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Thread: Diagnostic Module: Stored Codes Pre-Check Engine Light

  1. #1
    E500E Guru emerydc8's Avatar
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    Diagnostic Module: Stored Codes Pre-Check Engine Light

    Last year, I electrically disconnected my air pump due to a grinding noise. To keep the ("CEL") from illuminating, I connected an old module box blower to the two leads going to the air pump, hoping that it would trick out the computer and cause it to believe the pump was running on first start of the day.

    I've had it connected for two days now and haven't seen a CEL. I checked pin 19 and I'm not getting any codes (dtc 4 will usually cause the CEL to illuminate due to an inoperative air pump).

    I think it takes at least two trips with the fault before it will set off the CEL.

    Is it safe to assume that the experiment was a success if I'm seeing no codes on pin 19? In other words, are the codes leading up to a CEL visible with the home brew tester prior to the CEL illuminating?
    Jon D.
    1994 E420
    1995 E420

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    Re: Diagnostic Module: Stored Codes Pre-Check Engine Light

    Yes, stored codes leading up to a CEL illumination will be visible/readable prior to the light triggering (and after, if they are not erased).

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    E500E Guru emerydc8's Avatar
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    Re: Diagnostic Module: Stored Codes Pre-Check Engine Light

    Thanks, Gerry. It looks like I may have found a solution. Maybe I should just measure the resistance of the motor and buy an equivalent resistor from Radio Shack. It would definitely look better than the blower motor I have taped to the side of my windshield reservoir. It's also nice to know that these computers aren't that smart after all.
    Jon D.
    1994 E420
    1995 E420

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    E500E Guru emerydc8's Avatar
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    Re: Diagnostic Module: Stored Codes Pre-Check Engine Light

    It looks like I spoke too soon. The CEL is back with a dtc 4 on the Diagnostic Module (air injection pump). I am seriously considering removing the CEL bulb as a last resort because I refuse to pay upwards of $500 to pretend that this Japanese pump is somehow saving our environment. Moreover, I have no way of verifying this post below, but if true, the computer really is looking for a drop in O2 sensor voltage to satisfy itself that the pump is operating when it should. It sounds like the poster knows what he is talking about.

    The system uses the o2 sensor to diagnose the air injection system. After the car has warmed up, it will activate the air pump for approx 60 seconds and watches the o2 sensor voltage-as it blows air into the exhaust system it will drop the o2 sensor voltage-it needs to drop below 40mv within 60 seconds or it will trigger the light. When the air injection system is working correctly it will drop below immediately, if there is a minor restriction it may take a while: 15-30 seconds. The same is true of the o2 sensor is getting lazy, then it may take a while for the voltage to drop, it may not drop at all if the o2 sensor is really bad. Good luck.
    http://www.peachparts.com/shopforum/808677-post9.html
    Jon D.
    1994 E420
    1995 E420

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    Re: Diagnostic Module: Stored Codes Pre-Check Engine Light

    Grrrrrr. If that is true (and that poster does appear to have a clue) then yes, it will be very difficult to bypass. You'd need an early non-KAT LH module (or, the relevant portions of the EPROM code to make it stop looking for the O2 signal).

    BTW - based on the poster's name and city, I strongly suspect that is Steve Geyer of SG Motorsports in Renton, WA. He's a dealer tech and has probably forgotten more about MB's than I will ever know. Super nice guy, btw.


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    Re: Diagnostic Module: Stored Codes Pre-Check Engine Light

    So it's just got a bad coil on the pump? I though a rebuilt pump was ~350? I got a used one for $75.

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    Re: Diagnostic Module: Stored Codes Pre-Check Engine Light

    I think it's the bearing in the pump itself. I might look for a used one, but I'm not sure it's worth the hassle. I'd rather spend the money on more worthwhile parts or tools like a few of the specialty tools listed on Steve Geyer's site. This 17 mm offset wrench would have made things a lot easier to change my motor mounts.
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    Last edited by emerydc8; 11-14-2012 at 09:47 PM.
    Jon D.
    1994 E420
    1995 E420

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    Senior Member JordiC's Avatar
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    Re: Diagnostic Module: Stored Codes Pre-Check Engine Light

    Hey Jon, any update on this? My cars PO cut the wires to the air pump and I'm also getting ticked-off by the persistent CEL pin 19-4. I never have actually reconnected the wires just to see what would happen, maybe I'll do that soon. Could there be a way to measure the resistance from a working pump and then attempt to fake it? I know this issue has been talked about here before with no solution. It seems the only 2 outcomes are living with the CEL or coughing up $$ for a new pump.
    "I think that in any race engine, the nearer you are to it disintegrating, in general, the better it's performance will be." Keith Duckworth

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    E500E Guru emerydc8's Avatar
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    Re: Diagnostic Module: Stored Codes Pre-Check Engine Light

    Hi Jordi:

    I gave up after thinking that connecting an electric motor (actually it was a spare module box blower) to the wires for the air pump would solve the problem, but the light came on after all. I think the computer really is smarter than I thought. In fact, I can't remember where I copied and pasted a post from another MB forum, but there was someone with apparent authoritative expertise who actually knew that the computer was looking for a change in resistance of the O2 sensor for x-number of seconds on start-up. That would require an operable air pump. I am living with the code for now. Sorry it took me so long to reply by the way. I just happened to notice your post.

