probe of the meter on a good chassis ground point. Then check each of the leads going to the pump motor. When the pump should be running one of the leads should have 12 volts on it and the other should be close to zero, which indicates a good ground connection. If both sides have voltage on them then something is wrong with the return side of the circuit.


#8

I’ve run into this problem several times over the years with DC motors such as fuel pumps, heater fan motors, wiper motors and cooling fans. You would assume that reading 12VDC at the motor…the motor should work. The problem is that the motor is usually binding and the amperage in the circuit is not strong enough to turn the motor. If you connect it directly to a battery…it spins away. This only confirms that the motor has the ability to rotate if given enough amperage. The real test is when the motor will operate on a circuit that was designed for it. In short…and others have already said this…replace the motor if the circuit is in good repair.


#9

No matter what upstream circuitry controls it, a good pump should run as long as there’s battery voltage going to both terminals at the pump connector. This has to be measured when the pump is connected, not by removing the connector and measuring the harness voltage with the pump disconnected, b/c the connector could be faulty. Assuming the connector at the pump is ok – these are known to corrode and fail, so be sure about that – then the pump must be binding or getting clogged with sediment inside the tank.