I have 1991 350 D.When I turn heat on air come thru center vent,when ac is on no air thru center vents .What is possibly problem.Thank you.

the car should function that way.
if you want a/c air out the center vent, I think you have to modify the vacuum pods for it to work this way.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

It was different till month ago.Air was coming thru center vent when ac is on.I have two other MB cars and cold air is only coming thru center vents.