1985-1986 MR2 MK1A Radio Headunit Wiring - MR2 Owners Club Message Board
 
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post #1 of 9 (permalink) Old June 20th, 2015, 11:50 Thread Starter
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1985-1986 MR2 MK1A Radio Headunit Wiring

So I'm making this thread because I just replaced the headunit on my '85 Mk1a, and the wiring on it is dramatically different from the Mk1b (late '86-'89). I couldn't find a good, accurate source for wiring this up correctly after searching around a bit on the internet, so I'm writing it up here to say definitively: "this is how to wire this strange system and I just did it and it definitely works."

So,

If you have a Mk1a MR2, the wire bundle will look like this:

BLUE (constant 12v power)
PINK&BLUE (ignition controlled 12v power)
RED (right speaker +)
BLACK&RED (right speaker -)
YELLOW (left speaker +)
WHITE (left speaker -)
GREEN (subwoofer amp power-on signal)
GREEN (12v power when headlights are turned on (AKA dimmer))
^
Yes, there are two green wires. Yes, they are the exact same color. You really don't want to wire them up backwards or leave them disconnected, because if you do either, you won't have a dimmer function, and your under-seat sub won't work. I'll get to that in a second.

Wire everything up to the correct cable from your headunit. Your Mk1a doesn't have any rear speakers, so leave all of the rear speaker wires from your headunit disconnected.

If you have a dimmer function wire, it will need to be connected to one of the green wires. See below.

In order for the underseat sub to work, you need to connect your headunit's 'amp power on' (or 'power antenna') wire to the other green wire. See below.

SO YOU HAVE TWO IDENTICAL GREEN WIRES... The only way to VISUALLY tell which one is which is if they're still wired into the original harness. In that case, the GREEN wire DIRECTLY UNDERNEATH the PINK&BLUE wire is the (amp power-on signal) wire, and the GREEN wire with nothing wired in above it is the (12v dimmer) wire.

If you (like me) opened up your dash to find that the original harness had been cut out, don't despair! You can still figure out which green wire is which, it just takes a few extra steps:

1) Disconnect the negative terminal of your battery, and wire everything else up, but leave your dimmer and amp power/power antenna wires disconnected.

2) With the battery still disconnected, connect your amp power/power antenna wire to one of the green cables. Just grab whichever one and connect it up. You have a 50/50 chance of getting it right, and nothing bad will happen if you mess this up. At least, nothing bad happened to my system when I guessed wrong. You could maybe blow a fuse but I didn't.

3) Reconnect the battery terminal, power up the headunit, and put on some tunes. Check to see if you hear anything coming from the sub. Anything with any amount of bass and at any decent volume, and you'll be able to tell if its thumping or not. If it is, congrats! You guessed correctly. Disconnect your battery again, finishing wiring everything up and reinstall the dash, etc.

4) If you didn't get anything out of the sub, don't worry - you just wired the dimmer function up to the sub power, but that shouldn't mess anything up. Disconnect the battery, switch the green cable you connected to your amp power/power antenna wire and connect it to your dimmer wire if you have one. Then connect the other green wire to your amp power/power antenna wire. Reconnect the battery and test. If it still isn't working, your sub speaker or amp might be shot, or you may have other electrical issues. Sorry, this is where my help ends for you.

DISCLAIMER: my headunit didn't have a dimmer function, so no dimmer wire, so I can't personally confirm that the green dimmer function wire actually works for your headunit's dimmer function.

Good luck! Feel free to message me with questions.

BONUS:
Here's the original 1985 MR2 Mk1a radio wiring diagram:
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post #2 of 9 (permalink) Old June 20th, 2015, 21:52
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Great write-up. I'll be working on my '86 before long, and not having a BGB for it yet this will definitely make things easier. I knew there was a definite difference between MKIa and MKIb, but didn't have the references available.

What would be handy is to include the various colors for, and the types of (L/R - polarization), wires that split off from the oem harness and run to the oem underseat sub, especially for those that have a really badly hacked harness.
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post #3 of 9 (permalink) Old June 21st, 2015, 10:57 Thread Starter
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Quote:
Originally Posted by Picaro View Post
Great write-up. I'll be working on my '86 before long, and not having a BGB for it yet this will definitely make things easier. I knew there was a definite difference between MKIa and MKIb, but didn't have the references available.

