Here's my fix for the Astra mk4 temp gauge/engine conversion problem.
Cheap £15 LCD temp gauge and a couple hr's modding the standard dials end up with this
Personally I wouldn't bother emulating the signal for the mk4 astra dials.
Normaly the ecu feeds the fan control module with the temp signal, the fan control module then feeds the temp gauge, this makes the rad fans work I dependant of the temp gauge.
Now if you make some board to use With some form of temp sensor to feed the Astra temp gauge you still end up with te big problem of no fans. Seeing as the mk4 Astros don't he a fan switch in the rad it makes things very awkward.
What needs to be done is you need something to emulate the correct temp aigbal that the ecu would give out, which would go to the fan control module, the module will then work as it should so the fans would work as they should and then the fan/cooling control module sends its own signal to the Astra temp gauge.
Hope this makes sense as it would be a millions times better to do it that way, also saving people much time and money tryin to sort something out to then get their fans working properly...