Tachometer Signal 1Z Engine: Questions & Troubleshooting

Hello,
I installed a 1Z engine in my 1991 VW T3 van and want to connect a gasoline-powered tachometer. There has to be a way to do it; it's been done before.
The gasoline engine tachometer from the bus receives its signal directly from the ignition coil KL.1.
Now, the TDI control unit has the tachometer output connected to pin T68/2, which is supposed to be circuit class 1.
Nothing happens when connected directly.
Does anyone know what kind of signal the TDI-STG outputs and how I might be able to convert it? The STG is from a Golf manufactured in 1993.
I hope you can help me.
Regards,
Carsten.