91 928 dash msg, "Differential control inop"
 

I hope someone may have seen this problem before.....
after starting my display shows "Differential Control off" and then Brake service required.
the dealer says both are fine and operational....
I have noticed that when I run the AC I get low voltage and I am wondering if this is result of low voltage to the control module.
I only have 23,000 miles and the car runs great otherwise.
Thanks for any help
Daryl

when was the last time you flushed out the PSD unit?? what is the charge rate at the hot post with the engine running?? use a volt meter to find out