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    1990 Mercury Grand Marquis 5.0L FI 8cyl

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    1720 S RIDGEWOOD A
    (386) 427-1210

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    Glow Plug

    Inspect/Test/Replace

    A test light can be used to determine if the glow plugs are working.

    • With the test-light grounded, insert the test light probe into a glow-plug wire to check for continuity.
    • If the light comes on, check the other cylinders in the same manner. If the test light failed to light, check for continuity at the glow-plug relay.
    • If no power is present, check the control circuit of the glow plug relay.
    • If the control circuit is working, the relay may be defective (Be aware that manufacturers may have the system fused and fusible links may be located throughout the system).
    • If the glow-plug circuit is working, the glow plugs themselves may be defective.
    • With an ohmmeter, connect one lead to the terminal and the other lead to the body.
    Checking glow-plug resistance. Courtesy of KUBOTA Corporation.
    • Read the resistance and compare to specifications.
    • A visual inspection may also be necessary to check for excessive carbon buildup and physical damage to the tip.
    • A glow plug that has excessive carbon deposits on its tip can give the correct ohmmeter reading but still not work. Clean the tip with carbon cleaner.

    CAUTION
    Do not wire brush the tip.

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