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  1. #1

    Default Mercruiser 7,4l wiring issues

    Hi all.

    I’m trying to connect back together a Mercruiser 7,4l engine. I have bought the Mercury service manual, that has pointed me in the right direction. I have figured out from the wiring diagram that I either have a Thunderbolt IV or V.

    But here comes the issue I have, in the diagram in the manual there are 2 cables comming from the ignition module (wht/red,wht/green) that is suppose to be connected to the distributor. It looks like I dont have these 2 connection points on my distributor..? Maybe the distributor has been swapped..? But what do I do with these to wires then..? If anyone can point me in the right dirrection would be great! 😉😉

    have a nice day!

    Rgds Tommy

  2. #2
    Join Date
    Jul 2008
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    Northboro, Ma, USA
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    Default Re: Mercruiser 7,4l wiring issues

    Those wires go to the sensor inside the distributor.
    If your sensor has two threaded studs it is an old sensor.

    I am assuming the two wires you are speaking of come from the timing control module?

    What are the ends of the wires? Ring terminals or bullit connectors?

    If bullit connectors I would suggest buying a new sensor as it should come with the same connectors.

    If ring terminals then your manual should tell you which color goes to which threaded terminal on the sensore which will be on Side of sensor.

    Answer questions and we can assist more from there
    Jack
    Mass.

  3. #3

    Default Re: Mercruiser 7,4l wiring issues

    Hi

    It is ring therminals on the end of the cables. Are they then going to the inside of the distrubitor?

  4. #4
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    Default Re: Mercruiser 7,4l wiring issues

    No,
    If the distributor has the correct sensor you will see two threaded studs sticking out of the side of the base of the distributor.

    Take the cap off and look inside distributor.
    Describe what you see.
    Jack
    Mass.

  5. #5
    Join Date
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    Spotswood, NJ, USA
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    Default Re: Mercruiser 7,4l wiring issues

    This a TB IV
    click to enlarge

  6. #6

    Default Re: Mercruiser 7,4l wiring issues

    Hi all, thanks for all the help. I have attached pictures from my engine.

    Click image for larger version. 

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  7. #7
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    Default Re: Mercruiser 7,4l wiring issues

    What is that mess?

    Remove the rotor and take a pic of whats under it.

    What year is boat,
    Where did that engine and ignition come from?

    Is that a marine engine? Marine distributor?

    It is NOT a Thunderbolt Mercruiser.

    The pic Boat Dr posted is exactly what it should look like
    Jack
    Mass.

  8. #8
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    Island Heights, NJ,
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    Default Re: Mercruiser 7,4l wiring issues

    Thunderbolt cap and rotor



    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280

  9. #9

    Default Re: Mercruiser 7,4l wiring issues

    So then I probably dont have an Thunderbolt engine, I am sorry for misunderstanding. I have only read the Service manual and assumed it was a thunderbolt since the wiring looked right.

    It is a marine engine, it came with the boat i bought a couple of years ago and I just wanted to see if I could get some help getting it running, I know it doesnt look very nice. But this is what I have for now. If I can get it running than thats great, if not I will look at the options. I am new to this game and was hoping that someone could help me.

  10. #10
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    Default Re: Mercruiser 7,4l wiring issues

    looks like an Delco HEI distributor from an auto or truck - Pic #1 - bottom RH corner looks like a vacuum advance can....

    You really need to install a rated marine distributor.....

  11. #11
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    Default Re: Mercruiser 7,4l wiring issues

    Quote Originally Posted by makomark View Post
    looks like an Delco HEI distributor from an auto or truck - Pic #1 - bottom RH corner looks like a vacuum advance can....

    You really need to install a rated marine distributor.....
    I was also thinking a HEI Delco but due to his name I felt he was in northern Europe.
    I wasnt sure they would have that distributor available there.

    Where do you live ? USA or somewhere else?
    Jack
    Mass.

  12. #12

    Default Re: Mercruiser 7,4l wiring issues

    That is correct, I am from Norway.

    I found the cover on te engine today, I ve posted it in this post. Can you guys identify it for me then? Is it a Thunderbolt ? or Not? What would the HP be on something like this?

    Click image for larger version. 

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    And also I found the old distributor (which was in the old engine that was in the boat, VP engine) This distributor looks exactly the same. Same brand. Same connections and everything. Pictures below.

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    So where do I go from here..?

  13. #13
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    Default Re: Mercruiser 7,4l wiring issues

    The id tag is very difficult to read.

    A 7.4 =454 CUBIC INCH

    They are 325 hp and higher depending on model.

    There is no doubt what you have for an ignition is an Automotive not Marine, GM HEI distributor.

