Passat TDI: ECU Pin for Speedometer Signal

Hello,
I retrofitted a GRA (automatic headlight range adjustment) to my Passat TDI 35I from 01/95. According to VAG Com, the part should be working (all buttons and switches are recognized).
Unfortunately, it's not working.
According to VAG COM, the speed signal is always 0 km/h. The speedometer is still mechanical.
Regarding the questions:
1: How does the ECU actually receive the speed signal in mechanical speedometers?
2: Which pin on the ECU connector provides the speed signal, and what is the nature of that signal (signal shape, frequency of a square wave proportional to speed, or amplitude proportional to speed, and at what level)?

One idea I have is that if it's not possible to obtain the tachometer signal, we could use the speed signal and process it accordingly. If the clutch isn't slipping, this should work.
Thank you very much for your efforts.
Best regards, Harri.

1. The speed signal is pin 43 of the MSG. It is a frequency-proportional signal representing the speed.

"With mechanical speedometers, there's likely an intermediate piece on the transmission where the V-signal is tapped from the speedometer cable connection. If you only see the speedometer cable connection there and no cable, then that's the cause of your problem."

By the way: Do you have a "implausible speed signal" error message?

Regards,
Guste.

Is it possible that the speedometer signal is also coming through the ISO connector of the radio, and is an aftermarket radio installed?

If this is the case, the tachometer signal can be interfered with by the radio, and as a result, the engine control unit (ECU) no longer receives the V signal.

(This is just a guess - but these kinds of problems are more common than you might think.)

Just a moment...

'If the speedometer signal were present at the MSG (presumably a control module), it would display a value. Since there's only a mechanical speedometer installed, perhaps the instrument cluster panel generates a digital signal that is only sent to the speedometer, but the wiring to the MSG is simply missing?'

Or have I made a mistake in my thinking?

The idea about the accessory radio also came to mind.

I believe the Passat 35i has the Hall sensor located in the speedometer. Therefore, it doesn't have an intermediate piece between the shaft and the speedometer.

Best regards, WarLord.

Hello, thank you for the many tips. I took a closer look, and there's actually a small socket at the back of the speedometer, but there's no plug in it. It could be the Hall sensor. Does anyone have the pinout for that?
Best regards and thank you, Harri.

No, this Tacoho doesn't output a speed signal.

I tested by connecting a rectangular wave generator to pin 43 (speedometer input) of the control unit, and VAG-COM actually displays a speed value.
I became a bit bolder and connected the speed signal to the speedometer using a 270 Ohm resistor (not that bold, actually). Now, the speedometer shows a speed of 28 km/h when the engine is idling.
The GRA system is finally working. However, the engine now vibrates at certain RPMs (it doesn't run smoothly, but jerks), and I think it sometimes vibrates even without the GRA system engaged. There are no error codes.

Is it possible to change the controller parameters somewhere? What type of controller is it (P, PI, or PID?), and what are its settings?

Could someone please tell me what the target values for the VAG Com parameters should be under different operating conditions?

Best regards, Harri.

How many kilometers are on the car?

Best regards, WarLord.

The car has 210,000 kilometers on the odometer. I can't say for sure if that's the actual mileage. I bought the car a year ago for 3500 euros. The engine was running really well back then, very smoothly and quietly, and it started immediately. Now it takes about 2 seconds to start. That's probably not normal. In terms of performance, it's okay, I would say.
'The compression seems to be not good (Hoppel test: the engine rotates in 2nd gear every 8 seconds at approximately 10% incline). If someone could provide me with the target values that the VAG_COM tester displays, I could check that. This weekend, I'll remove the injectors and examine the spray pattern.'
Best regards, Harri.