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Thread: Check engine light stays on after removing ignition

  1. #1
    Senior Member kre8tive202's Avatar
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    Check engine light stays on after removing ignition

    Hello all,

    Ok I need help. My 1992 500E has a check engine light that stays on after I remove the ignition key. I can get it to turn off but I have to cycle starting the car on and off until the light stays off. What is going on? Any help would be greatly appreciated.

    Thanks,
    Mario

  2. #2
    .036 Hoonigan™
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    Re: Check engine light stays on after removing ignition

    The one time I experienced this condition, it was a failed diagnostic module (DM), located under the passenger footwell area. In my case the CEL remained on, and the DM itself was warm to the touch. Replacing the module with a used one cured the problem. For a quick test, just disconnect it, the car runs fine without it. If the issue goes away, try installing a different DM.

    Dave M.
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    Re: Check engine light stays on after removing ignition

    Used as from an E420/400E?
    1994 E500
    249/275 - 8F19 or 8F32 or 8320

    1991 560 SEC
    199/268
    2014 E350 Cab 799/264

  4. #4
    Senior Member kre8tive202's Avatar
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    Re: Check engine light stays on after removing ignition

    The DM module is the module under the floor board on the passenger side correct.

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    .036 Hoonigan™
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    Re: Check engine light stays on after removing ignition

    Quote Originally Posted by nocfn View Post
    Used as from an E420/400E?
    Yes - identical on all years of USA-spec 124.034/.036... possibly the same on other chassis or models (need to check EPC).


    Quote Originally Posted by kre8tive202 View Post
    The DM module is the module under the floor board on the passenger side correct.
    Yes, it's the silver shiny metal one. NOT the black ones. You need to twist the round metal knob and the connector will separate itself from the DM module.


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    Senior Member kre8tive202's Avatar
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    Re: Check engine light stays on after removing ignition

    Ok so here is an update. I discovered today that my fuel pump is intermittently cycling after the ignition key has been removed. So along with the check engine light illuminating, my fuel pump is cycling on and off. Is this still the DM module?

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    .036 Hoonigan™
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    Re: Check engine light stays on after removing ignition

    The fuel pump relay is triggered by the LH module. It is highly unlikely that the CEL remaining on is related... but, if you have some really unusual electrical gremlins, there's a faint possibility. I've never heard of fuel pumps cycling after the key is removed - that is bizarre. Any change if you disconnect/unplug the DM?


  8. #8

    Re: Check engine light stays on after removing ignition

    Is there any chance that this is ignition switch/tumbler related?

  9. #9
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    Re: Check engine light stays on after removing ignition

    Quote Originally Posted by Stevester 500E View Post
    Is there any chance that this is ignition switch/tumbler related?
    Good point, Steve. For the fuel pumps, I'd say yes, there's a possibility that could be related to the ignition switch, maybe. For the DM, I don't think so, but we'd need an expert like Klink to weigh in.


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    Senior Member kre8tive202's Avatar
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    Re: Check engine light stays on after removing ignition

    Thats what i was thinking too, that it was my tumbler. Just got a free moment, let me go unplug the DM module

    Sent from my SAMSUNG-SM-G890A using Tapatalk

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    Re: Check engine light stays on after removing ignition

    Keep in mind that the tumbler is just the lock assembly, and is totally separate from the electrical switch. The tumbler is an easy swap and should be done as preventive maintenance if it's the original tumbler. The ignition switch is much more difficult to access, and requires a fair amount of disassembly of the interior knee bolster / instrument cluster... not fun.


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    Re: Check engine light stays on after removing ignition

    Ok just unplugged my DM Module and that took care of both my check engine light and fuel pump cycling on and off. What does this thing actually do? Confirm guys I can operate the vehicle without this module until I get a replacement. Where is a good place to get one?

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    Re: Check engine light stays on after removing ignition

    I just replaced my tumbler a year ago.

    Quote Originally Posted by gsxr View Post
    Keep in mind that the tumbler is just the lock assembly, and is totally separate from the electrical switch. The tumbler is an easy swap and should be done as preventive maintenance if it's the original tumbler. The ignition switch is much more difficult to access, and requires a fair amount of disassembly of the interior knee bolster / instrument cluster... not fun.


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    Re: Check engine light stays on after removing ignition

    Quote Originally Posted by kre8tive202 View Post
    Ok just unplugged my DM Module and that took care of both my check engine light and fuel pump cycling on and off. What does this thing actually do? Confirm guys I can operate the vehicle without this module until I get a replacement. Where is a good place to get one?
    The DM is only used in North America, for EPA/DOT regs. It monitors various signals from the engine management system, and when it detects faults that could adversely affect emissions, it triggers the CEL on the dash. Euro and Japan models do not have either the CEL nor the DM. I believe you can operate the car without it; however the CEL will not function and therefore would not pass the visual emissions test. As for a source... the DM is an awfully rare failure. I'd probably just get one off eBay with a matching part number; or ping David Hendy, or Jeff (captruff) and see if they have one floating around.


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    Senior Member kre8tive202's Avatar
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    Re: Check engine light stays on after removing ignition

    Ok just picked one up from EBAY for $78.00.

    Thanks everyone

    Quote Originally Posted by gsxr View Post
    The DM is only used in North America, for EPA/DOT regs. It monitors various signals from the engine management system, and when it detects faults that could adversely affect emissions, it triggers the CEL on the dash. Euro and Japan models do not have either the CEL nor the DM. I believe you can operate the car without it; however the CEL will not function and therefore would not pass the visual emissions test. As for a source... the DM is an awfully rare failure. I'd probably just get one off eBay with a matching part number; or ping David Hendy, or Jeff (captruff) and see if they have one floating around.


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