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Thread: Wiring diagrams for '85 MGM

  1. #1
    Lost and driftin' Arquemann's Avatar
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    Default Wiring diagrams for '85 MGM

    Mainly looking for how the engine harness is tied to other wiring in the car.
    I'm going carb on my car so I have stuff to figure out.

    Mostly if the AC/heat and cruise control is tied to the engine harness. (basically: does my AC and cruise work after removing the ECM and harness?)
    Also I'll need to figure out a key-on power source for a the fuel pump relay and ignition.

    So if you guys have wiring diagrams as pictures or in scanned forms, those would be greatly appreciated. I haven't managed to get myself an EVTM since shipping to Finland hasn't been available or the shipping prices have been atrocious.
    Alternatively answers or explanations to my question(s) would be fine aswell.

    I'm trying to get all the puzzle pieces ready before pulling the car apart when spring comes. Like making actual plans and shit.
    1985 Mercury Grand Marquis LS, "Maisa"
    2008 BMW 530d Touring, "Femma"

  2. #2
    Wagon Addicted Tiggie's Avatar
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    I don't have the scans you need but the ECM didn't get the cruise until 88, so I am fairly certainly you will find it stand alone in your car. Will the wires be loomed together in some places... likely, but you will be able to figure it out.

    Different car but not that far apart in age. My 82 ran without the MCU plugged in (carb'd computer). Everything worked fine.
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  3. #3
    Lost and driftin' Arquemann's Avatar
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    I'm guessing the EEC power relay gets key-on power straight from the ign switch? Could it be a good place to get key-on power for the fuel pump (I'll keep the factory relay) and for ignition coil? Is the factory coil positive a part of the engine harness or alternator harness?
    1985 Mercury Grand Marquis LS, "Maisa"
    2008 BMW 530d Touring, "Femma"

  4. #4
    Beater gonna beat sly's Avatar
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    I'm pretty sure the ECA relay already feeds coil power for the fuel pump relay. I know it does in EFI cars. Power to the switched sides of the relays comes from the battery and should stay that way in the grand scheme of things.

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    Quote Originally Posted by gadget73
    ... and it should all work like magic and unicorns and stuff.
    Quote Originally Posted by dmccaig
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  5. #5

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    Iíll get you pictures of the appropriate diagrams from my Ď85 EVTM this weekend.


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  6. #6
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    I wouldnít worry about rewiring anything or cutting wires for a carb swap. The po of my 87 carb swapped it and all he did was unplug the salt and pepper shakers (which your car doesnít have). Pull the connectors to unused sensors and tie them out of the way. Get a fuel pressure regulator to step down the fuel pressure and check the tv cable for correct movement or get a lokar bracket.


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  7. #7
    Beater gonna beat sly's Avatar
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    Yeah... grounding the switched side of the fuel pump relay coil should leave the fuel pump on at all times the key is on. Swapping out the fuel pump with a low pressure pump or putting in a low pressure regulator... or even putting in a "surge tank" that loops back to the fuel tank and putting a low pressure fuel pump on that would sort the fuel needs to a carb.

    Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
    rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)
    Quote Originally Posted by gadget73
    ... and it should all work like magic and unicorns and stuff.
    Quote Originally Posted by dmccaig
    Overhead, some poor bastards are flying in airplanes.

  8. #8
    Lost and driftin' Arquemann's Avatar
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    Today I did a little digging into the driver rear corner of the engine bay. Basically just verified that the EEC power relay gets a key-on signal even with the ECM disconnected. And that the fuel pump relay gets its signal from the ECM, ie the pump doesn't prime with the ECM connector disconnected. Only thing I forgot to check is if the coil gets voltage with the ECM disconnected, but my guess is NO.

    Quote Originally Posted by sly View Post
    Yeah... grounding the switched side of the fuel pump relay coil should leave the fuel pump on at all times the key is on. Swapping out the fuel pump with a low pressure pump or putting in a low pressure regulator... or even putting in a "surge tank" that loops back to the fuel tank and putting a low pressure fuel pump on that would sort the fuel needs to a carb.
    I intend on getting a return type regulator for the carb and using (most of) the factory fuel lines. I'd like to keep the old pump & relay setup since it's already there and should be fine with a carb and appropriate FPR.
    Most likely I'm going to usethe key-on signal coming to the EEC power relay and split it to the fuel pump relay and ignition coil. No need for any ballast resistor nonsense since I'm going with HEI.