    What you should have is what BTDoctor picture shows. The distributor, seperate stand alone coil and timing module (module specific for 454 model).

    Marine disributors do not have a vacuum advance.

    So either you find good used parts, buy new replacements or take your chances and use what you have knowing it is not MARINE APPROVED.

    Many have made that change and it can work,

    The mercruiser timing module does get used at all.
    The HEI has a built in coil (on top of cap) and a mechanical advance i believe or it is controled by the ignition module in distributor.

    It is a basic two wire hook up, Side of cap is a 3-4 tang connector.
    Power in and tachometer out.
    Forget if it also has a ground..


    Easy to look up on the internet.
    I am using my phone for this so i cant look it up right now.
    Jack
    Mass.

  14. #14

    Default Re: Mercruiser 7,4l wiring issues

    Hi Jack

    Thank you soo much for the reply! It makes sence now that you're telling me this.

    The ID tag is OD762636

    So then it is probably only the purple wire comming from the ignition module that goes into the distributor. Everthing else can be isolated then…?

    If anybody can find me a wiring diagram for that, that would be great. Its not so easy for us Norwegians to find stuff like that (we dont know what to search for to be honest)

    I think I will try to make it work like this, if it doesnt work then buy used parts.

    Can i use the distributor that i found of the old VP engine? it looks a bit nicer, better condition. Is it a simple swap? they look the same....
    And also the alternator on the mercruiser engine looks like it had better days, is that the same as the VP engine also?

    Hopefully I dont have to many stupid questions!

  15. #15
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    Default Re: Mercruiser 7,4l wiring issues

    Ayuh,..... That automotive distributor in a boat motor, is a Bomb, waitin' to explode,.....

    Search for a Delco Voyager kit, or find a Merc Thunder Bolt distributor that belongs there,..,....

    Distributors, Starters, 'n Alternators Must be Marine units, or yer buildin' a floatin' Bomb,....
    Any Grease is Better,... Than No grease at All....

  16. #16

    Default Re: Mercruiser 7,4l wiring issues

    Quote Originally Posted by bondo View Post
    Ayuh,..... That automotive distributor in a boat motor, is a Bomb, waitin' to explode,.....

    Search for a Delco Voyager kit, or find a Merc Thunder Bolt distributor that belongs there,..,....

    Distributors, Starters, 'n Alternators Must be Marine units, or yer buildin' a floatin' Bomb,....


    I understand, the motor is high and dry in my garage. I just want to see if I can get it going without spending to much money on it. If/when I get it going I will of course follow your advice.

  17. #17
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    Default Re: Mercruiser 7,4l wiring issues

    http://www.marineengine.com/parts/me...ts/sn/0d820560

    Our sponsor... these are the parts that your engine left the factory with
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280

  18. #18
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    Default Re: Mercruiser 7,4l wiring issues

    you can search delco HEI distributor wiring to get info on making what you have run...like Jack said, the PURPLE wire should provide +12VDC with the key in RUN...

  19. #19

    Default Re: Mercruiser 7,4l wiring issues

    Okay guys. So the wiring is sorted. I think.

    Now there is a new problem... the engine does not turn.. it turns slightly.. so it is not stuck.. but it stops maybe after half a round on the flywheel. I can turn it back, but it is also stuck in the other direction, its a bit difficult to explain... but when it comes to this point its a complete stop and the engine will not turn beyond this point..

    anyone got any idea what it is?

    I was thinking that one of the valves had broken of and fallen down on the sylinder head causing it not to stroke all the way. But I opened the top covers and had a look on the rods and lifters and they looked good...

  20. #20
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    Default Re: Mercruiser 7,4l wiring issues

    a valve or valves are stuck in the open position, hitting the piston causing it to stop turning.
    use a 5/8 ths socket , loosen and remove the rocker arms . All the tops of the valves must be at the same height . Any that are lower are the stuck ones

  21. #21
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    Default Re: Mercruiser 7,4l wiring issues

    or something else inside is binding up.....either way, doesn't sound promising.

    Has the engine ever run since you purchased the engine & hull?

  22. #22

    Default Re: Mercruiser 7,4l wiring issues

    Hi you guys.

    «Bt doctur» that sounds like a good explanation. So then I will try that and see.

    «Makomark» No, this is the first time I’ve tried to get it going. According to the previous owner it was running when he tested it, but that is some years ago. I was just gonna check if I could get it running, if not then I will go for other options.

  23. #23
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    Default Re: Mercruiser 7,4l wiring issues

    I'd be inclined to pull the plugs and get a borescope to take a look inside of each cylinder....

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