Using a UPS as a inverter

Hello,
I got a 3KVA 48vdc-230 vac 60Hz UPS card, made by PowerWare and in base of this card, I could rebuilt the entire UPS, it works perfectly but I need it as a stand alone inverter.
It has a 25 pin connector interface and found that rising pin #18 to +5v enables the UPS function, but if there is not previously 230 vac present in the input power connector, it will not start, because it is tracking the phase of the source to synchronize its internal oscillator.
The only way that I have found to start it, is using a little 12 to 110v
inverter to feed the input thru a 110-220 transformer, then stopping the inverter and the UPS starts maravillosamente, sorry,marvelously well.
Please, I will appreciate very much any kind of help in this problem, thanks in advance.

If You can make some reverse engineering on PCB, can discover the opamp input on low AC voltage side for mains present detection. Build a small 50(60)Hz sinus oscillator, and inject this signal to detector input with pushbutton.