FUEL PUMP HELP

sdotapel11

Original poster
Member
Dec 9, 2019
18
Minnesota
when i removed the fuel pump relay and direct jumped the power circuit, the pump turns on and gas flows. therefor the control circuit is not actuating the relay? We tested two other relays and they aren’t the issues. We tested for voltage and that isn’t the issue. We checked grounds and there were loose ones and we tightened but still nothing. Something isn’t giving the relay power. We are thinking PCM? The pcm is also making a quiet “dog whistle” sound with the key off, and with the key on, it gets slightly louder.
 

linneje

Member
Apr 26, 2012
404
Just a thought (from experience), did you check for voltage under load. It is rare, but in some cases the voltage will test correctly, but when you place it under load and check the pump will not run because there is a massive voltage drop. I had this happen with my water pump in my RV and was baffled when my pump ran in a different circuit. But checking the voltage under load showed the drop.

If this is the case, you may have a corroded wire or loose wire, probably on the ground side of the circuit. Check that first before moving on to other possibilities.
 

mrrsm

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Supporting Donor
Member
Oct 22, 2015
7,641
Tampa Bay Area
Along the same lines... @MAY03LT covers problems with Diagnosing Fuel Pump Circuits on Trailblazers and Envoys that may reveal other things. This one is worth Downloading and Reviewing because he shows and explains the Circuitry and How to Test the System:


Again...along the same line of @linneje 's thinking... THIS VOP (Video Original Poster) finds a PHYSICAL CONNECTION PROBLEM that solved this Mystery. Please watch the ENTIRE Video.

If necessary... Elevate the SUV safely on Jack Stands high enough to be able to maneuver under and around the Vehicle and have enough room to gain access to the Wiring and not feel like Human Pretzel. This is where having Large Truck Jack Stands(with Pin Safety Locks) can be very helpful:


One last idea... A Faulty Ignition Module Switch can be involved if the gliding contact points inside are either missing their "sweet spots" or going past those contact points. If you have had any sketchy starts just after turning the Key...Keep that thing in mind.
 
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sdotapel11

Original poster
Member
Dec 9, 2019
18
Minnesota
Along the same lines... @MAY03LT covers problems with Diagnosing Fuel Pump Circuits on Trailblazers and Envoys that may reveal other things. This one is worth Downloading and Reviewing because he shows and explains the Circuitry and How to Test the System:


Again...along the same line of @linneje 's thinking... THIS VOP (Video Original Poster) finds a PHYSICAL CONNECTION PROBLEM that solved this Mystery. Please watch the ENTIRE Video.

If necessary... Elevate the SUV safely on Jack Stands high enough to be able to maneuver under and around the Vehicle and have enough room to gain access to the Wiring and not feel like Human Pretzel. This is where having Large Truck Jack Stands(with Pin Safety Locks) can be very helpful:


One last idea... A Faulty Ignition Module Switch can be involved if the gliding contact points inside are either missing their "sweet spots" or going past those contact points. If you have had any sketchy starts just after turning the Key...Keep that thing in mind.
I did change the ignition switch within the past two months
 

mrrsm

Lifetime VIP Donor
Supporting Donor
Member
Oct 22, 2015
7,641
Tampa Bay Area
Okay... But if that component was not an ACDelco replacement part... still keep it in the back of your mind... The Story goes that many an Experienced Owner of the GMT-360's "Keeps a Brand New ACDelco OEM Ignition Switch in the Glove Box"...Just in case of future problems with Non Start conditions raising its Ugly Head.
 

budwich

Member
Jun 16, 2013
2,027
kanata
turn your key to "ON/RUN" and leave at that position, do not attempt a start, what dash lights do you see?
 
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