CAN Bus wiring diagram, signal A3, Golf 4 1.8T engine swap into a Golf 2

Hi everyone, I'm new here and also relatively new to the world of CAN bus. I've already read quite a bit, and it's definitely a lot to take in.

I have a question that is probably quite simple for experts. But first, let me tell you about my problem.

I installed a 1.8T AUQ engine with 180 horsepower, along with its drivetrain and Haldex all-wheel-drive system from an Audi TT/Golf 4 4motion into a Golf 2.

The engine is running and everything seems to be working fine. Now, I'm planning to activate the all-wheel drive. This is what I wanted to implement using a CanChecked display with Haldex control.
I have CAN bus cables, specifically orange/black (High) and orange/brown (Low), originating from three different sources: the engine control unit, the Haldex unit, and the CANChecked display. I have connected all three sets of cables together and connected them to the OBD2 port. As far as it goes, or perhaps not, values from the control unit, such as engine speed and throttle position sensor readings, are displayed on the CanChecked screen.

Then, a button is configured on the display to control the Haldex system, allowing the user to lock or unlock it. So far, so good with the Haldex system.

"My problem now is that when I turn the ignition on or start the car, the display shows the CAN signal disappearing every few seconds. This causes the Haldex system to blink on and off every few seconds, indicating an error and going into limp mode. Also, the RPM reading disappears for a fraction of a second every few seconds."

"Due to a lack of knowledge, I have only been able to perform a few measurements so far. The resistance of the Haldex control unit is 2.45 kOhms, and the resistance between the CAN bus lines is approximately 65 Ohms, which should be within the normal range."

Then I measured the voltage on the CAN bus lines. With the ignition off, I got around 2.5V, and with the ignition on, I measured approximately 2.2V on the CAN Low line and 2.6V on the CAN High line. I also observed that the voltage drops briefly every few seconds.

What can I do? Am I missing a component, such as a CAN gateway? As far as I know, this is a common issue with the A3 Golf 4 models, specifically with the speedometer. Or have I missed something? I'm a bit lost right now. I've already invested a lot of money in this car, and now I've bought a new display, but unfortunately, it's not working properly because of the glitches.

I hope you don't bombard me with criticism because of my lack of knowledge. Best regards, Patrick.

Hello Patrick,

Are you perhaps using too many termination resistors or have they been activated? 120 ohms would be the correct value.

Are the CAN lines twisted together (which is important for longer lengths) and not routed near sources of interference?

I have no idea what kind of display you have. I would first look at the CAN traffic and see if any bus errors occur or if the same CAN ID is being sent by multiple participants.

Best regards, Rainer.

Okay, the CanChecked display has a 120-ohm resistor for enabling and disabling it, but I'm not sure how to check that in the engine control unit (ECU) or Haldex control unit. I can take another look at the issue of potential sources of interference in the wiring. Actually, the wiring harnesses were mostly original.

I'm not familiar with the details about CAN traffic and CAN IDs, so I need to read up on those topics again. Do I need a sniffing tool for that?