What would be handy is to include the various colors for, and the types of (L/R - polarization), wires that split off from the oem harness and run to the oem underseat sub, especially for those that have a really badly hacked harness.
The underseat sub wires are shown in the picture above, but I honestly don't know what each wire connects to in the amp, other than that the one green wire sends the 'power on' signal to the amp circuit when the headunit is powered. I was fortunate in that my underseat sub circuit was functional and unmolested. I should have taken a picture of where the cables ran to on that thing... I always could take it out again if you need a reference.
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post #4 of 9 (permalink) Old June 21st, 2015, 12:56
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Sorry, but must not have had my glasses on, or not paying proper attention when I looked at the diagram originally. It does show everything, and that is all that is needed in case rewiring needs to be done above stream of the sub, thanks.
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post #5 of 9 (permalink) Old June 21st, 2015, 18:41 Thread Starter
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Quote:
Originally Posted by Picaro View Post
Sorry, but must not have had my glasses on, or not paying proper attention when I looked at the diagram originally. It does show everything, and that is all that is needed in case rewiring needs to be done above stream of the sub, thanks.
no problem! and no worries haha, its a pretty ambiguous and unclear diagram.
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post #6 of 9 (permalink) Old June 26th, 2015, 02:26
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Cool that you posted this, but I would recommend using new wiring. The only factory wire I ever used in 20 yrs of ownership was the 12v switched. Modern head units require a lot more juice than they did in the '80s. The last few new head units I bought actually wired up differently then years ago. Head units used to have a 12v switch to turn on and supply power and 12v constant for memory retention. The last few modern units I bought Also had a 12v switched to turn the init on and off but was not the main power supply. They used a much heavier wire that the manufacturers recommended running straight to the battery for not only memory but also for the main power supply. This heavier wire was also fused.
Also, the factory wiring in MK1s is pretty small and very old. I also always ran new heavier speaker wire since new head units push so much more power.
Just my 2 cents.
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post #7 of 9 (permalink) Old January 8th, 2016, 23:26 Thread Starter
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The old wiring has been working perfectly in my setup. I dropped in a replacement for the OEM sub and put some Pioneers in the dash, and running them from this headunit has worked just fine with the old wiring. Granted, I don't think I have (or will) ever crank the volume all the way up...

Always a good idea to replace the old wires and tubes on these cars with modern stuff, especially if you're trying to run more power etc. But I will say in this case if you're just looking to get your tunes going again, its by no means necessary.
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post #8 of 9 (permalink) Old February 5th, 2017, 15:03
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To add to this...

If you find your oem wiring cut I bought new "wiring pigtails" with the oem connectors on ebay.

1989 MR2 SC

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post #9 of 9 (permalink) Old July 18th, 2018, 12:33
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A few tips:

1) You mentioned two blue wires on the new head unit, one for remote amplifiers and one for power antenna. Don't mix these up as they are not interchangeable on most head units. The power antenna lead is not going to send power to your subwoofer when you are in different modes on the head unit. It usually only sends power when the radio mode is active.

2) As others have suggested, constant power (yellow) AND ground (black) wires on newer head units are typically thicker gauge wire than factory. The factory wires are probably around 16-18ga while most high output head units will have a 12-14ga wire. These cars aren't old enough to the point where wires are bad, but the smaller wire cannot carry the same amount of current and will see a greater drop in voltage across the length of the wire, and there is a small chance the wire could overheat if you pull too much current through it. If you plan to use the head unit's onboard amp that claims 45 watts x 4 or more and you really want to get that wattage to your speakers, that's when you need to consider running a dedicated power and ground wire. If you're installing an aftermarket amp and running new speaker wires and power wires for it then the factory wiring is probably fine as the head unit itself will not draw very much current. Power wire should go to battery with a fuse within 18" of the battery. Ground wire can go to a good spot on the chassis or all the way back to the battery. If you hear any sort of interference or alternator whine from your speakers, it's usually because you chose a bad ground point.
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