Unknown red/blue Button
 

Whats it do? I never noticed two buttons , in the center of the dash air vents. One red, the other blue. I intutivetly understand that red is heat and blue is cold but what is the function? Are the on when depressed or off ?
This is a 1990 500SL. The same one I posted about where the stealership wanted $4600.00 to repair the AC and I posted that the ac started to work great all by it self when I declined the repair. BTW, ac still working great, even better than I thought it could.

The buttons regulate hot or cold air to the center vents.

The buttons are toggle switches, so you have to look at their little LEDs to see their status.

All air that enters the passenger compartment through the climate control system passes through the evaporator. Air that exits the system through the side, foot, and defrost vents passes through the heater as well. But air from the center vents can pass through the heater or bypass it.

When the automatic controls are cooling, the center vent air will normally bypass the heater; and when heating all air will pass though it. But when transistioning between heating and cooling the automatic controls will blend the air for a brief period, as some air passes through the heater and some does not.

The buttons are a manual override of the automatic controls. The red button forces all center vent air to pass through the heater; and similarly, the blue button forces it to bypass.

These buttons seem to have limited utility. When the current SL was introduced in '01 its climate control included them, but after several years they were omitted.

Brian, if you have an owners manual pg 22 will help you but if you don't please feel free to go to http://www.benzworld.org/forums/2391320-post1.html for a free PDF download.