Fuel Pump Sending Unit too?

[kevvan]

The oil sensor is a backup to the fuel relay, make sure its connected.

is this what you mean?

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[man-a-fre]

Seems like i remember something about jumping the fuel pump at the aldl connector to get power to it to see if pump works.

 

[Jglew82]

Seems like i remember something about jumping the fuel pump at the aldl connector to get power to it to see if pump works.

Yes, there’s a fuel pump test pin on the ALDL that runs through the fuel pump relay. Give it 12v and see if it makes it run.

 

[man-a-fre]

Fuel pump on aldl pin g bottom left on aldl.

 

[7Brandon07]

Hey sorry to intrude but I have a 1991 c1500 fuel pump isn’t working I did test the fuel pump fuse with a test light it has no power with the key in accessory is it supposed be that way? My real question is do you guys know anything about the blue wire that leads from the fuel tank along the frame into the engine bay then leads through the firewall into the cabin ? I have one and it’s not connected to anything I find it odd, I also don’t even see a blue wire coming from the fuel pump on any of the schematics I’ve looked at they are all either tan or pink, anyways thank you if anyone can help me, I did post my own thread but I see that the poster here has a 90 figured I may find help here thanks

 

[Schurkey]

I did test the fuel pump fuse with a test light it has no power with the key in accessory is it supposed be that way?

The pump won't run in Accessory. The fuel pump needs to run in "Start" and "Run".

My real question is do you guys know anything about the blue wire that leads from the fuel tank along the frame into the engine bay then leads through the firewall into the cabin ? I have one and it’s not connected to anything I find it odd, I also don’t even see a blue wire coming from the fuel pump on any of the schematics I’ve looked at they are all either tan or pink, anyways thank you if anyone can help me,

The way I remember it, the gas tank should have four wires--a ground wire, and three that go into the connector. Power and ground for the pump; power and ground for the gas gauge/sending unit.

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For the record, I replace the pump, the strainer/sock, the hose between the pump and the metal part of the "hanger", and the two clamps on the hose get replaced with "Fuel Injection" clamps that don't damage the rubber.

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Sometimes there's no hose, on some applications GM uses a "pulse damper"; and if the damper doesn't leak, I don't replace it.

I also replace the in-tank wire harness. Typically about twenty bucks, available at any parts store. The replacement is kind of a "generic" replacement, and the wires tend to be extra-long.

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As often as not, I keep the molded rubber seal for the hanger assembly, because the original seal is molded more elaborately than the plain O-ring replacement. If the original doesn't have a billion tiny cracks, I keep it. This is a comparison of the O-ring seals on a Trailblazer pump module.

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[7Brandon07]

Hey thank you for the reply thanks for the picture I was trying to find one that good, I haven’t actually dropped my fuel tank yet but I could only find one wire coming from the fuel tank which is that blue one I knew there had to be more I think I’ll try to get a better look at it tomorrow. So if I understand right if I go to crank the engine over then the fuse should light up my test light? Also something I read a few replies back on this post, if I were to take a wire from the battery and plug it in to pin G on the ALDL if my fuel pump is still good it will turn on ? I’m just looking for a way to know my fuel pump is to blame before I go through the ordeal of messing with the tank and buying the new assembly, it probably is the fuel pump but wanna be sure, it had no signs of failing but sometimes that happens. Thank you

 

[Schurkey]

I will not replace a fuel pump until AFTER I have checked fuel pump voltage supply, and fuel pump amperage draw.

Amperage draw can be done anywhere in the fuel pump circuit, but the voltage testing has to be done as near the pump as you can get. This is generally at the molded plastic connector just before the wire harness disappears on top of the tank.

Ideally, I want to connect a low-amperage probe to the fuel pump + wire, and then view the amperage draw on an oscilloscope. Once I have the amperage pattern on the 'scope, I know the amperage draw of each bar of the fuel pump armature, and I can calculate the pump RPM.

There's no point to replacing an electric fuel pump until you absolutely know that the pump is receiving all the voltage it's intended to run on; and it's not drawing excessive current.

Of course, a fuel pressure test should also be part of the diagnosis--but I want to assure that low fuel pressure is due to a faulty pump, and not a faulty wire harness supplying the pump with power.

 

[7Brandon07]

Thank you, I lost my multimeter during a move a couple years ago, I’ll have to buy another in the event that it is a faulty fuel pump, would you say it’s easier to slide the bed back or drop the tank? Seems like most people say the bed is but lowering the tank seems easier to me I’ve changed mechanical fuel pumps before most my cars been carbureted but this would be my first electric pump and first time lowering a tank, I sorta got lucky compared to some people as I currently only have 3-4 gallons of gas in my tank, thanks

 

[Schurkey]

I've done fuel pumps on my truck both ways. The first time, I dropped the tank. The second time, I pulled the bed entirely off. I think pulling the bed is more work; in my case I had to remove the receiver hitch, and the bumper before the bed would lift. The 18mm (heads on the) bolts that hold the bed to the frame were tough to remove in part because my impact wrench was getting weak--the screen on the compressed air inlet was getting plugged.

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Once the bed was off, the pump was very easy.

Dropping the tank is probably the better way since your tank is almost empty.