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  1. #1

    Default 1985 Factory Stereo wiring

    So I am attempting to sort the Stereo wiring in my 85' GT while the dash is out and on my bar undergoing surgery (hence the lovely Jim Beam sign in the background). I just wanted to get clarification on the factory Amp that with a modern head unit would be completely useless so I'm trying to just bypass it completely. Below is the diagram I came up with. I also can't seem to find where the front right and left speakers wires run but I'd imagine they were in some way routed through the factory amp. The Previous owner had replaced the factory front speakers and just ran new wire to them; however, the factory plugs are still in tact. Also is it safe to just cut the amp out completely as labeled in my diagram?

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    These wires were all cut and I need to identify exactly what they were for so I can label them but I'd imagine they were cut as part of bypassing the factory amp and just don't get used/need to be properly capped:

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    It looks like most the wires coming off the factory amp are identified on this site and just cutting it out completely would be safe?:
    https://www.the12volt.com/installbay...etail/887.html

    I have the wiring diagrams if necessary but I have not read through one of these in years and I'm not very good at it. Figured someone on here would probably be very familiar with the stereo wiring on these things.
    1997 Mustang Cobra - Very First Car
    1985 Mustang GT - RestoMod Work in Progress
    2006 F-150 FX4 - Tow Vehicle

  2. #2
    FEP Super Member gr79's Avatar
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    Top of this 'Electrical' forum has a 'Sticky' for the 1985 wire diagram. Shows wire trace routing, connections.
    Page 14 of 17 shows stereo wiring.
    Compare wire codes to the other site to double check.
    When using aftermarket radio head unit, was/is easier to leave entire factory harness intact and connect all new speaker and power wires directly off the aftermarket radio harness.

    Have done many stereo change outs, additions, wire repairs, in my cars since forever. Factory and aftermarket.
    Wiring diagrams are not something one deals with everyday. Nor have primo access to like on a worktable.
    Takes hours of study when starting out cold again, depending on state of mind at the time and time since last attempt.
    Current home stereo config is 10x more complicated. Keeping notes is a must. Even what switch does what.

    Am using a factory stereo radio with aftermarket 4 channel amp and 8 speakers. Working clearly and reliably.
    All wire connections are permanently labeled with special tags in my handwriting. Many zip tags. Takes up room.
    RF+, LF+, radio LT+, amp rear LT+/-, front speaker LT+/- , 12v radio power, amp power, ground etc.
    Can be 24 or more connections to id.
    Even after trying to keep everything simple, still gets confusing every time repair or mod is needed.

    Proper stereo component and wiring work is of the type that is highly rewarding and pleasurable to all in the end.
    Last edited by gr79; 03-05-2019 at 01:18 PM.

  3. #3

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    So I've been looking over the diagram and I'm rather confused as I thought if it had that Amp device mine has it was "Premium Sound"; however, after going though the wiring diagram looking for the following disconnected bare wires that were left:

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    I noticed these wires were probably for the "AM Radio Receiver" which I'm guessing has long been removed. The "Yellow/Black" wire on this connector appears to go back to the power dist/fuse panel; however, this plug is labeled "C-701" in the diagram but if I'm reading this thing right because I have the premium sound my plug should have another wire coming off (0-LB or "Light Blue") and would be labeled "C-708" in the diagram but it does not? Honestly I could be reading this wrong I have not reviewed a wiring diagram in years. I also don't have the original stereo/cassette and some of the original plugs have probably been removed.
    1997 Mustang Cobra - Very First Car
    1985 Mustang GT - RestoMod Work in Progress
    2006 F-150 FX4 - Tow Vehicle

  4. #4

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    So I've been doing some more digging and reading through the factory diagram and I think I may have removing the factory amp mostly figured out.

    I'll start out with the plug labeled C-701 as I believe that's what feeds power to the unit:
    - The Wire I have circled in blue (LB-R) is labeled as "Illumination Lamps" at the top of the wiring diagram so I'm guessing that's for illumination
    - The wire I have circled in Yellow (Y-BK) is labeled "Power Distribution - Fuse Panel" and I have traced it back to the fuse panel so I'm pretty sure this feeds the 12v constant power.
    - Lastly the wire I have circled in Green (LG-Y) is a mystery because it just runs off the top right side of the diagram and I'm not sure what that means (runs into another page maybe?)

    Moving on to the factory amp itself. After Identifying the top plug as simply just being the direct connection to the front speakers (only the front speakers were amplified) I know I can simply just unplug and directly connect those wires to whatever aftermarket head unit I decide to go with (The previous owner had just run new wires as I'm guessing they didn't know that's where those wires go). Now the plug below the front speaker plug appears to be where power goes into the amp via the yellow and orange wires. The previous owner had just wired these two wires directly to the yellow (Y-BK) wire coming off C-701. My guess is all this is doing is feeding power to the unused factory amp and potentially feeding the 3 unidentified wires hanging off to the left that had long been cut (Pretty sure that was done to remove the amp by the previous owner).

