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http://www.bbbind.com/free_tsb.html Free wiring diagrams . Enter vehicle info. Year , make , model and engine . Under system click engine , then under subsystem click fuel controls . Click the search button , then click the blue link . 6th diagram down fuel pump electrical diagram .
Ran a hot wire to what fuel pump or the control module ? Which of these is yours ?
Figure 9: Fuel Controls - Fuel Pump Controls (LY6)
Figure 10: Fuel Pump Flow Control Module (Except LY6) Does yours have fuel pump flow control module under the vehicle Fuel Pump Flow Control Module (except LY6)
Mounted to the left side of the frame assembly below the driver seat ?
Fuel Pump Flow Control Module (FPCM) - LMF
The fuel pump flow control module (FPCM) is a serviceable GMLAN module. The FPCM receives the desired fuel pressure message from the engine control module (ECM) and controls the fuel pump located within the fuel tank to achieve the desired fuel pressure. The FPCM sends a 25 KHZ PWM signal to the fuel pump, and pump speed is changed by varying the duty cycle of this signal. Maximum current supplied to the fuel pump is 15 amps. A liquid fuel pressure sensor provides fuel pressure feedback to the FPCM.
Electronic Returnless Fuel System (ERFS) - LMF
The electronic returnless fuel system is a microprocessor controlled fuel delivery system which transports fuel from the tank to the fuel rails. It functions as an electronic replacement for a traditional, mechanical fuel pressure regulator. A pressure vent valve within the fuel tank provides an added measure of fuel pump flow control module (FPCM). Desired fuel pressure is commanded by the engine control module (ECM), and transmitted to the FPCM via a GMLAN serial data message. A liquid fuel pressure sensor provides the feedback the FPCM requires for Closed Loop fuel pressure control.
If your vehicle has this system check it for DTC'S - diagnostic trouble codes . Will take guess work out of it .
DTC P0191: Fuel Rail Pressure (FRP) Sensor Performance
DTC P0192: Fuel Rail Pressure (FRP) Sensor Circuit Low Voltage
DTC P0193: Fuel Rail Pressure (FRP) Sensor Circuit High Voltage
DTC P0230: Fuel Pump Relay Control Circuit
DTC P0231: Fuel Pump Control Circuit Low Voltage
DTC P0232: Fuel Pump Control Circuit High Voltage
DTC P023F: Fuel Pump Control Circuit
The engine control module (ECM) supplies voltage to the fuel pump flow control module (FPCM) when the ECM detects that the ignition is on. The voltage from the ECM to the FPCM remains active for 2 seconds, unless the engine is in crank or run. While this voltage is being received, the FPCM supplies a varying voltage to the fuel tank pump module in order to maintain the desired fuel rail pressure.
DTC P025A: Fuel Pump Control Module Enable Circuit . Ford Electronic Returnless Fuel System Diagnosis Part 1 this for ford but the GM system works the same . You'll need a wiring diagram so you know what wires are what . Free wiring diagrams http://www.bbbind.com/free_tsb.html
Does the starter spin the engine normally when you turn the key to the start position ? Do the headlamps shine bright ? If any of these work correctlly the battery is fine . If the engine turns over normally an doesn't start an run it doesn't mean the fuel pump is bad either .There could be any number of thing's wrong , testing first electrical circuits of igintion system , fuel system . Hooking up a fuel pump pressure gauge , pull a plug wire an check for spark . I'd say your best bet would be to take your to a qualified repair shop before you replace parts you don't need .
This truck uses a high pressure fuel system, key must be on for the electric fuel pump, which is in the tank, to work. Fuel pump senses when pressure drops, so that it will shut off if there is full pressure, and engine not running. If there is lower pressure, such as a broken line, it will continue to pump, if the engine is running. If you're asking if it has a safety feature shutting it off in an accident, no it does not, except if the engine shuts off due to the accident, fuel pump will not work, unless key is turned off and then back on, and will then work for about 5 seconds and shut off, unless engine is started. If you're asking why the fuel pump doesn't work after an accident, it's most likely damaged, or a wire has been damaged, OR the computer maybe damaged (therefore telling it to turn on/off when key is on and engine running with key on and pressure is low, or at the right level to run the engine. Again, if you turn the key on and do not start the engine, fuel pump will work briefly for about 5 seconds to bring pressure up to where it can be started, then the fuel pump will work again when the engine is started (so the fuel pump isn't running with the engine off for very long.)
Testing the fuel pump electrical circuit would be the way to see if the fuel is the problem ! Learning how to test automotive electrical systems would also be helpful . Fuel Pump Electrical Circuits Description and Operation
Also looking at a wiring diagram for your vehicle would be helpful as well . You would do that to see what all is involved in the fuel pump electrical circuit on your vehicle. Also pulling a spark plug wire while cranking the engine over to check for spark .