    Quote Originally Posted by 87GrandMarq View Post
    I wouldnít worry about rewiring anything or cutting wires for a carb swap. The po of my 87 carb swapped it and all he did was unplug the salt and pepper shakers (which your car doesnít have). Pull the connectors to unused sensors and tie them out of the way.
    This would be the easiest, but I'm going to remove everything I can, engine harness, thermactor crap, solenoids, vacuum rats nest. I absolutely do not want to leave the mess there after the swap. Simplify things as best as I can, there's enough wires and lines going all over the engine bay even without all the engine related stuff.
    1985 Mercury Grand Marquis LS, "Maisa"
    2008 BMW 530d Touring, "Femma"

  9. #9
    Lost and driftin' Arquemann's Avatar
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    Apparently the fuel pump relay has TWO power sources, one that's hot at all times and for some reason there's another one that's only hot when cranking??
    Caused me some headache to figure out why the "I" post on the starter relay had 12 volts at all times, before I realised that the other power source was backfeeding the post on the starter relay...
    Even weirder was that I really couldn't figure out where the other power is spliced in from, as I followed the wire all the way from the starter relay to the fuel pump relay, it's somewhere there I guess.

    -Seems like the fuel pump relay is activated by grounding where as the EEC power relay is activated by a power signal from the ignition switch. I should be able to just swap the output wire from the FP relay to the EEC relay, but I'll have to check the amperage on the EEC relay first, FP relay was 30A I think.
    -Coil negative goes into the TFI and ECM, but the positive goes to TFI aswell as the cabin, so it most likely it gets power from the ignition switch somewhere in there and would probably work without the ECM.
    -If the coil doesn't get power without the ECM, I'll probably repurpose the EEC relay for ignition and rewire the fuel pump relay to work without the ECM. I have no clue how many amps the coil pulls, but I doubt the fuel pump relay is enough for the pump and coil.
    1985 Mercury Grand Marquis LS, "Maisa"
    2008 BMW 530d Touring, "Femma"

  10. #10

    Default Wiring diagrams for '85 MGM

    Hopefully these wiring diagrams help. They show the fuel pump and ignition system power sources.





    I missed page 45 which has the continuation of the EEC power relay circuit. Iíll get that when I get back home.

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    Last edited by matth825; 09-19-2021 at 02:33 PM.

  11. #11

    Default Wiring diagrams for '85 MGM


    Hereís everything the EEC power relay feeds.


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  12. #12
    Lost and driftin' Arquemann's Avatar
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    Thank you very much for those, I do remember seeing atleast 2 of those pages before, but I couldn't find them. I'll be sure to save these ones.

    I can see exactly what I found out with the fuel and EEC relays, one is ground-activated and the other isn't.
    The ignition system page is really helpful, looks like I should be able to keep the coil wiring functional while removing the ECM and harness.
    Lastly I'll have to figure out what goes where with the 8- and 6pin connectors on the driver fender, as atleast some wires go from the 8-pin to the ECM connector.
    Probably a couple grounds to figure out aswell while removing the harness.

    My goal is to remove as much of the CFI-related wiring in the engine bay, so I'll pull the ECM connector and start following wires to their ends.
    The single power source to the ECM splits off into a million pins and seemingly quite a few grounds are spliced together.
    1985 Mercury Grand Marquis LS, "Maisa"
    2008 BMW 530d Touring, "Femma"

  13. #13

    Default Wiring diagrams for '85 MGM

    Glad those helped! Hereís a picture of those connectors with their designations

    The start circuit goes thru C232 to the TFI module along with the switched hot to the EEC relay coil. I didnít see anything concerning running through the other two connectors.

    Also, here is a third page showing more EEC wiring from the ECM connector.



    Removing the ECM connector and working your way from there is definitely the way to go. Other than cutting a few wires at splices it should all come out without much fuss. Iíd keep it all intact and stash it away just in case.


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  14. #14
    Lost and driftin' Arquemann's Avatar
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    Thanks!
    Yeah, hopefully those connectors on the driver fender apron don't cause too much headache, other than those, removal seems pretty simple for now. Of course it's going to get more complicated the further I dive into it, but that's to be expected.

    Weird, I thought I followed both coil negatives to the C530 2-pin connector, but it's supposedly something else. On the diagrams the two coil neg wires go through a C455.
    1985 Mercury Grand Marquis LS, "Maisa"
    2008 BMW 530d Touring, "Femma"

  15. #15

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    Sure thing. If you need any others let me know and Iíll post a picture.

    Are you going to go with a Duraspark 2 for your ignition system?


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  16. #16
    Lost and driftin' Arquemann's Avatar
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    Quote Originally Posted by matth825 View Post
    Are you going to go with a Duraspark 2 for your ignition system?
    I didn't actually mention it in this thread, but in the RR thread. I'm gonna go with a Duraspark dizzy but I'll be pairing it with a GM HEI module.
    Compared to a full duraspark system it's simpler, cheaper and just as good or even supposedly a bit better.
    No wiring harness needed (only a few wires to run), no ballast resistor to install, uses the factory coil and a spare HEI module takes less space in the glove box

    Module will need a heatsink which will be a bit of an eyesore but oh well.
    http://www.carbdford.com/tech/HEI/hei.htm
    1985 Mercury Grand Marquis LS, "Maisa"
    2008 BMW 530d Touring, "Femma"

  17. #17
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    Some great info here.
    Very helpful.
    ..

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