    So my question is can I just undo the wire nut, label the remaining wire coming off C-701 as the 12v constant and then just unplug the plug up at the factory amp and voila it's deleted (cleanly). The only thing that bothers me here is what the heck is the orange wire for?

    Also I had a hell of a time even locating the two plugs on that factory amp in the diagram because they don't match any of the plugs labeled in the bottom half of the diagram.

    The only thing I can't figure out is where the 12v switched power comes from or does it even exist?

    Hopefully someone can help me out I'm not great at reading these diagrams and I really don't want to start a fire in a freshly restored car I've spent countless hours on. I have a feeling the previous owner might have been running that risk with the butchered factory amp that was still getting power with 3 lose uncapped wires just hanging out.
    1997 Mustang Cobra - Very First Car
    1985 Mustang GT - RestoMod Work in Progress
    2006 F-150 FX4 - Tow Vehicle

  5. #5
    FEP Super Member gr79's Avatar
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    Default switched power is Y-BK, LG constant hot?

    Intro:
    Describe, translate, guess best i can. From what i read, you are better at it than you think. No need to be a pro to do this.
    I have a EVTM for 85 LTD/Marquis. Wire codes may be same as Mustang. Wire connector id, fuse positions are different.
    They had no separate amp on/off switch under dash.
    Same amp deal. Only FT speakers are amped, common ground thru amp (7 wires: 2 input from radio, 4 out, one ground back to radio).

    Power:
    Shows two or three fused power circuits for radio system. Depends on radio. Two hot in accy or run, one constant hot.
    One for illumination, one for radio (and amp if equipped), one constant hot for radio memory (if electronic)

    Fuses:
    Electronic radio has extra fuse connection (LG-Y) for constant hot. Preserves clock and station memory settings when all power is off.
    Electronic radios use 2 powered fuses, conventional radios 1. Conventional radio needs no extra fuse to preserve any settings.

    Wire codes:
    Electronic radio: LG-Y is hot all times from fuse panel. Connects direct to radio. 15a fuse.
    All radios: Y-BK is hot in accy or run. Power for both radio and amp if equipped. 15a fuse.
    All radios: LB-R is radio illumination. Part of instrument illumination circuit. 5a fuse.

    EVTM diagrams show:
    Amp (premium sound option) connected to conventional or electronic radios.
    Amp unit has two-wire connector Y-BK and O-LB near the amp.
    O-LB is connected from amp to radio. Prob for standby or boost mode.
    Conventional radio has 2-wire connector Y-BK (from fuse box) and O-LB (from amp).
    Electronic radio has 3-wire connector LG-Y and Y-BK (from fuse panel) and O-LB (from amp).

    Mod note:
    Wiring the LG and Y-BK wires together would allow power to the amp and radio to be hot all the time.
    Power to radio/amp with no key in ign. No standby mode for amp.
    Can power down radio at radio. Amp prob on 24/7, if no amp on/off switch.
    I agree. Bare wires, hot power= ng.
    Best was run new wires from aftermarket radio to speakers, but the power wire circuit was not completely sorted out or understood to take the amp totally offline.
    Ida just unplugged it's 2-wire power connector.

  6. #6
    FEP Senior Member tbirdman's Avatar
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    There were two amps used in 1985 and 1986. One was a four channel amplifier used with the two post radios, and a two channel amp that was used with the electronic radios. The two channel amp powered the front speakers only. The rear speakers were powered from the radio and bypassed the amp.
    If they take my stapler, then I'll set the building on fire...

    85 Mustang GT

  7. #7

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    I'm pretty sure I had the two channel only amp. I traced Y-BK wire back to the fuse panel and it is linked to position "11" which is labeled "Radio/Premium Sound" so I believe that is simply the 12v supply for the radio. Now the other wire that is cut in my photos that I'm not sure of is the LG-Y wire and is "12" in the fuse panel diagram which is "Power Door Locks" at least according to what I can find on an 85' GT; however, I do not have power door locks. This would probably be a lot easier if i still had the old radio components but I don't

    Also the fuse for location "12" has been pulled or never existed. gr79 mentioned above it is probably always hot and connects direct to the radio. I'm not entirely sure what to do at this point but I believe having some EVTM's in front of me might help. I currently don't have those but I can try to dig them up.
    1997 Mustang Cobra - Very First Car
    1985 Mustang GT - RestoMod Work in Progress
    2006 F-150 FX4 - Tow Vehicle

  8. #8
    FEP Super Member gr79's Avatar
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    Fuse 12 may have been removed to shut off the circuit.
    Could plug a fuse in, then connect meter to LG-Y.
    If power is found with key out, then its constant power.

