GRT Avionics

Hello,
I have a GRT modified EIS4000 to work with 24 V system. Unfortunately the display reads only 20-21 V and with teh engine running max. 24 V.

So this is at least 3-4 V to low. My GARMIN 430 reads proper 23 V and with engine running 28 V.

Any sugggestion what can be the problem ?

Thanks,

Peter, Lancair 360 MKII, 99%