Gm Fuel Pump Module

Gary Technical Support Specialist New Jersey Posted   Latest  
Unsolved
Driveability
2015 Chevrolet Silverado 1500 4.3L (LU3) 5-spd
P2635 — Fuel Pump "A" Low Flow/Performance
Check Engine Light On

Good evening everyone. I have this Chevy Truck with a 4.3l that set a P02635 everytime. The low pressure fuel line is running around 85 - 90 psi and and scan tool shows Desired to be at 65psi. They have changed the low pressure sending unit, no change , if you relive all pressure it will go down to 0psi on the scanner so im sure the pressure is true. They have also changed the Fuel Pump Control Module same problem. The FPCM has its own High Speed network, just PCM to FPCM. I scoped the network, it does show mirror data on both lines. The only thing I noticed is that it seem to be starting at 3.0V not the normal 2.5V although CAN high and CAN low do change 1V each. GM info says that the output to the fuel pump is cycled at 25KHz with a varying PWM to control the pumps speed on the + side to obtain desired pressure . On both the new and old module are running a straight 14v to the pump no cycling (scoped). Ground to the pump is also controlled through FPCM but that not cycled, it also is around 0v when the pump is running. I put my low current probe on the + feed to the pump, look like it is running around 16 -17 amps, you can feel the module and wiring harness getting warm as it runs for awhile. My question is what is stopping the module from controlling the pump duty cycle? with key off everything does shut down, The ECM and FPCM each read around 120 ohms each and 60 ohms plugged in.

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Interesting
William Technician
West Virginia
William
 

No other codes?

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Gary Technical Support Specialist
New Jersey
Gary
 

The original module was setting a U0074

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William Technician
West Virginia
William
   

Sounds like a communications issue as you exactly described this bulletin. I would follow Caleb’s recommendation.

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Gary Technical Support Specialist
New Jersey
Gary
 

William I saw that TSB and thought great the answer. Then I read it hoping for some hand holding testing to isolate the problem, but nooooo

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William Technician
West Virginia
William
   

Lol typical GM for ya. As so many parts have been replaced, it basically has to be one of a few things. A loss of continuity in either the 7493, 7494, or 5660 circuit, a module ground/power/ignition or serial data ignition, or a faulty ECM. My course of action would be to light a test thru the circuits using a fused jumper wire at the other end. If it lights on all circuits, likely an ECM.

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Gerry Owner
Louisiana
Gerry
 

Hi to all these post and answers help me greatly to diag my U0074 no communication with fuel pump control module don't want to go over everything but my powers and grounds where good and my com wires at the FPCM with connector unplugged KOEO 2.5V each circuit plugged in and engine running the + circuit was steady at 3.3V and the - circuit steady at 0.1 volt the same with the FPCM unplugged and…

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Caleb Diagnostician
Indiana
Caleb
 

Awesome, good deal man.

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Saifullah Diagnostician
Ontario
Saifullah
 

I would think that if it sets a “ low flow” code it will go into fail safe and provide a full continuous ground instead of a PWM control.

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Agree
Glenn Owner/Technician
Texas
Glenn
 

Just going of of electrical theory, I would say you have a high resistance on the ground circuit. Check the grounds carefully. Clean and retighten all you can find. On some models there is one located at the front driver side motor mount that has proven to cause issues. If the module is seeing to high of a voltage, typically the ground is weak. I have used this approach for years and it has…

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Gary Technical Support Specialist
New Jersey
Gary
 

I did find the ground on the frame , I lossen it and retightened it , I’m showing almost 0 volts on the pump ground

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Caleb Diagnostician
Indiana
Caleb
 

Glenn if the fuel pump is pulling 16 amps and the FPCM grounds the pump how could there be a voltage drop on the ground?

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Glenn Owner/Technician
Texas
Glenn
 

Going off of general experience with these issues fuel pump circuits seldom require a 20 Amp fuse. He is measuring 16 Amps, that is a sign of high resistance which usually involves a poor ground due to high resistance or a poor connection issue. He is measuring over 50% of the rated fuse capacity, that is not good for any fuel pump.