    My 79 EVTM says power locks have power all the time thru an in-line CB.
    Same for 85 LTD.

    79 Premium Sound amped rear speakers only. The under dash switch turned the amp on and off.

    Fuse 11 may be switched power. Works with key on only? Or all the time?

  9. #9

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    Unfortunately I can't really test any of that because the dash is pulled out of the car atm. This is a resto project so it may be awhile before the dash makes it back in with a proper battery connection. I was trying to sort the wires before re-installing as once the dash is back in I really don't want to have to pull it again. I guess I will just extend these wires so they are easily accessible once the dash is back in and hooked to battery power to see if one is switched power and the other is constant. I was trying to track down an EVTM last night but it looks like I'll have to buy one off ebay as most the ones that were posted or shared on sites have long since disappeared. My plan is to extend the Y-BK and LG-Y wires and cap them for now until everything is back together and I can determine what kind of power runs through them. I guess I should extend the illumination wire as well.

    I'm also going to remove the wire nut holding the Y-BK/Orange wires together and pull the hacked up amp since I think all this is doing is feeding power to a dead amp.

    On a side note is the illumination wire usually just used for dimmers? I hooked it up in my beater truck and it did more harm than good as it would have been better just to always have the stereo at full bright since modern stereos have built in dimming.

    Thanks for all the help and let me know if you think of any reasons this may not work.

    Thanks again!

    -Rob-
    1997 Mustang Cobra - Very First Car
    1985 Mustang GT - RestoMod Work in Progress
    2006 F-150 FX4 - Tow Vehicle

  10. #10
    FEP Senior Member tbirdman's Avatar
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    Scan of the wiring diagram for the electronic radio and amplifier setup...

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    The amp for the conventional (2 post) radio has two 8 pin electrical connectors. The amp for the electronic radio has one 8 pin connector and one 4 pin connector.
    If they take my stapler, then I'll set the building on fire...

    85 Mustang GT

  11. #11
    FEP Super Member gr79's Avatar
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    Good plan extend wires, cap, mark for later.
    Like plumbing or house wiring.
    Dimming happens in concert with all panel lights or just radio when park/headlights are turned on.
    Have not figured out how to wire that way when i installed the 90's ele din Ford.
    So many different wire codes and terminal position variances for each type of radio option.

  12. #12

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    Thanks all this is exactly what I needed and I think I have it all sorted now. I def had the electronic radio originally (8 and 4 pin connectors). Now that I know what the 12v constant and 12v switched wires are I should be good to go. Thanks again this has been extremely helpful!

    Now I just need to figure out what the + vs - are on the front factory speaker wires but I have been told this doesn't really matter much on modern speakers? Either way it should be really easy to sort that out.

    Thanks again for all your help!
    1997 Mustang Cobra - Very First Car
    1985 Mustang GT - RestoMod Work in Progress
    2006 F-150 FX4 - Tow Vehicle

  13. #13
    FEP Super Member gr79's Avatar
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    Connections matter a lot.
    The wider of the 2 speaker terminals is +. May also have a paint dot, striped/coded wire, etc.
    Factory plugs insert one way.
    Connecting all speakers correctly gives max bass and clarity, especially if connecting 2 together on same channel.
    For sure to the correct +/- output wire of the radio or amp.
    Once powered up:
    If sound sounds weak or off, switch one speaker's connection. Then listen if music sounds right compared to the others.
    All should blend together. Out of phase is subtile but noticeable over time. Can disregard +/- if it makes a difference.
    If a speaker is not in phase with the others, it will sound weak, especially bass notes.

    Attachment 126790
    Last edited by gr79; 03-15-2019 at 12:22 AM.

  14. #14
    FEP Senior Member tbirdman's Avatar
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    gr79 is correct, speaker polarity does matter. If you reverse the polarity you're not going to blow anything up, it just won't sound right.

    One way to determine which wire is positive and which is negative (if you still have the factory speakers connected to the wiring harness) is to use a battery. Cut two small lengths of wire and strip the ends. Take some electrical tape and tape one end of each wire to the positive and negative terminals of the battery. You can use a 9 volt, but I've done it with a "D" battery also. Take the other ends and touch them to the bare terminal connectors on the speaker. If the cone pops out, you're good. You've correctly identified the positive and negative terminals on the speaker. Just trace back to which speaker wire leads to which terminal, and mark the wires with a small piece of tape + and -. If the speaker cone pops in, reverse the wires and you should then see it pop out, which is what you want it to do.
    If they take my stapler, then I'll set the building on fire...

    85 Mustang GT

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