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Thread: First TBI Conversion

  1. #1
    Fuel Injected!
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    First TBI Conversion

    I'm working on my first TBI conversion, with the donor truck being a 91 Chevy 1500. I have the wiring diagram and have been able to trace everything needed for the conversion. I have not started disassembly of the factory harness yet as I have a couple questions on how people have handled the following. See in the first pic the large block of (epoxied?) wires where the harness runs through the firewall on the passenger side. What did you do with this? Has anyone cut and re-spliced this for easy of handling and trimming the individual sensor wires to an appropriate length?

    Also - on the second pic you can see the ECM connections along with a white harness plug. Not sure what this plug is for but some of the ECM wires route through this plug, appears to be ground related. Can I eliminate this plug?

    I see some forum feedback suggesting "tuning out" such things as EGR and others if not needed. My TBI conversion is going into a 78 K15 with no emissions required, so I would like to make this as simple as possible. If I eliminate the EGR, for example, is it best to pull out of the harness (including ECM plug) completely, or does it need to be plugged into the ECM and the wire capped off?

    Lastly - I want to confirm all I need for the conversion is the wires connected to the ECM harness plug ins. We ended up pulling the entire harness which of course has a whole lot more wires that what is connected to the ECM plugs.

    Thank you.
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  2. #2
    Fuel Injected!
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    Quote Originally Posted by mtbraun View Post
    I'm working on my first TBI conversion, with the donor truck being a 91 Chevy 1500. I have the wiring diagram and have been able to trace everything needed for the conversion. I have not started disassembly of the factory harness yet as I have a couple questions on how people have handled the following. See in the first pic the large block of (epoxied?) wires where the harness runs through the firewall on the passenger side. What did you do with this? Has anyone cut and re-spliced this for easy of handling and trimming the individual sensor wires to an appropriate length?

    Also - on the second pic you can see the ECM connections along with a white harness plug. Not sure what this plug is for but some of the ECM wires route through this plug, appears to be ground related. Can I eliminate this plug?

    I see some forum feedback suggesting "tuning out" such things as EGR and others if not needed. My TBI conversion is going into a 78 K15 with no emissions required, so I would like to make this as simple as possible. If I eliminate the EGR, for example, is it best to pull out of the harness (including ECM plug) completely, or does it need to be plugged into the ECM and the wire capped off?

    Lastly - I want to confirm all I need for the conversion is the wires connected to the ECM harness plug ins. We ended up pulling the entire harness which of course has a whole lot more wires that what is connected to the ECM plugs.

    Thank you.
    You can melt the glue out of that and pull it apart with the careful use of a heat gun.

  3. #3
    Fuel Injected!
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    Quote Originally Posted by mtbraun View Post
    I'm working on my first TBI conversion, with the donor truck being a 91 Chevy 1500. I have the wiring diagram and have been able to trace everything needed for the conversion. I have not started disassembly of the factory harness yet as I have a couple questions on how people have handled the following. See in the first pic the large block of (epoxied?) wires where the harness runs through the firewall on the passenger side. What did you do with this? Has anyone cut and re-spliced this for easy of handling and trimming the individual sensor wires to an appropriate length?

    Also - on the second pic you can see the ECM connections along with a white harness plug. Not sure what this plug is for but some of the ECM wires route through this plug, appears to be ground related. Can I eliminate this plug?

    I see some forum feedback suggesting "tuning out" such things as EGR and others if not needed. My TBI conversion is going into a 78 K15 with no emissions required, so I would like to make this as simple as possible. If I eliminate the EGR, for example, is it best to pull out of the harness (including ECM plug) completely, or does it need to be plugged into the ECM and the wire capped off?

    Lastly - I want to confirm all I need for the conversion is the wires connected to the ECM harness plug ins. We ended up pulling the entire harness which of course has a whole lot more wires that what is connected to the ECM plugs.

    Thank you.
    Most cut the epoxy off and clean the wires. You can cut and trim the wires to the correct length, just make sure you have a proper connection and they are waterproof and won't pull apart.

    For removing things you don't need, that is all in programming/tuning the chip. You can remove the wires to clean up the harness as well.

    You won't know what wires you need unless you're looking at the wiring diagram for the ECM you have. The wires on that white harness plug could be for things like the fuel pump, VSS, park/neutral switch, brake switch, ignition, ground, and power wires. You need a wiring diagram, and to trace each wire to see what you need.

  4. #4
    Electronic Ignition!
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    I did a similar swap about a year ago using a 91 c3500 as the donor into a 85 c10.

    Document everything. go to Home Depot and buy a set of wire number stickers (Electricians use them for labeling wires for which circuit numbers go where) I found this incredibly helpful when I had wires I didn't know the purpose. Tag it with a number and write in a notebook what it is.

    I didn't tune anything out until after I had gotten the vehicle running as it was in the original vehicle.

    That white plug I believe my run to the speedometer. The transmission will send the speed signal to the speedometer and then the speedometer will output a signal to the computer (It will do a signal modification based on your tire size and gear ratio so that the computer receives a standard input. This process is performed by a DRAC or VSSB module in other years)

    If you are using the transmission from the donor vehicle the speed signal is digital instead of a cable so the best option for me was to get a speedometer out of a 91 or 92 suburban or blazer and use a DRAC module that way it fits in the factory cluster.

    I have pictures and documentation for most everything from when I did mine if you would like to see any of them. I even maintained the 91 factory keyless entry which was easily set up for remote start as well.

  5. #5
    Fuel Injected!
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    Thanks everyone for your suggestions so far. I am using a TH350 so will need to figure out how to adapt the speed sensor to a cable speedo. If you could pm me pics that you feel would be helpful I would be grateful.

    Matt

  6. #6
    Super Moderator Six_Shooter's Avatar
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    That white plug is the connector to the dash wiring. This is where the CEL, ALDL and ignition wires run through, and few others.
    The man who says something is impossible, is usually interrupted by the man doing it.

  7. #7
    Fuel Injected!
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    If you have the other side of that white plug then you could use it for the interface. If you don't then cut it off and use a terminal strip on the wires that you need from it.

    You can remove the wiring for the stuff you're not using. It makes no difference to the ECM if the wires are there and not connected or completely removed.

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