The Testbook / T4 is a bit akward in that although the main unit powers from Mains Etc, the vehicle communications interface gets power from the OBDII socket. The T4 interface does have an LED to indicate it is getting power, but the Testbook interface does not, which always struck me as about dumb ass backward and cheap skating as you can get. First mod i ever do is to dril a hole in the casse and put an LED and dropper resistor in (total cost less than 10 pence) then if i plug into a vehicle with no power on the OBDII i dont even waste time by trying comms.
On your L322 the pins of interest are 16 for 12 Volts power, 4 and 5 give ground and 8 is the Data line for all the DS2 bus ECU's (Thats everything except the EMS and Gearbox) The EMS and gearbox are on pin 7. BMW also put an IGN feed to pin one but thats not used.
Frankly anyone with absolutely any experience on a Testbook / T4 at all, would first check the power and ground on the OBDII socket. Ideally you need to try it with something that draws a bit of current like a Bulb as sometimes the earth can be iffy and it reads fine with a meter but as soon as you try to draw some current it dissapears.
If Power is fine, then its down to the data bus wire on pin 8. This goes to a junction point that then splits it to several ECU's. It could be you have an open circuit between the OBDII connector and the junction point.
You would pick this up by not having continuity between pin 8 and any ECU's DS2 connection point.
It is possible however that you also have a short to battery voltage or ground somewhere on the DS2 bus wiring or its is unlikely but still possible that an ECU has gone faulty and is jamming the bus preventing any comms over it. For that you need to satrt isolating the ECU's on the BUS.
