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Update 4 plug TT Legacy to 3 plug WRX Ecu Conversion

Guest MasterTm27

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Guest MasterTm27
 kamineko said:

wonder what the * (astrix) was referring to for the grounds? i think its missing a note

The astrix was just to show the grounds and earths sorry i forgot to remove them before putting that up. Also Note wire B95,4 under the Twin Turbo Is shown as Secondary waste gate this is because i used that particular wire for the wire B137,6 Pressure Exchange Solenoid but any of the wires on that plug could be used i have posted a link to a pic of that plug. https://acrobat.com/#d=Sr6m4ct8S8KrYz8Gw6XMXA
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  • 2 months later...
Guest MasterTm27

Also note that the 97 ignitor dosnt have to be used with the earlier model Twin Turbos if your useing the spark pack as they allready have a ignitor that can be used not totally shure when they changed from useing that late 96 to 97 i think.

Wires: Twin Turbo

B44,7 Ignition Control #4 (Black/Red) Not needed when useing spark pack.

B44,8 Ignition Control #2 (Red) Not needed when useing spark pack.

B44,9 Igniton Control #3 (Yellow/Purple)

B44,10 Igniton Control #1 (Yelllow/Blue)

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  • 1 year later...

the thread lives!

Nah.. i used this thread orginally to get my BG5a going, so may as well make this the place for more info.


Stay tuned for more info on the earthing for this conversion (BG5a)

The doco in this thread may work but i believe it is slightly flawed when it comes to the earthing.

BG5a docos here (incl wiring diagrams) https://skydrive.live.com/embed?cid=CFA30F34A9604D4A&resid=CFA30F34A9604D4A%21161&authkey=APhR-3DViSPDvno

The earthing points i raise are that with WRX v3/4 some earths are supplied by the ECU (dediacted) some are comon earthing points that go to ground (body/chassis) others are like the shield wires that protect the signal wires on certain sensors.. these have a comon earthing joint connector but DO NOT go directly to ground, the wiring diagram for WRX v3/4 reveals all so track n trace for yourself to see what i mean.

I will cover the The shielding for the AFM signal wire and other sensors later in this thread.

Watch out with the TT wiring as some earths go to ground when they should not! But these findings should help.

My last BG5a conversion i cut ALL the wires off at the ECU and started from fresh tracing all wires and mirrioring the V3/4 WRX wiring diagram to the TEE,

i never documented it but now i am doing another BG5a conversion.. I am not cutting the ECU plugs off (yet) so here are my findings.


(1)Note 1 - AFM

Plug B43 Pin 11 MAF negative (GND) on the TT loom states to convert it to the v3/4 it should now be the MAF GND on the WRX B136 pin 7.

According to the WRX v3/4 wiring diagram the AFM has a direct GND wire from B136 pin 7 on the ECU directly to the AFM itself. i.e. it does not just go to any old earth point.. it has a dedicated ‘point to point’ earth wire to and from the ECU.

The reason i have brought this up is that in the doco in this thread the wire you are told to pull from the TT ECU plug B43 Pin 11 and re-pin into 136 pin 7, but this wire actually has shared/common earths from other sensors on the TT system. i.e. its not the singular dedicated AFM earth wire.

So i say, sticking to the WRX system diagram.. run a new AFM GND dedicated ground wire. I wouldn’t pic any old random earth wire either as its bound to be grounded else where.

Note the pic below showing the pins in question from the doco:


Someone queried about the * in the doco.. after my findings i can see via the WRX v3/4 wiring diagram that WRX B136 pin12 and B136 pin 18 (both with asterix’s) are both earths and are have a common joint. That being the case join that wire from B43 pin 11 to both pins 18 & 12 in B136. Makes sure it goes to the engine earth.. i.e. test continuity

Then run the new AFM GND wire on its own (don’t use any other suspected earth wires, it needs to be dedicated).

Note 1 summary

The TT wire stated to use for the dedicated AFM GND wire is NOT dedicated.. it is connected to ground!

Yes it will more than likely work if you follow the original doco, but why would you?? The AFM is a very sensitive sensor and if the WRX V3/4 diagram shows it has a dedicated point to point earth wire its for a reason.. i.e. the ECU supplies the AFM its earth via its internal noise suppression components as opposed to random earth point on the engine or body of which is can potentiality cause induced noise on the wire and/or signal corruption.


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i brought some more of this like what i used for the last BG5a conversion. $16 from redpaths for 3 meters.

You can use this double shielded, wiz bang cable for the AFM wiring i.e. run the dedicated earth and the signal wire in this and earth the outer shield as will be explained soon.

You can use one of the existing 12v feeds close by, just make sure its a desent size thickness cable, teh STI orange AFMs have a bigger than normal 12v feed cable.

Prolly waay better than the factory shield too!


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NOTE 2 - SHIELD EARTHS & the Joint earth connector (incl AFM)

included sensors with shields and some that have no shield but simply get their earth from joint connector incl:

- AFM sheild

- Oxygen sensor shield

- Knock sensor shield

- Cam sensor shield

- crank shield

- MAP sensor (refered to as a 'boost sensor') earth wire (is not a shield)

- eng temp sensor earth wire (is not a shield)

- TPS earth wire (is not a shield)

Via the joint connector all of the above get their earth from the B136, 13 Pin, they are NOT earthed directly to ground/chasis.

