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@002 Trailblazer ls . Now I know there is something hidden in plain sight that we're not seeing so I'm going to tell my tale and whether parts are revelvant
or not I'm including them.
I was driving my car at night with low beams that would work and for some silly reason would quit so I'd drive with my brights on until the lows came back ect. One day watching utube learned of a relay, replaced it and BOOP the lows were back. Got to my destination , went to leave, the car would
crank but no start. Acting like it was getting no fuel. Towed it home , replaced the relay , good. Didn't hear the fuel pump at all. Had a full tank of gas.
Bought a new fuel pump , put it in , still no sound of it working. Dropped it again. Checked power from fuse box , checked plug that goes into pump.
Got a little confusing . so lets just say there is power. Did plug that into the pump and tested it with a pin and got nothing. So ordered another pump
gotta wait till tuesday. Don't know what else to do. Is there a shut off doo dah?
We had a time putting the new one in . Had to squish it down pretty hard and hold it because the ring wouldn't cooperate. Any advice? This has been going on 3 weeks now.
It looks like the PCM provides power to the fuel pump relay coil, which closes the switch to the fuel pump. How are you testing for power at the fuel pump? I ask because some ways are more accurate than others. With the tank back up it might be hard to get to that pump connector, but I'm not sure. The gray wire at the pump should have battery voltage on it with the key on and the black wires should have less than .5v on it with the key on. It may only energize for a few seconds unless you crank the engine, or it's running. If you can't get to the pump with the tank up you can at least check the 20a fuse under the hood that needs to be good and we can do some tests to see if the relay is doing its job.
Put in a second better pump by far but still nothing. Now am I to understand that the
fuel pressure sending unit? the second round deal on the gas tank, if thats closed the pump will get no power? Is there a cut off switch?
jsjj4x4 wrote: Checked power from fuse box , checked plug that goes into pump.
Got a little confusing . so lets just say there is power. Did plug that into the pump and tested it with a pin and got nothing..
Are you saying you had voltage at the fuel pump plug with it disconnected? Then when you plugged it in and backprobed you didn't have power? If so that sounds like high resistance somewhere in the fuel pump power circuit.
man, I don't know anymore. Could you walk me through a kind of checklist in the order of occurence? Because I've looked at so much stuff I xcan't think anymore.
watched scannerdan and his explanation about how the oil pressure switch acts for the pump when the relay doesn't work and how dumb gm was for leaving it like that. So now am trying the oil pressure switch. I can't afford to trade all these sensors out and I've heard it said that it could be any number of sensors and switches etc. My ex keeps saying it's something stupid we've overlooked. But I think he probably knows as much as me and that's less than 0
The gray wire going to the fuel pump connector should have power on it when the engine primes for about 3 seconds and when you're cranking the engine over. Test that with a test light or voltmeter.
Ok so the female end of the plug that comes from the fuse box to the pump .... the only readings that showed was the purple wire that with the key on or off flux between 5.16/5.18 . Nothing anywhere else
We are working with a purple on the left outside , grey after that then black with the last being orange. The purple with key on or off tests at 5.17 +/- . The grey has nothing cranking or in the short time. This is looking at the plug from behind.
We went further up the grey wire to see if it had power on up (about 7 inches) and nothing still at cranking.
We are working with, from behind, far left purple wire, grey next to that, black then orange. The purple with or without engine cranking reads 5.17 +/- . The grey ) . On , cranking or whatever. Probed to at least 7" above wire , still 0.
The gray and black are what actually go to the pump. You should have 12v on the gray and near 0v on black while cranking. Did you check the 20a fuse #10? Maybe called ecm b? that supplies power to the gray wire through fuel pump relay #41. Check the fuse, if it's good see if there is another relay that is the same as that fuel pump one and swap them. That's an easy starting point. If you don't find any issues there we will have to see if the relay is getting what it needs to work.
We checked ALL the fuses at least 8x's, changed, swapped , voltage checked the slots everything except an exorsism. More than one relay makes a clicking sound when turning the key. From the inside it sounds more like I would imagine a fuel pump priming would sound. I never heard anything like that when it ran , regarding that. But might be a little confusion at the fuse block where the relay goes. looking from the outside in from drivers side there are 4 prongs , the bottom right is a power side and upper right is power too, correct? and if you put a wire from upper left to bottom right is suppossed to bypass relay right? then what?
Where are you grounding your meter? I ask because I've had a bunch of problems with broken/rotted body and frame grounds causing no start and when the current traveling through the pump with end up putting voltage through the body and having your meter attached to that will instead read voltage drop which could be near zero if ground is gone.... moral of the long winded story is test your grounds if you're not connected to your battery (which is the preferred place to connect for testing) which is reason #323 why I love my power probe!
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The following user(s) said Thank You: Noah, Tutti57, jsjj4x4
with the fuel pump relay out, you should have power on two of the slots for 3 seconds after the key is on, during cranking, and running. jumping the switch side of the relay from the one with power to the side without should send power to the pump. If it doesn't, your problem is between there and the pump. Two legs of the relay are the coil and the other two are the switch. There might be a diagram on the switch that shows you which legs are which. You can use a test light and go from B+ to the switch slot that doesn't have power and that should power up the pump if the wiring is good to it. Paul has a few relay testing videos you can watch on youtube where he does this exact test. I recall one where he does it in a honda pcm relay.
the pump and car run when it is hard wired directly to the battery and using the grey
plus ground off the pump plug ( the 2 center wires ) .
so what you're telling me is that if you jump the relay the grey wire is only powered for 3 secs to prime and not on all the time? i thought that was what the lower voltage wire did (purple) (?)
because i did just that. i went poking along the grey wire going towards the fuse block with the relay jumped and got 3/4 there and never got n e kind of reading because i did it wrong, right? i need a fairy car mother. i told scannerdanner i need to physically use his brain for 5 mins. i've watched soooo many videos and up to a point i get it then they take a diff road to recovery and i'm left hitchhiking again. why cant they be exactly like mine from start to end ? wouldnt that be a lil slice of heaven
It is power for 3 seconds in accessory position but is on all the time while cranking or running.
By jumping the relay, you are acting like the relay. The side of the relay with the coil pulls the switch side closed when power and ground is applied to it. If you jump the load/switch side and the pump runs, you either have a bad relay or the control/coil side of the relay circuit is missing power or ground.
With the relay removed, key on, two of those slots should have power. Do they?
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