2000 Chevy astro 4.3 fuel pump issue

I think you're looking at a bias voltage from the PCM? You've very likely set a fuel pump relay code in the PCM, not that that'll cause your problem.

This video is what you want to watch if you want to understand this better. I also attached the wiring diagram for reference.



You're better off partially plugging the relay into the socket and carefully probing the pins. This will complete the circuit, but still give you access to the terminals.

Key on, with the relay partially installed, retake your readings. One of those 12V readings should be gone. Let us know what you find, and we can go from there.

No worries, a test light and DMM will work perfectly. I forgot to ask - with the pump relay bypassed (and the pump running), does it start and run? If not, then it's possible that the pump control issue is a symptom of a bigger issue.

I dunno that replacing the alternator would have caused these symptoms, unless the battery is weak (from getting discharged) and causing other issues. Or, is it possible that there's a poor battery connection? How's the cranking speed?

I'm sorry to nag, but it's worth being specific. With the key on, relay partially installed, what are the voltages at all four fuel pump relay pins? And the same readings during cranking. That'll spell out what's happening at the relay.

Sure, we can do that! Here's a link to the engine performance diagram, and a shot of the fuel pump wiring:

www.scannerdanner.com/media/kunena/attac...17/DownloadPdf27.pdf



The diagram shows C310, the main connector that goes from the vehicle to the tank and pump itself. This connector is on the drivers frame rail, just ahead of the tank, if I remember correctly. It contains the fuel pump power and ground, pin A grey and pin G black. I'm not sure where you've been testing before this, but C310 makes an idea testing location if you can put your hands on it.

Grey should have 12V (or close to whatever battery voltage is) whenever the relay is energized. Black should have less than .5V at all times (relay off, relay on, whatever). 4-5V would indicate a voltage drop, or weak source voltage, on the grey wire. Paul has several great videos out there on fuel pump diagnostics, but you could probably start out with these two:

Possible, but I think Cheryl is right, powers and grounds are a good place to start. I can't pick them all out right now, but they'll be in the PDF diagram I posted earlier.

Did you happen to watch that last video I posted? The symptoms sounds VERY similar to yours.