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Caleb Diagnostician
Indiana
Caleb
 

Ok, correct me if I am wrong. I was always under the impression that when resistance goes up, amps go down. When resistance goes down amps go up. I thought this was basic ohms law?? If there was a v drop/high resistance on the ground that would cause lower amperage in my experience.

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Caleb Diagnostician
Indiana
Caleb
   

Gary, I have had the same fault on a 16 Silverado. The 100% ground is normal cause the FPCM has a comm problem with the ECM so it can not receive data from ECM on what speed to run the pump so it fully grounds the pump in order to keep the truck running. The 16 amps is normal for the pump when its “ON” 100%. Can you please post your scope captures of the CAN signals? Be sure your scope leads…

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Gary Technical Support Specialist
New Jersey
Gary
 

Sure does Caleb. According to GM the positive side is duty cycled 25k. The ground side is just grounded through the FPCM when it received a signal from the PCM . Now I see there there is also a “turn on or wake up” signal that comes from the BCM, I did not check yet. Is it possible the CAN signals are incorrect from the PCM? I was wondering if the FPCM would default the pos side to full on, no…

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Caleb Diagnostician
Indiana
Caleb
   

Ok gotcha. Doesnt really matter which side is duty cycled. If yours is full power and full ground then its definitely in default. Just test the CAN lines like I said KOEO and post them up here. I bet that will tell the story. The wake up signal is just a low current 12v IGN feed that wakes up the module. It does not turn on though when the key is turned on. If you watch it on a scope it will…

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Gary Technical Support Specialist
New Jersey
Gary
 

I will be heading back to that store tomorroW. Will get some scope shots of the CAN lines. i wonder if it would be a good idea to replace the missing module with a 120 ohm resistance During testing?

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Caleb Diagnostician
Indiana
Caleb
 

You could but it should not matter. Usually when one resistor is missing the waveform is slightly less crisp and not as defined.

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Dale Diagnostician
South Dakota
Dale
 

Would love to see this whole thread as a walk thru in a video format. Helps with the learning process in some areas I'm weak on but that's why we want to learn it. Would make for a good case study possibly?

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Bernie Mobile Technician
California
Bernie
 

Are you having …mm issues when any modules? Are you getting the fuel pressure reading of 85-90psi from the “actual fuel pressure” data PID on the scanner or from a fuel pressure tester attached on the line? Yes, I read the part about reading 0 psi, but I would think you would want to …nfirm the fuel pressure with an actual gauge under load especially when the …de sets “every time”. The fuel…

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Gary Technical Support Specialist
New Jersey
Gary
 

Hi Bernie you are right on with the sensors location. This is the 2nd sensor installed all with the same results. But we can install a gauge up by the manifold just to be sure. There are no other modules on this network other than the PCM and FPCM. Earlier versions had the FPCM part of the bigger high speed CAN network, replacing the module required it to be programmed. This setup is plug and…

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David Technician
Indiana
David
 

Okay just did one of these I had no data pack coming out of PCM but termanating resistor was good new pcm works great now

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Helpful
Caleb Diagnostician
Indiana
Caleb
 

Nice job David. You said everything I was trying to say in one sentence lol. Someday ill be able to do that haha.

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Gary Technical Support Specialist
New Jersey
Gary
 

Ok update time. Spent another couple of hours on the Silverado today and saved a couple of screen shots from the u-scope. Taking some of Caleb's advice remeasured the terminating resistors in each module both in around 123 to 125 ohms together 61 ohms. Caleb you were right the “wake Up” wire (vo/yel) does come alive when you open the door this turns the FPCM module on and communication starts…

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Caleb Diagnostician
Indiana
Caleb
 

Nice work! Looks like you nailed it!

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Geoff Diagnostician
Hawaii
Geoff
 

off-topic --- I had no idea the uScope could upload screen shots.

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Ruben Owner/Technician
Ohio
Ruben
 

following this

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