Mercedes S320 Climate problem

1995 Mercedes S320 Auto
I have just replaced the climate control unit due to the original's buttons broken and failing, the air con is working ok.
I found one second hand 150 (new Merc price 470) had it sent to me in the
post, it works perfectly apart from the air con cooling fans run all of the time when the engine or ignition is on, even with the air con off, no fault codes were registered when It was scanned.
When I put the old unit back in it was back to normal, so it is the unit problem, is it faulty or as it has been mentioned to me does it need programming to new car, as it is set up for the car it once belonged to.
Any help appriciated
John
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Definitely the unit is at fault. That is one of the problem with MB unit... fan on high all the time...
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Thanks for the reply I will send the unit back and hopefully get another, they seem to be scarce.
John

unit...
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