    EDIT: It was posted above:

    Code:
    After the car has warmed up, it will activate the air pump for approx 60 seconds and watches the o2 sensor voltage-as it blows air into the exhaust system it will drop the o2 sensor voltage-it needs to drop below 40mv within 60 seconds or it will trigger the light.
    I guess you could get really creative and install some sort of relay with a resistor that would temporarily drop the O2 voltage on start-up.
    Jon D.
    1994 E420
    1995 E420

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    Senior Member JordiC's Avatar
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    Re: Diagnostic Module: Stored Codes Pre-Check Engine Light

    Ok, so today I reconnected the smog pump wires, started the car and heard no odd noises. I cleared the code and will now wait a few days to see what happens. Could it be that simple?
    "I think that in any race engine, the nearer you are to it disintegrating, in general, the better it's performance will be." Keith Duckworth

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    Senior Member JordiC's Avatar
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    Re: Diagnostic Module: Stored Codes Pre-Check Engine Light

    Quote Originally Posted by JordiC View Post
    Could it be that simple?
    No, of course not. The following day good ol' Smoggy turned back on. I checked today and DTC 19-4 showed it's familiar face again. I'm going to have to keep living with it while I hunt down a used air pump.

    BTW, I don't think I mentioned this but the PO had physically cut the wires, there are no original connectors present. Maybe the pump is not original or the connectors became corroded.

    "I think that in any race engine, the nearer you are to it disintegrating, in general, the better it's performance will be." Keith Duckworth

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    E500E Guru emerydc8's Avatar
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    Re: Diagnostic Module: Stored Codes Pre-Check Engine Light

    I'm considering permanently deactivating my check engine light. My wife says it puts me in a bad mood. I think she's right. They don't check it in Arizona when I take it through emissions testing anyway.
    Jon D.
    1994 E420
    1995 E420

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    Re: Diagnostic Module: Stored Codes Pre-Check Engine Light

    Quote Originally Posted by emerydc8 View Post
    I'm considering permanently deactivating my check engine light. My wife says it puts me in a bad mood. I think she's right. They don't check it in Arizona when I take it through emissions testing anyway.
    Just cover the light with a happy face sticker....

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    Re: Diagnostic Module: Stored Codes Pre-Check Engine Light



    I was thinking I like this one better.
    Jon D.
    1994 E420
    1995 E420

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    Re: Diagnostic Module: Stored Codes Pre-Check Engine Light

    That "give a damn" smilie really needs to be updated from a Windows 95 box style to Windows 7 or Windows 8... JELMER ?!?!?!?!

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    E500E Guru emerydc8's Avatar
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    Re: Diagnostic Module: Stored Codes Pre-Check Engine Light

    If it was updated to Windows 7, it would say:

    16 bit MS-DOS Subsystem. Virtual Device Driver format in the registry is invalid. Choose 'close' to terminate the application.
    I just bought an older HP Mini netbook on Ebay that runs Windows XP since the Windows 7 developers apparently don't give a rat's ass about anyone trying to run older programs.
    Jon D.
    1994 E420
    1995 E420

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    Re: Diagnostic Module: Stored Codes Pre-Check Engine Light

    Quote Originally Posted by gerryvz View Post
    That "give a damn" smilie really needs to be updated from a Windows 95 box style to Windows 7 or Windows 8... JELMER ?!?!?!?!
    Can't believe I wasted half an hour on this.
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    "But if you really must have the Porsche — if you really must have a Porsche sedan — you can buy a 500E and have enough left over for something air-cooled"

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    Re: Diagnostic Module: Stored Codes Pre-Check Engine Light

    Quote Originally Posted by Jelmer View Post
    Can't believe I wasted half an hour on this.
    Oooh ooh ooh HELL YEAH !! Thanks mate !


  19. #19
    E500E Guru emerydc8's Avatar
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    Re: Diagnostic Module: Stored Codes Pre-Check Engine Light

    I like it.
    Jon D.
    1994 E420
    1995 E420

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