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2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

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  • #16
    Re: 2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

    The wire harness runs from the tank, down the left side of the truck, across the fiirewall and terminates at the right front fender were the FICM (fuel injector control module--I transposed some letters in the first post) is located. It passes through four connectors starting by the tank with C009 and passing through C008, C002 and ending at C001 which is the connector that plugs into the FICM. If it was my truck and wasn't sure what some guy is posting will work, I would most likely try to connect a test switch near the C009 connector by the tank. It should be easy to get to after removing the tank cover and easy to see and splice the wires (or repair them if this doesn't work). If you want to do it at some other point, the wire colors stay the same through all harnesses. If it works out, you could tap into the harness under the drivers door area or extend the wires into the cab from the C009 connector area. If you want to mess around with this kind of stuff, the CNG supplement for your truck would be a good investment from www.helminc.com.

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    • #17
      Re: 2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

      [QUOTE=siai47;35370]The wire harness runs from the tank, down the left side of the truck, across the fiirewall and terminates at the right front fender were the FICM (fuel injector control module--I transposed some letters in the first post) is located. It passes through four connectors starting by the tank with C009 and passing through C008, C002 and ending at C001 which is the connector that plugs into the FICM. If it was my truck and wasn't sure what some guy is posting will work, I would most likely try to connect a test switch near the C009 connector by the tank. It should be easy to get to after removing the tank cover and easy to see and splice the wires (or repair them if this doesn't work). If you want to do it at some other point, the wire colors stay the same through all harnesses. If it works out, you could tap into the harness under the drivers door area or extend the wires into the cab from the C009 connector area. If you want to mess around with this kind of stuff, the CNG supplement for your truck would be a good investment from [QUOTE]

      siai47,
      Have you used this circuit before? Just wondering if it has been successfully used either in the pickup or another car.
      Last edited by Highmarker; 03-25-2010, 07:28 AM. Reason: Tried to fix the quote

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      • #18
        Re: 2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

        Just wanted to report that I was finally able to give my truck a trial pull on both CNG and gasoline. I didn't build the switch yet since I wanted to get a feel for the difference between fuels and it was significant. I noticed what I would guess was a 25% increase in power on gasoline over CNG. Would love to have gasoline on demand switching now for steep inclines.

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        • #19
          Re: 2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

          Now that boat towing season is back, I've gone back to unpluging the CNG connector under the truck when pulling. When plugging it back in it usually take several start cycles (and several miles) to get back on CNG. I've found that you can immediately get back on CNG by disconnecting the battery for a second. If you do it quick, you don't lose your radio presets or clock but it will reset the system back onto CNG instantly.
          2003 GMC Sierra K2500 ex-cab, factory bi-fuel
          -Todd

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          • #20
            Re: 2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

            Originally posted by siai47 View Post
            You might want to try this if you are adventurous . CNG switchover occurs when the fuel level drops to a certain level and will restart when the fuel level is raised above a certain level. Changing the fuel tank pressure sensor circuit might allow you to change fuel type on the fly without getting a DTC. The fuel tank pressure signal is sent from the tank to the FCIM as shown in the drawing. When the tank is full the sensor has low resistance causing the voltage seen at pin 25 to be nearly 5 volts. When the pressure is low, the resistance is high causing the voltage at pin 25 to be close to the low reference level (not ground). By cutting the Dark green with a white tracer wire and installing a two position three pole switch you should be able to either send the correct pressure signal from the transducer to the FCIM or a false 0 pressure signal making the vehicle switch to gasoline. The remaining pole on the switch needs to be attached to the grey wire. I am not sure if returning the switch to the normal position will restart CNG operation or an ignition cycle wil be required. Again, TRY THIS AT YOUR OWN RISK !
            Just wondering if anyone has tried something like this: Installing a switch on the CNG tank sensor circuit to manually for operation of a Chevy truck on gasoline.
            Jared.
            Mountain Green, Utah
            2003 CNG Cavalier
            2003 CNG Silverado 2500HD

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            • #21
              I have had my 2003 2500HD for about 2 years now and I want to know the same thing. I also have a check engine light when not running CNG, it goes off when running CNG. The power is slightly less with the CNG but it's not really bad, I haven't noticed too much hauling but I don't haul often. I have a switch in the dash but I have to hold the switch down to go to gas.... as soon as I let go of it, it goes right back to CNG if there is CNG in the truck. Any help making it switch back and forth would be great.

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