let me start by saying I've read several related threads.
my problem is: bought a 99 2door jimmy, didn't run. replaced distributor + cap & rotor, it ran, but rough at idle. ran code check, cleared some old codes, nothing new..
the next day i turn the key and hear nothing. it turns over but no whirring from pump. i order a pump/sending unit, before i change pump it starts! so one day it works, next it doesn't, then it rises again on the 3rd day!(pun intended) i change sending unit anyway, then hear nothing when i turn the key.
bench test the unit and it's good.
i know gray is 12v, black is ground, purple 5v, black w/ stripe ground
a test light shows the black wire has power. both black and grey light up when plugged in to the unit.
there is a large pin style diode that comes from the same harness the pump wires come from it has a long blue cap with several pins. it looked faulty so i took the blue cap off. I had no power on grey or black pump wires.
Then plug blue diode cap back in now the black wire is hot again.

what is that wiring diode and is there a relay control other than under the hood, which i changed with the horn relay and it was okay?
I read about a fuel pump control behind the glove box but only found a signal flasher relay and wiring. i was thinking this might be for earlier models.

to simplify:
1.how does my ground have power?
2.is there another control relay other than under hood?
3.does ignition or oil pressure switch factor in power to the pump?
4. i'm open for suggestions

 

what happens if you jumper out the fuel pump relay?

can you meter the wires at the pump while wiggling them around to make sure the connector is in good health.

oil preasure is tied in i believe but should not factor in on intial key start.

keep googling for a schematic for the priming ckt for the pump. it bypasses the oil preasure switch on intial start just to verify those wires.

on your 2 door look to see were the pump is in relation to the body floor. if you can cut in an access panel, do it. best mod you can ever do. my 97 4 door is in a bad spot for me to do.

 

i've found a thread that says there's a relay on firewall near the distributor. i'll check it out tomorrow. I think there's a pinched wire somewhere. if I could find the problem and fix it an access thru the floor would be great, but until my wiring issue is resolved I'm leaving the tank out. i have a 2000 jimmy i could swap relays with to see if that's part of the problem. thanks any other ideas out there!

 

The fuel pump relay is located in the underhood fuse/relay center. It was only located on the firewall on older models. There are no diodes in the fuel pump circuit nor is there any direct interaction with the oil pressure switch on the 98+ trucks like there was on the older models.

 

Fixed the wiring problem! It was the plug that T's at the driver side under bumper. Only 2 door models have this. It has 11 wires that run into a connector that has a blue cap. I replaced it because it was shorted out,which somehow made my ground wire hot. Thanks for the replies hope this helps someone else that thinks they need a fuel pump. I replaced mine while i was in there, but the old one worked fine. Also, don't know how long it will last, I paid about $50 for it on ebay. Beats $300+ at the parts stores.

 

yeah, well hopefully your $50 ebay pump lasts, dad put one in the '97 my wife has now. he did that shortly before we got it (almost 2 years ago) about 3 weeks ago the fuel pump died. yep, i paid $300 at autozone for a new one. hopefully it lasts the rest of the life of this 190,000+ vehicle. i thnik this is the 3rd pump overall thats been in this vehicle since the time dad first got it 5 years ago.

 

I wouldn't use anything but GM or Delphi. I wish you guys that like changing fuel pumps several times lived close to me. I need help often.

 

The reason you showed power on the grey and the black wires at the module is because the black wire lost its ground. Voltage from the grey wire was going through the windings in the pump motor, and coming out the black wire. An ohmmeter on the black wire at the module connector would have shown infinite resistance to ground, (open circuit) and it needs to be less than 5 ohms.

The "T" under the bumper is a ground buss. Several grounds attach to it then one black wire goes to ground. On 2 doors, it grounds to the top of the frame, left rear corner. Might want to check that Wiggling around under there you may have disturbed it and it's making contact now.