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Thread: "check rear lights" message only when lights turned on

  1. #1
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    Question "check rear lights" message only when lights turned on

    Hello All ! My 2003 xk8 dash message says "check rear lights" only when the exterior lights are switched on. Message goes away once lights are turned off. I do not have any messages about cruise control. Brake lights, cruise control, reverse, running, headlights, fog lights all work fine with light off or on.

    No operational issues, only the message showing up on dash when exterior lights turned on.

    I read the posts about the brake light/cruse disable switch but I do not have the symptoms the other posts indicate.

    Any ideas will be appreciated.



    Super88diesel

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    Senior Member astromorg's Avatar
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    The bulb failure indicator system covers only rear lights, specifically side lights (two bulbs each side), side marker lights (US only), brake lights and number plate lights.

    You didn't mention the number plate lights so hopefully.....!
    2005 XK8 4.2-S Coupe and 2006 Auto Sovereign 3.0 X Type Saloon

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    Replaced all rear and side lights, found plate bulbs wrong wattage and replaced them too...no change...message "check rear lights" still shows up. Cleared message with button on end of turn signal lever...comes back after restart.

    Schematic shows security & locking control module as possible fault.

    Anyone have experience changing this module? Is it possible to avoid reprogramming issues with using matching label numbers? Any known easy repairs (resister, diode, solder...)?

    Any info is appreciated,

    Thanks

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    Senior Member Nerja's Avatar
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    Quote Originally Posted by super88diesel View Post
    Replaced all rear and side lights, found plate bulbs wrong wattage and replaced them too...no change...message "check rear lights" still shows up. Cleared message with button on end of turn signal lever...comes back after restart.

    Schematic shows security & locking control module as possible fault.

    Anyone have experience changing this module? Is it possible to avoid reprogramming issues with using matching label numbers? Any known easy repairs (resister, diode, solder...)?

    Any info is appreciated,

    Thanks
    You are going down the wrong road thinking about changing the SLM - they are hard to come by and a replacement won't necessarily fix your fault. You need to investigate why you are getting the message with a multimeter. One or more of your bulbs/bulb holders has a bad connection. I would go through each of the tail lights in turn checking what resistance value you are getting. Unplug the connector at the SLM and check from there. You need the wiring diagram for your car, which shows the wire colours/pin numbers and also shows which bulbs are monitored by the SLM (there is a resistor shown in series with them on the wiring diagram).

    Compare the resistance found at the connector with the resistance of the bulb itself and take it from there.

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    Senior Member astromorg's Avatar
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    Depending on the outcome of following Nerja's guidance, you may have to go deeper. There are, in the bulb sensing section of the SLM, those resistors, one for each bulb circuit, to which he refers. They do on occasion burn out, but being of the thro-the-hole circuit board type, can be replaced easily. Your incorrect wattage bulbs come to mind.
    2005 XK8 4.2-S Coupe and 2006 Auto Sovereign 3.0 X Type Saloon

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    I pulled out one plate bulb and message/yellow dash light went out. Pulled the second one out, no message. Replaced both bulbs with LED bulbs drawing 1/2 watt each and all is well. Plate is illuminated and no dash message or warning lights.

    Symptom corrected but the root defect cause is not. I can live with that for now.

    If symptom returns, I will dig deeper to find the root cause.

    Thank you all for your input, much appreciated.

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    Senior Member astromorg's Avatar
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    To help understanding of what's happened, do you know the (incorrect) wattage of the bulbs you removed versus the correct 5W?
    2005 XK8 4.2-S Coupe and 2006 Auto Sovereign 3.0 X Type Saloon

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    Phillips 194, 12volt, 3.6 or 3.8 watts....hard to read. I just bought the car and am still de-bugging it so who knows what was installed in the past. Would probably need much higher wattage bulbs to cause SLM resisters to cook but I know resisters can go out of tolerance up or down in value from age and use.

    I have a Texas / California rust free car but I did remove the battery and clean up a little acid corrosion. I then applied rust prevention goo (not paint) on the floor and some may have contacted the SLR or connectors. Possible slight conductivity between connector pin for the plate bulbs and ground pin causing increased current draw when lights turned on???? I did not check the conductivity of the goo, doubt there is much but anything is possible.

    If anything shows up again, I will check the SLR connectors and do a resistance check while unplugged to see if there is a low resistance reading with the bulbs removed. Should be infinity, anything less could be an issue.

    I hope all stays operational without anymore warning messages.

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