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Thoughts Or Links On Combining 75 Cj Body Wiring With Chevy Tbi 350 Engine

Discussion in 'Intermediate CJ-5 and CJ-6 Tech' started by fesser, Apr 15, 2019 at 11:05 PM.

  1. Apr 15, 2019 at 11:05 PM
    fesser

    fesser Napa, CA Sponsor 2019 Sponsor

    Napa, CA
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    I have been doing a lot of searching for information regarding wiring my Jeep to coexist with a Chevy engine and TBI. I have found much info but nothing that is specific enough for me since I don't have a lot of wiring/tbi experience. Much of the information I have found deals with different year Jeeps or swapping the TBI onto a Jeep engine.

    The Jeep is a 1975 CJ6.
    The engine is a 350 with TBI. The engine, computer (ECM) and engine related wiring harness are all out of a 1991 full size Blazer. The trans is a TH400.

    I don't plan to eliminate any sensors, or to try to minimize the engine harness. I plan to keep as much of the engine/computer wiring intact as possible and as much of the Jeep dash and body wiring intact as possible. My problem is where the two systems intersect.

    I am hoping someone here has done this or knows of a 1975 specific link.

    Thanks.
     
  2. Apr 15, 2019 at 11:19 PM
    zila

    zila I throw poop

    Rock Springs,...
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    I installed TBI in my 75, but kept the 304.. I believe what you are doing should not be too hard, as all the TBI stuff should be already there.. And it is already programed for your engine. You may need a different distributor that has a reluctor in it and is compatible with a GM ignition module..
    Try this link for instructions on conversions. Your wiring should be pretty much the same..

    FWIW I mounted my computer on the back of the dash with Velcro. Right next to the glove box


    Installation Instructions
     
  3. Apr 16, 2019 at 10:02 AM
    fesser

    fesser Napa, CA Sponsor 2019 Sponsor

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    Thanks Zila. I have seen those and other instructions that are similar for the TBI end of things. I guess I am more at a loss for how to adapt the Jeep harness to things like the GM charging system, and what parts of the original setup can be eliminated (sounds like pretty much anything related to ignition). My 350 has the stock (new, because I smashed the old one moving the engine around) 1991 distributor. That should be fine, right?
     
  4. Apr 16, 2019 at 10:43 AM
    timgr

    timgr Jeepin' Nerd Sponsor

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    GM charging system should be just a 10SI or 12SI alternator. Look at the wiring diagram for the 1975 or 1976 CJ and copy that. The sixes had a 10SI from the factory, so the scheme for them is mixed in to the same diagram for the V8s. Look at the diagram with this in mind and it should be clear.

    There are lots of systems that depend on the ignition system. You will need a connection to ignition power to turn the ECU and fuel pump on. You need the original distributor since its advance is controlled by the ECU. I don't know where the TBI/MPI Chevys take ignition power from. If it's from the ECU, then you will not need any connection to the ignition, since you've covered that by sending power to the ECU. If it's separate, than you will need to arrange a power lead to the ignition, likely from your new relay-driven panel that turns on the ECU and fuel pump (see next paragraph).

    You will be eliminating everything that goes along with the Prestolite ignition. The Prestolite stuff has a power lead to the module and a coil lead. Both of these plus power for the regulator come from splice "G" hidden inside the loom under the hood. This is shown clearly in the TSM wiring diagram. I would take those wires back to that splice and use that lead to my new EFI power panel. That may not be enough power to run the fuel pump; typically a separate wire is run from the solenoid that powers the panel and the original switched power is used to turn the panel's relay on and off. The Prestolite ECU needs nothing from the starting circuit, so leave that unchanged.

    You will need to figure out where to put the GM ECU and how to run the wires. As I recall, there are a lot of extra wires in the GM harness that you can prune out. I know nothing specific about this, but there are articles on-line that discuss it. I expect you will at least need the harness diagram from the donor, maybe the pinout and description of the ECU to complete this part.

