cruise speed input signal
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Thread: cruise speed input signal

  1. #1
    FRESHMAN ROVER
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    cruise speed input signal

    Hi, I dont know why in my 1998 4.6 p38, at connector c255 pn 20 there is no reading voltage (no 5v stationary no 3.5 v wheels turning, with ignition key in pos II). My speedo works fine, so the output signal from the Z108 (ABS ecu) pin 28 to the becm (road speed) is working.
    some ideas?
    The output signal from the becm connector 256 pin 16 (road speed to instruments pack) is the same (exactly the same) as the output signal from the becm connector c255 pin 20 (road speed to cruise control)? (both are yellow wires!!!, both are road speed signal....)
    thanks

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  3. #2
    SOPHOMORE ROVER
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    Re: cruise speed input signal

    It really needs an oscilloscope to check the signal properly. You might get away with an LED & Resistor to see the pulse with wheel turned by hand.

    Something like this will easily do the job with a laptop.

  4. #3
    FRESHMAN ROVER
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    Re: cruise speed input signal

    thanks. But... easily with a laptop???? ....how???
    My idea was to connect then connector 256 pin 16 directly (via wire) to the connector c243 pin 11 (Cruise control ecu) instead of connect the from connector c255 pin 20 to the connector c243 pin 11..... what do you think??? may be i will cause a becm shortcut or fire some where???
    sorry for my poor english...

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  6. #4
    SOPHOMORE ROVER
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    Re: cruise speed input signal

    Simple first check is to unplug both connectors, and check the wiring for continuity end to end. While you're doing that also check the connectors for corrosion, and clean the contacts if required.

  7. #5
    FRESHMAN ROVER
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    Re: cruise speed input signal

    thanks, but remember i posted "I dont know why in my 1998 4.6 p38, at connector c255 pn 20 there is no reading voltage (no 5v stationary no 3.5 v wheels turning, with ignition key in pos II). My speedo works fine, so the output signal from the Z108 (ABS ecu) pin 28 to the becm (road speed) is working.
    some ideas?" ...then my real problem es that i dont have speed signal to the cruise control...

  8. #6
    SOPHOMORE ROVER
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    Re: cruise speed input signal

    Measurements of the speed signals using a multimeter are challenging at best. It depends a lot on what Averaging or RMS adjustments the multimeter actually does.

    Also, many of the outputs in the P38 electrics depend on pull-up or pull-down resistors in the destination ECU, so often you need both connected for the measurement to work.

    If your readings from BECM on "C255 Pin 20" and "C256 Pin 16" are "exactly the same" then surely both are working ? In this case the issue is either the Cruise ECU, or the wiring between this and the BECM.

    If the readings from C255 & C256 are different, then the problem maybe the BECM, but is more likely to be a short in the Cruise ECU, or a bad connection in the wiring to the cruise.

    In any case debugging pulsed waveforms really needs an Automotive Oscilloscope.

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