What precisely does the TCU Isolation switch do?

I believe it opens the wastegate when it’s over boosted.

Here is the information I found in the Rotax installation manual.
"The isolating switch serves to break the circuit of the servo motor for a short time in case
of surging of the TCU."
I have attached a wiring diagram (found in the same manual) "914 Servo Sw". See item 32 circled in yellow, this is the servo disconnect switch.
As you can see, it disconnects the servo motor (item 8) from the TCU (connector 1, pins 2 and 14). Once the switch is opened I believe it will
lock the servo motor in its present position. But I have not tested this. I will try to test this hypothesis the next time I am at the airport.

Allen found the right data.  I know it works exactly like it says. 

It just turns off the power to the TCU.  in doing so it will lock the turbo in whatever position is is in when you turned it off.  This is to prevent uncontrollable power fluctuations in flight or on landing due to sticking wastegate or any other inputs that may cause unstable RPM due to boost problems.. 

Cheers

on the Stemme motor glider the light was activated by a timer, when the throttle was advanced to 100%  the light would illuminate (come on ) after 5 min or so, and thats all.  It was only to be 115 hp for a short time. Basically and idiot light so maybe to remind you to bring the throttle back a tad. No map or even RPM info was used to turn on the lite.

hi Gary and all

the TCU sees the RPM and Boost.  When you take the engine to power (anything over 108% throttle position) it then starts an internal timer.  If you stay WOT for 5 mins the timer will automatically reduce the boost so you do not detonate the engine. 

Throttle position works like this on the 914.  Take off is WOT and this is 115% throttle.  Max continuous is 100% throttle position.  The limitation of the power (boost) at max is for 5800 at 40 inch HG.  The max continuous is 5500 at 33 inch boost.  These numbers work for constant speed propellers.  If you use a fixed pitch you must compromise and only use 5500.  

The manual switch is in case the turbo becomes uncontrollable by "hunting".  This is the overshoot and undershoot of a turbo so it swings the power.  The switch turned to the off position will lock the servo motor in its position, whatever that is when you turned it off.  At the very least you then have stable power to carry out a landing. 

If the lamps are set up according to the Rotax installation manual you will have an orange or red lamp.  Orange gives you the information on the sensors input.  Flashing orange says you have a communication error or failed sensor.  The red lamp when flashing indicates an overspeed of the engine.  If the red lamp is on steady then that is the most serious as this indicates you have exceeded boost. 

Cheers