    This should give you a place to start. (y)
     
    Last edited: Apr 16, 2019 at 10:55 AM
  5. Apr 16, 2019 at 10:59 AM
    45es

    45es Sponsor Sponsor

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    I doubt very much you will find anything specific to your '75 CJ6 and the '91 Chev 350 install although you may get lucky. That in itself is not a problem. You are integrating a modern EFI controlled engine into a vehicle with a very rudimentary electrical system and anything you find whether it be a 2A or your CJ6 would basically be the same. I suggest you print a copy of your jeep's wiring diagram. Also print a copy of the '91 Chev's wiring diagram and then study the two. You will see some commonality such as power distribution, charging circuit, electrical grounding, etc. You are trying to get a basic understanding of the two systems and this will help lead you to understanding where/how the two systems intersect.

    "I don't plan to eliminate any sensors, or to try to minimize the engine harness."
    This may or may not be doable. For example, are you maintaining the computer controlled EGR system or removing it.

    Study the two systems and ask questions. This is not rocket science and many can help.

    Also, you do not need to buy a new distributor. The existing distributor has a reluctor and ignition control module built into it.
     
  6. Apr 16, 2019 at 11:29 AM
    zila

    zila I throw poop

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    If it is the OEM distributor and ignition etc you should be fine. The charging system is a separate matter.. I converted mine to a Delco unit. Now that I think about it, your donor engine should be set up for a GM alt … I used a CS130, which is three wire. There are one wire alts out there too that many like because of simplicity of wiring. If you go to the link I provided you will see what you need to get your TBI system running. As I recall I needed a source for 12VDC like a keyed source. You will also need a fuel pump relay, a MAP sensor, and a O2 sensor. All should be on your 350. If not they are readily available at NAPA. One more thing, your fuel pump needs to put out ~13 psi. You can mount an external or do what I did and mount it in the fuel tank.

    Read thru the install instructions I linked. Yours is going to be very similar.. There is a pin out list near the end that will help.
     
    Last edited: Apr 16, 2019 at 3:06 PM
  7. Apr 17, 2019 at 10:16 AM
    fesser

    fesser Napa, CA Sponsor 2019 Sponsor

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    Thanks Timgr. I might start with a magnifying glass. That TDM diagram has some small print. Good info on the splice. I am going to be running an ECU dedicated power relay next to the fuel pump relay

    Thank you 45es. I have the diagrams but I find they can be a little confusing to me, but I am working on figuring them out. I could have been clearer about where I was hoping for 75 specific information. The 75, unlike earlier years, had a bulkhead fitting that joins the fuse box to the underhood wiring through the firewall. In my searches I found information for later year Jeeps that "decoded" the engine side of the bulkhead fitting with specific information "run a wire from terminal X to terminal Y on the ignition relay". That doesn't seem to be out there. I agree that I need to get more familiar with the two sets of diagrams so that I can address the duplicate systems.

    I have been reading the link and it does help. One problem is that the instructions are for their aftermarket harness and although it does say that most colors are universal, I am still reluctant to assume pink on theirs is pink on mine, so to speak. I know I can verify colors by tracing and comparing and likely will do that when I get started. I did install the fuel pump in the tank, referencing your build. That was very helpful and that is the one area of the swap that I fell pretty confident about since I am using the stock fuel pump relay from the Chevy (should just be one wire to deal with). Thanks for your help.

    Based on everyone's advice, I think what I will do is make photocopies of my relevant wiring diagrams, highlight or otherwise mark them up to help me see and remember what I am doing, and start a list of more specific questions as I run into trouble.
    Thanks all.
     
  8. Apr 17, 2019 at 11:12 AM
    zila

    zila I throw poop

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    Read the pin out diagram on the linked instructions.. They have the color list and what it is for.. I believe they stayed with the original colors..
     
  9. Apr 17, 2019 at 11:56 AM
    fesser

    fesser Napa, CA Sponsor 2019 Sponsor

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    That is handy. Thanks.
     
