PD08 and Spark Timing
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HT4100 4.1, 4.5, 4.9 Discussion, PD08 and Spark Timing in Cadillac Engine Technical Discussion; This is most likely a very novice question, but I'll ask anyway. If the 4.x engines require a base timing ...
  1. #1
    area310dude's Avatar
    area310dude is offline Cadillac Owners Member
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    PD08 and Spark Timing

    This is most likely a very novice question, but I'll ask anyway.

    If the 4.x engines require a base timing of 10 deg BTDC, how come the typical advanced degree values coming from PD08 in the PCM Data are so much higher?

    The factory manual says you should see the same values with a timing light as in the PD08 display. And yet people typically see values vary as high as 49 degrees and never drop to less than 13 degrees. What do those PD08 degree values actually mean? How come we never see the 10 degrees displayed?

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    aeronca36606 is offline Cadillac Owners Enthusiast
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    Re: PD08 and Spark Timing

    The timing displayed is relative to the base timing. The computer does not know what the actual timing of the engine is. It only knows what it is calling for. If the bast timing is set correctly, Then the computer will follow the actual timing of the engine.

    Timing changes with engine load and speed. The computer controls this and sets the correct timing advance.

    During startup, the computer is not controlling the timing until the engine reaches about 400 rpm. When this happens the computer sends a signal to the distributor to hand control over to the computer. Before this the timing runs with a pre set advance table in the ignition control module.

    This has 2 effects. It allows for a stable slightly retarded value for engine start-up. It also allows the Distributor to operate in the event of the computer failing to take over control. (Limp mode)

  4. #3
    area310dude's Avatar
    area310dude is offline Cadillac Owners Member
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    Re: PD08 and Spark Timing

    Interesting! I've read there is a pre-programmed "spark curve" for these engines that obviously changes with engine rpm and torque.

    So I guess we would only see the 10 deg timing when we go through a special procedure to set base timing (never done it myself) or else when we are in "limp home" mode. We would never see 10 deg ordinarily since the engine quickly reaches 400 rpm when we turn the ignition key.

    Am I saying all this correctly?

  5. #4
    Sevillian273's Avatar
    Sevillian273 is offline Cadillac Owners Connoisseur
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    Re: PD08 and Spark Timing

    Yes. In short, the numbers you see in PD08 already include the base timing. For example, if you see 37 on the display, 10 of that is the base timing and the computer is adding an additional 27 degrees.

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