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Thread: Flex Fuel Sensor and 0411 PCM

  1. #1
    Lifetime Member
    Join Date
    Mar 2007

    Default Flex Fuel Sensor and 0411 PCM

    I am working on making a circuit to simulate the flex fuel sensor output (0-5V square wave ~50 to 150 Hz) to utilize the additional timing / fuel tables in the L59 OS's. I have a board made, and tried to see if I could see it using EFI Live. I started with a 2000 Blazer 0411 PCM (happened to have it laying around). I started up the Scan tool .... neither of the flex fuel PID's showed up. Makes sense, the OS in there isn't looking for it

    Flashed a 2002 L59 (5.3L) Silverado tune into the PCM ... changed the VIN # to make what is in the OS ... then started up the scan tool. Validated the PIDs and low and be hold, there are the flex fuel PID's .... yeah!

    Now, I tried to plug in a square wave generator to the flex fuel sensor input wire (C1 (blue) pin 56) to see if I could watch it on the scan tool. Didn't work.

    I am using a bench harness (not a full wiring harness), so I thought maybe I didn't have a ground hooked up. LS1B PCM bench harness only hooks up a couple of ground wires. I grabbed all of the grounds that I could find (ECT, TPS, MAP, etc...) and hooked them up to the same ground as my bench harness..... still didn't work. I verified that the square wave generator is putting out a signal by hooking it up to an oscilloscope, but the PCM doesn't show anything.

    Has anyone used the flex fuel PID's on a 512 kB 0411 PCM??? Were the L59 PCM's different than the rest of the 0411 PCM's? I am actually planning on using this on a 1 MB PCM, but I don't have the vehicle available to play with. That is why I was hoping to get this working on my bench setup 1st.

  2. #2
    Forum tyhee Site Admin GMPX's Avatar
    Join Date
    Apr 2003


    Personally no but I was talking to someone just yesterday who uses that 2002 L59 OS in conversions, with a real sensor it does display the ethanol percent.

  3. #3
    Lifetime Member
    Join Date
    Mar 2007


    Thanks for following up on that PID working.

    I think my problem is now my "simulator". I made a simple circuit using a 555 timer chip. Verified it was producing a 0-5 volts square wave on an oscilloscope. But after reading the shop manual (2004 Silverado books), I am now concerned that I may be sending the PCM the "wrong" signal. From the shop manual:

    "The Fuel Composition Sensor (FCS) produces a square wave digital signal with variable frequency and variable pulse width. The frequency is linearly related to the ethanol content in the fuel. The pulse width is linearly related to the temperature of the fuel.

    The PCM provides an internal pull up to 5 volts on the signal and the sensor pulls the 5 volts to ground in pulses."

    I am now confused on just exactly what kind of signal I need to send it, and how to hook it up. I found a simple testing circuit for the sensor itself and it does talk about using a pull up resistor on the output. This appears to match what the shop manual says is going on inside the PCM (internal pull up to 5 volts). But what am I supposed to do for the "sensor pulls the 5 volts to ground in pulses"???

    Any insight would be greatly appreciated.

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