  10. Apr 17, 2019 at 11:58 AM
    sterlclan

    sterlclan Member Sponsor

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    Fwiw the blazer harness will bolt through that hole in the firewall. I used the whole harness when I retrofitted my 57chevy pick up with the later r running gear didn’t do fi but it is all wired for it.
     
  11. Apr 17, 2019 at 12:21 PM
    fesser

    fesser Napa, CA Sponsor 2019 Sponsor

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    That is good to know especially if I get to a point where I give up on the Jeep half of the wiring. My original wiring isn't too bad under the dash though so I am going to try to live with it. I also didn't keep that half of the Blazer wiring.

    I have been reading a lot of F.I. stuff on the Binderplanet site. A guy named Hamilton posts a lot of really good information. This is the most helpful link I have found so far because he takes it slow and doesn't assume that the reader is already up to speed on TBI systems.
    www.binderplanet.com/forums/index.php?threads/step-by-step-real-world-build-up-stop-read-faq-first.48769/
     
  12. Apr 17, 2019 at 12:51 PM
    timgr

    timgr Jeepin' Nerd Sponsor

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    From this, I would assume the Blazer already has a fuel pump relay which you plan to use.

    If you are having trouble reading the '75 diagram, the '76 diagram (part of the '76 TSM) is a much better transfer than my '75 TSM's diagram. It's a PDF so you can zoom it without a magnifying glass too. Splice G is called splice K, and there is a Motorcraft alternator instead of a Motorola, but the under-hood wiring is substantively the same.

    Bill Hamilton is a great resource. He also has a company called "Hamilton Fuel Injection" that sells turnkey systems for whatever vehicle you own.
     
  13. Apr 17, 2019 at 6:25 PM
    fesser

    fesser Napa, CA Sponsor 2019 Sponsor

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    Yes, I kept pretty much everything that was under the hood, including the fuel pump relay.
    Good info about the 76 TSM. I think I have one of those here someplace too.
    By "turnkey", do you happen to know if it includes the Jeep side of the wiring?
     
  14. Apr 17, 2019 at 7:53 PM
    timgr

    timgr Jeepin' Nerd Sponsor

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    Mmm. Maybe. I expect he would talk you through it if needed. I know he promotes himself as working with the customer to get the system installed properly. In most cases that's about tuning the PROM to the customer's engine. He sends new PROMs in response to log files that the customer sends him.
     
  15. Apr 17, 2019 at 8:40 PM
    fesser

    fesser Napa, CA Sponsor 2019 Sponsor

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    I see. I really don't think I need another harness unless it was built to work with the Jeep wiring. I took everything connected to the engine or the ECM off when I pulled the engine. I already bought a VSS and am waiting on the plug that the OBD1 reader plugs into. Once those things are connected then I have to get power to the ECM (which I believe I can do with the link that Zila referenced and the Hamilton ones on the Binderplanet site. Getting some of the Jeep wiring worked out is likely to be the bigger half of the project since I will be deleting things there.
     
  16. Apr 17, 2019 at 9:10 PM
    sterlclan

    sterlclan Member Sponsor

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    this is throttle body injection right? I seem to recall that the ecm only needs power from the jeep harness when the key is on and in the start position.it also gets battery power I added all the injection stuff to an 86 blazer many moons ago there was only like 3 wires different in the two rigs, although they were only a few years apart. two to feed ecm one to fuel pump. you got this.
     
    Last edited: Apr 17, 2019 at 9:16 PM
  17. Apr 17, 2019 at 11:06 PM
    fesser

    fesser Napa, CA Sponsor 2019 Sponsor

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    I hope I got this. I am starting to see it your way. I might be making it a bigger issue than it is since I am still thinking about what it entails instead of just getting started on it.
     
  18. Apr 18, 2019 at 7:27 AM
    sterlclan

    sterlclan Member Sponsor

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    Don’t over think it get the Jeep end working then add the necessary stuff for the ecm.
     

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