The TT AFM shield on the WRX V3/4 wiring diagram goes to the joint connector, as do shields from multiple other sensors. This earth junction point/connector gets its earth from pin B13 on the ECU.

If you use the above cable i recommended for the AFM just make sure you earth this AFM shield wire into pin B13.

If you are using the existing AFM signal wire, it is wrapped in a shield wire, this shielding must still connect to B136,13 (however you decide to do it).

Your current TT AFM, Knock, Oxygen sensor shields terminate on pin B43,18

Your current TT Crank shaft shield is on B43,19

Your current TT Camshaft shield is on B43,20

In the WRX v3/4 scheme all of the above go to the joint earth connector of which gets it earths via B136,13, what you have to do is grab the wires from B43, 18, 19 & 20 and solder them onto the wire on B136,13 (not using a joint connector here, but you can if you wish), once soldered make sure none of the shields have earth to the chasis/engine. Remember.. these sheilds are ECU supplied earths only!

The following diagrams are WRX v3/4 i.e. the end result of what your Legacy wiring should look like




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NOTE 3 - Cam and crank sensor earths ** talking about the earth wire now, not the shield **

Your current TT cam sensor earth is on B43,9

Your current TT Crank sensor earth is on B43,7

On the v3/4 WRX set up these go to a comon joint earth connector (of which on a v3/4 is sometimes located in side the main earth connector, and is presented as two seperate earthing circuits, if you take the cap off that joint connector you will see two circuits)

So yea, the crank and cam earths in the WRX setup go to earth pin B136,9.

Note the shielding from the cam and crank goes to 'G' which is the main earth joint connector (seperate to the cam and crank earths) as per note 2

So remove wires from B43, 7 and solder it into B136,9 and you are done!

Always double check your work!


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i used the same document's as these when i did mine .

i got everything sorted but must have missed the afm earth as mentioned by gazz. i normaly dont have any problems but occasionally ill get a cell (23) .

big ups to gazz ! i knew there was an earthing problem but i now know what to change :)

i say Ruben should add this info into the the conversion thread then theres a complete how to in one place that covers all the bg/bd's

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NOTEs - BG5a 3 to 4 plug

The earths and shields are probably the most confusion part of the 4 to 3 plug conversion. To apply some structure to the overall process.. i find Remembering the 3 types of earths that the WRX v3/4 setup requires is helpful:

1) shield earths (dont go to ground)

2) earths that dont have a shield but DONT go directly to ground either (i.e. supplied by the ECU and DONT go to ground via the chasis)

3) earths that actually do go to ground directly via the chasis

So if you focus on getting these right, the rest of the pins are pretty straight forward 're-pining'. Hopefully the above posts (Notes 1-4) will help.

The GO is:

1) research/read the V3/4 wiring diagram for how/what sensors get their earths and how their shields are earthed and where they go etc.

2) locate your current TT shield earths (you can strip back the loom at the sensors in question a tad to expose the begining of the shielded wire then use a continuity test to track back to the ECU. Note 2 has this information for you!

3) once you have found all the shields (see above notes or study the BG5a 4 to 3 plug doco/diagram) snip or remove them to make sure they go where they should i.e. shields get an earth from B136,13

Coil and ignitor

If using a WRX manifold and singular WRX coil pack, you will need the ignitor, mount this new ignitor in the same place as the old TT ignitor as the wires you need are there from ECU to the ignitor, then from there i find its easier to run new wires required for the coil pack on the manifold, then find the correct pins in the plugs that go to the ignitor (by the battery needed) and wire in/solder the newly run wires there.

I dont recommend mounting the ignitor on the actual engine/manifold as heat and vibrations will kill it in time.

If using your TT coil over plug setup with wasted spark, "the single turbo conversion thread" has all the info on this.

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  • 10 months later...

*edit, Cleared codes but now it is majorly overfueling smoking out the garage!!

firstly I would like to say this thread is an absolute godsend, thanks to everyone who has contributed!!

Ive followed every instruction on here to the letter and finally fired up the engine.

the only codes i have left over are 31 and 45. Is there anything I have done wrong to get these codes?? engine runs pretty rough and eventually dies.

Im hoping it is something simple ive done wrong, but does anyone have any ideas?

I have converted BG5A TT to a V3 sti motor.

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Firstly gazzy2000 you are a champion this info you\'ve posted in this thread has been invaluable in getting some issues sorted in my car!

I\'m still having some troubles with the ground pins in my conversion that\'s using a 3 plug v3/4 ecu. Am I right to assume that i\'m NOT supposed to have continuity on my ecu between B136, 7,9,12,13? I think my ecu has shat itself if this is the case, the car struggles to start and doesn\'t run the best when it finally starts. I\'ve been troubleshooting the shields on pin 13 because everytime I plug in b136 pin 13 shows continuity between itself and the chassis. The wiring doesn\'t so i\'m thinking the ecu is to blame?

These 4 pins on my ecu show continuity?


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