Update regarding this issue... borrowed a used PBCU from a SL 500 of the same model year; part # installed was the same as the one removed. With this unit installed the center vents worked properly. Reinstalled the original PBCU and the center vents again no longer opened. Connected the M-B HHT, went to activation mode and the vents again worked properly. Logical conclusion is that the fault lies with the PBCU.

Therefore ordered and installed a remanufactured unit from Beckmann Technologies. Installed that unit, ensured it was properly coded and... the center vents still don't work. Fault code check reveals no fault codes for this Beckmann PBCU.

Any suggestions as to how to resolve this problem? Checked with my local dealer and they are not interested in working on this vehicle, due to its age. Apparently they no longer have the original HHT in their tool inventory and they don't feel comfortable with the emulation mode present in the current SDS.

I could try a new PBCU from M-B, but that costs over $1,000 and there's no guarantee it will work.

Any suggestions???