Date registered: Feb 2005
Vehicle: Two '87 570SECs, one '87 560SEL
Location: Las Vegas, NV
Mentioned: 6 Post(s)
Quoted: 256 Post(s)
Not closing the defrost vents is a function of a vacuum pod. These fail. I'd bet that this pod is dying or there are significant vacuum leaks elsewhere that won't allow enough vacuum to that pod. Pull off the right side of the console to get to the vacuum manifold, pull a vacuum on the line going to that pod and see if 1. it holds a vacuum (isn't leaking) and 2. the defrost vent moves properly.
Pull the center vent air direction control thingys (the vanes you move to blow the air right/left) and you can see this pod about a foot inside the dash.
I changed out that pod's rubber diaphragm without disassembling the dashboard to get the pod in/out, but it was a PITA.
US '87 560SEC with Euro headlights, clear markers, H&Rs, 17x8.5/Nitto's, ATE slotted rotors with ceramic pads, BergWerks FGS, 3.07 Limited Slip. '87 560SEL with Euro Headlights, lowered, 235/40/17's