# GTI VR6 OBD1 to OBD2 turbo swap



## GTI Jay (Feb 11, 2010)

Needless to say I am having a difficult time these past few days finding definitive information on a complete OBD1 to OBD2 conversion on a mk3 GTI platform. I've been searching on vortex as well as Google and finding almost nothing but dead end threads and mk2 VR swaps. I have found the a2resource ecu pin out chart. I know the parts I need, I have everything. 

The situation I am in: 
I want to go TURBO. OBD1 has very limited options on tuning. Before you say no, why not just tune the OBD1 ecu please read on....

I am currently driving a 1995 OBD1 GTI with a very healthy VR6. I just purchased a 1997 OBD2 GTI VR6 for very cheap that was previously turbocharged, and has all major turbo parts included. BUT the engine was blown, piston in the oil pan. The car sat for the last 10 years outside in the arctic conditions of Fairbanks, Alaska until now. That being said it needs a lot of work, wheel bearings, brakes, bushings, rubber brake hoses, fuel tank varnished blah blah BLAH and so on... This car came with a brand new obd2 c2 motorsports stage one turbo chip that was never installed, (hence a probable reason why the engine blew) I want to bolt on these turbo parts (including OBD2 ecu) to my currently running VR6 and in order to do this I need to convert it to OBD2.

With all this being said, I strongly want to turbo and drive my 1995 GTI with fresh suspension, brakes, and excellent mouse poop-free interior. Is there a diy I am not seeing? Do I HAVE to pull the dash and wire an OBD2 fuse block?  if so, does anyone have or know where I can get specific information or a diy? Someone please help me out here!


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## SpacedOutStar (Aug 23, 2013)

You don't see a DYI because it is so easy you don't need one. You have both cars, simply swap your upper intake and throttle body so the OBD-II one is on your car and then switch the engine compartment harnesses and ECMs. They are pretty much the same car so there would be only minor differences between the two of them. The both use the same wiring system so it would be a pretty simple swap. I have never done this on your specific car and because your car has the engine harness included with the body harness there may be some differences depending on the options but I would be surprised if it was not fairly simple. 

If I were you I would begin by removing the harness from the OBD-II car and then putting it on the ground and checking if the connectors under the hood are the same on both cars. Don't forget to switch the harness that is on the engine with the later one as well. Otherwise I think this should be pretty easy for you.


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## GTI Jay (Feb 11, 2010)

Thank you for your response. The only real reason I haven't tried what you suggested yet is that I'm afraid the wiring harness from the 42 pin cannon plug on the end of the cylinder head to the interior to the fuse block is different. And the reason I think it might be different is according to http://www.a2resource.com/electrical/management/motronicvr6aba.html
Obd1 and obd2 there are differences in the pin out with injector numbers. There's also a thread in which this guy was trying to do this same swap and was super confused wiring the fuse block. This just seems like something to good to be true to be this easy. 


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## bonesaw (Aug 8, 2004)

You will need to change intake manifold and down pipe. Need to change both the engine harness to fuse panel and harness on actual engine. Will want to swap over possibly headlight and fan harness. You will need to change the cluster harness. It is very straight forward.


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## GTI Jay (Feb 11, 2010)

And the headlight, fan and cluster harnesses all connect directly to the back of the fuse block if I remember correctly. Don't get me wrong I know my way around the a3 chassis extremely well but the only thing is I've never dealt with wiring harnesses through the firewall to the fuse block. I've had the dash out numerous times for heatercores and such and I've seen the mess of wires. Have you by chance done this before?


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## SpacedOutStar (Aug 23, 2013)

bonesaw said:


> You will need to change intake manifold and down pipe. Need to change both the engine harness to fuse panel and harness on actual engine. Will want to swap over possibly headlight and fan harness. You will need to change the cluster harness. It is very straight forward.


Do you need to do the cluster harness and why? When I did this on my Corrado I did not have to swap the cluster harness and everything in the cluster that worked before worked after. The 1994 Corrado used a MAP based MFA and the 1997 Passat harness I used had this as well so everything swapped across without an issue, When I originally did this swap I did not have a swap car to take parts off of so I just welded an O2 bung on my stock down pipe but if you have the proper one you may as well swap it over. 

