I have recieved many emails asking how i wired up the 91tt ecu/harness to a 94 NA car, as well as seeing posts on a regular basis on the same subject. Heres how i did it.
as Posted on US3S sept 04. I have to find some time to get the pics back online.........Keep in mind this was a 91tt harness/ecu in a 94 RT base
The following items need to be re-wired..........
1. Reverse switch wires
2. Front O2 sensor
3. Radiator fan control circuit
4. OBD diagnostic port
1.
Reverse switch
On all 3s's the reverse switch is shared with the control harness, on the 91-93 cars, these wires are tied into connector C-29, pins 10 & 11 (black/white & red/blue wires) inside the car next to the blower motor. On the 94+ cars, these wires are tied into connector C-30, pins 2 & 3 (same wire colors) also next to the blower motor assembly. You need to splice these 4 wires (2 pairs) together. The wire colors are the same so it should be pretty straight forward.
2.
Front o2 sensor
The front o2 sensor on 1g tt cars is wired to the control harness, through the body harness. It uses pins 1,2,7,8,10 of connector A-67 located at the underhood relay box near the air filter. The NA cars have the sensor connector already as part of the body harness, it's tucked away and taped to the harness that goes to the alternator and A/C compressor. Unfortunatly these wires travel PAST connector A-67 and continue into the fender near the abs pump. Since there was not enough slack to cut these wires and insert them into A-67, i chose to run a new custom built harness from the sensor connector to connector A-67. I got a 5 pin wether pack connector with contacts from NAPA and made a new connection near A-67. I cut the wires from the 5 pins (1,2,7,8,10) on the control harness side and used those 5 wires for my new NAPA harness so now there are 2 connectors near the relay box (i call the new connedtor "A-67b") I used 2 cables, first one was 18ga/2 cond. for the heater/ground, and 22ga/2 conductor with shield for the signal wire (i grounded the shield, and used the 2 main conductors for the signal) Heres a picture of this new connection.........
3.
Radiator fans
On the 1g cars, the fans are controlled by 2 switches mounted in the radiator, on 2g cars they are controlled through the ecu. This is how i wired them without modification to the fan harnesses, and keeping the 2g fans with a 1g tt radiator.......Take the black wire from each switch, tie them together, and attach a grounding lug to them. Next take the other wire from each switch and run them into the passenger compartment to connector C-29 near the blower motor. tie these wires into pins 3 & 11 of C-29 (pin 3 high, pin 11 low). Heres a couple pictures of how i did it. I chose to use another weather pack connector from NAPA so the connection can be unplugged to remove the radiator without cutting any wires.
4.
OBD port
This is pretty simple. Insert paper clips into the following positions (or wire up a 1g port)
- Signal/data + = Pin 7 of OBDII connector
- Signal/data - = Pin 1 of OBDII connector
- Ground = pin 4 of OBDII connector
That should pretty much cover it.