Free wiring diagrams herehttp://www.bbbind.com/free_tsb.html
Enter the vehicle info. year , make , model and engine size . Under system click on engine ,then under subsystem click on fuel controls . Click the search button ,then the blue links.
But first ,hook up a fuel pressure gauge instead of guessing if it's the fuel pump . You don't know any of this , suggest you take it to a qualified repair shop .
When you turn the key to on, not start, do you hear the pump turn on for a couple of seconds to pressurize the fuel system? If you do, check fuel pressure with a gauge, should be over 50 psi if multi port injection, about 20 psi if throttle body injection. If pressure is good, you may need to check the fuel injector circuit.
If no sound from the pump, you will need a voltmeter or a test light to check the fuel pump circuit. First check the fuel pump fuse. Then check for power at the gas tank on the fuel pump wire, when the engine is being cranked over. Power should be going to the pump then. If nothing, move back to the fuel pump relay-need to check for power at the relay.
WHAT DOES WORK ? WHEN YOU WASH DOWN ENGINE YOU NEED TO COVER/WRAP/ ENCLOSE ELECTRICAL ITEMS !!! COIL, ALTERNATOR, DISTRIBUTOR, CONTROL MODULES, ETC., OTHERWISE, YOU'VE GROUNDED OUT YOUR ELECTRICAL SYSTEM !!! USE COMPRESSED AIR TO BLOW OUT ALL MOISTURE FROM THESE ITEMS MENTIONED. POWER FOR FUEL PUMP SYSTEM COMES FROM BATTERY, TO A JUNCTION BLOCK TERMINAL, WHICH HAS FUSIBLE LINKS WHICH ACT LIKE FUSES,, TO PROTECT EACH SYSTEM . DON'T KNOW IF THATS YOUR PROBLEM , BUT STRANGER THINGS HAVE HAPPENED ! FUSIBLE LINK LOOKS LIKE A HEAVY GAGE WIRE, BUT COULD BE A SINGLE STRAND OF WIRE INSIDE INSULATION ! NEED TO BE REPLACED IN LIKENESS !!! YOUR VEHICLE NEEDS TO DRY OUT ! MAYBE I'M ALL WET ? BUT CHANCES FAVOR THE PREPARED MIND !!! YOU NEED TO UNDO WHAT YOU DID , MAKES SENCE, YES ? ITS WHEN YOUR PROBLEM CAME TO BE ! OTHERWISE, INSPECT EVERY WIRE IN THE FUEL PUMP CIRCUIT, ESPECIALLY THE GROUND WIRES !!! THEY ARE JUST AS IMPORTANT AS THE POWER WIRES, WITH OUT QUESTION ! YOUR RELAY GOT BACKSPLASHED LOOK HOW MANY WIRES GO THERE ! IT ALSO HAS ITS OWN FUSE. THINK I COVERED IT, GOOD LUCK , YOU JUST MIGHT FIX THE PROBLEM AND NOT REALIZE IT TILL IT STARTS !
http://www.bbbind.com/free_tsb.html free wiring diagrams ! Enter vehicle info. year , make , model an engine size . Under system click on engine ,then under subsystem click on fuel controls . Then click the search button , then click blue link .The 7th diagram down shows two fuel pump relays , This I have not seen before ! One is speed cont. relay an the other is just fuel pump relay ? The speed control one has a resistor in the electrical circuit . That is way your droping voltage .
Fuel Pump Electrical Circuit Diagnosis (VIN K)
Fuel Pump Electrical Circuit Diagnosis (VIN 1)
The PCM alters fuel pump speed by energizing the fuel pump speed control relay. Under normal conditions, the fuel pump speed control relay is energized. The applied voltage to the fuel pump is controlled by a resistor assembly. When higher fuel volume is required due to increased engine load, MAP sensor value over 90 kPa the PCM de-energizes the fuel pump speed control relay circuit. The increased voltage to the in-tank fuel pump allows a higher volume of fuel to be delivered to the fuel rail. The PCM also compensates for low system voltage by energizing the fuel pump speed control relay.
Have it check for codes !
Did DTC P0230 or DTC 1260 set?
Go to DTC P0230 Fuel Pump Relay Control Circuit or DTC P1260 Fuel Pump Speed Relay Control Circuit
Go to Step 3
Bypass the whole system and wire the relay for the fuel pump straight to an igintion on sorce.
Gm made that system so the fuel pump would shut down if oil pressure was lost, to try and save an engine!
Sounds like a neat project, what engine did you put into the truck. If it's a fuel injected engine the fuel pump for the old 2.8 may not have enough for it. you may end up needing a inline pump like you can buy at all of the "performance" web sites. Also if it's a carb engine then the pump may be to much, and keep unseating the seats! These are just thoughts going through my head, you may have already thought this through?
Good luck with the project, it will be a tire burning SOB. Oh and be careful with that little 7.25" rear end that was in there from the factory, they break with just a 4.3 and you are adding a couple more HP to the mix!