Like i had said above, I would start by pulling the engine compartment harness from the OBD-II car. You know where the 40 pin connector is, remove everything on the car side of this connector so the entire harness is sitting on the ground. Mark everything you remove and take a lot of pics as you go. On the A3 chassis this harness will go all over the place so take your time and do not cut any wiring. When you go to pull the wiring harnesses from the back of the fuse panel you will need to drop the fuse panel from the holder so you can access the back of it. There is a lock bar that holds the harnesses in place which you will need to slide over so you can get the plugs out of the fuse panel. All of the harness going into the fuse panel are indexed so they will only go in one way in one place so this should be pretty simple. There will be some wires that plug into harness under the dash so I would mark and tape both sides of these connectors for later reference if needed. 

Once you have the main engine compartment harness out of the OBD-II car you can lay it on the ground and then start comparing it to the one on your current car. It is a little intimidating at first but by the time you get the OBD-II harness out of the car you don't care about you will see it is not that bad. You are lucky in the sense that you will be mostly swapping parts and harnesses and no hard work is involved because it will all just plug in. I did have to change the ends of some of the connectors when I did mine but I don't think you will have this issue because yours are both the same car but different years. 

On my Corrado I only had to deal with the connectors on the fuse panel in blue below but I think you will have more. The red part is where the locking bar sits...


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## bonesaw (Aug 8, 2004)

Only need to do the cluster harness if you want entirely plug and play. The turn signal connector for cluster is a black 2 pin and mfa is a signal pin. Purple wire with white stripe. On obd 2 it's a single 3 pin connector. You could very easily connect the wires but I prefer the plug and play. 

In the corrado the headlight harness and engine harness are generally seperate. On mk3 obd 2 it's one harness. So most likely the headlight harness needs to be swapped. It is very straight forward. If you run into any issues take pics of the connectors and post in here.


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## GTI Jay (Feb 11, 2010)

Wow thank you guys for this information. I'm about to tear into this here yet today. I'll post back later tonight or tomorrow depending on how this goes 


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## GTI Jay (Feb 11, 2010)

So just to clarify, obd2 ECU will plug are work with obd1 harness?


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## bonesaw (Aug 8, 2004)

No. You need to use the obd2 engine harness. The engine harness and headlight harness are one piece so it is easier to do both. Just dive in. It's fairly simple. Especially that you have two complete cars.


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## GTI Jay (Feb 11, 2010)

Well, there's 2 engine harnesses. 

1. 40 pin cannon plug on the rear of the head- fuel injector, tps, coilpack, cam pos sensor, VSS, aux water pump, oil press and temp sensors, water temp, iat.
Then
2. ECU, lambda, knock, maf, crank pos, sai, alternator, and that's pretty much all I can see from this point, looks like it ties in to rad fan, reverse light switch, and so on and goes In through the firewall. I'm sure then ties into headlights and all that. 
So far everything plugs right in with only changing the 1st harness. Haven't put power to it yet though. And both mafs obd1 and obd2 are 4 pin and confirmed to both work fine on my running obd1 engine 


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## GTI Jay (Feb 11, 2010)

Has anyone tried not doing the whole harness too the fuse block? But changing the engine/fuel injector/throttle position sensor harness out?Maf plugs right in and works fine. ECU plugs in too


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## bonesaw (Aug 8, 2004)

You will most likely want to just change the harness. The throttle body has the idle valve built in and 2 more additional wires. It also has the sweep flipped compared to obd1. The engine harness to fuse block also has the provisions for rear 02.


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## GTI Jay (Feb 11, 2010)

You are correct about the rear o2, however, I'm not worried about a rear O2 since I'm going turbo and the c2 software does not use front or rear O2 sensor input. I did noticed the external idle valve is eliminated. But that's what I meant, will it run with just swapping the 42pin harness which includes obd2 throttle body, fuel injectors, cam pos sensor, coilpack, and oil press sensors? But not swap the ECU harness that goes all the way to the fuse block and use the obd2 ECU?


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## SpacedOutStar (Aug 23, 2013)

Why make this hard on yourself? You have everything you need and all you need to do is to swap harnesses. To answer your question, you can use the harnesses in your OBD-I car and run the extra wires needed. Take the wiring diagrams for both engines and find all the differences between the two set-ups, change the wiring needed, run the extra wires needed, and then hope you got it right and don't fry something. Or do it the easy way and just change the two harnesses and be done with it. You keep asking about doing it the hard way when the easy way is sitting right there,


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## bonesaw (Aug 8, 2004)

dkoglhk3 said:


> So just to clarify, obd2 ECU will plug are work with obd1 harness?


No


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## GTI Jay (Feb 11, 2010)

This is good information thank you bonesaw. I put the project on hold as I found out I had to move at the time but I'm actually about to start doing this again and be swapping the harnesses here in the next few weeks 


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