Automatic Climate Control System Response Sluggish or Operates Too Cold - 1977 to 1990

In M-Bs with automatic climate control, system response can become sluggish as the car (and the system) ages.  This can be due to a leaky air tube for the in-car temperature sensor in 107, 123 and early 126 models.  The sensor is located either adjacent to the right side radio speaker or in the center of the dashboard under a small intake grill.  In both cases there is a small (about 1/2" inside diameter) tube connecting the sensor housing to the suction side of the heater blower.  The tube allows air to be drawn from the passenger space into the sensor housing to continually expose the sensor to inside air temperature variations. A portion of the tube consists of foam rubber-like material which, over time, disintegrates.  This allows air from under the dash to be drawn directly into the blower, bypassing the air temperature sensor. Consequently, the sensor does not "see" the true passenger space air temperature, thus sending a false signal to the automatic climate control. This problem usually causes the system to run too cold on warm days because no cool cabin air is reaching the sensor.  

You can check air flow around the sensor by pushing the AUTO HI (1977 to 1980) or Blower HIGH button (1981 and up) while the system is running. Place a tissue over the sensor air intake and see if the paper is drawn onto the grill or extinguish a burning match near the grill and see if the smoke is drawn into the grill. If no air flow is present, the foam hose is probably disintegrated and needs replacement.   The fix is simple - use 1/2" inside diameter foam insulating tubing for water pipes. It is usually sold in 6-foot lengths and costing about one or two dollars at most hardware stores. 

Remove the glove compartment first - it is held in place with 6 or 8 plastic pin assemblies. First remove the inner pin from each assembly, and then withdraw the outer pin. Snap the glove compartment light out of its hole, and then carefully withdraw the box from the dashboard. Locate the sensor tube and check the foam rubber section for disintegration or blockage.  If the foam crumbles easily when handled, it needs to be replaced with a new piece of foam tubing.  Use rubber cement will help keep it in place.  While you're there, check the condition of other rubber hoses on the various climate control components - replace any that are cracked or broken.  (Caution: In 1977 to 1980 M-Bs (Type I ACC), don't be alarmed at the large coil of electrical wire you may find laying under the glove box inside the dashboard - it is supposed to be there. It is a controlled vent for the vacuum system. Don't try to find a place to connect it - one end is plugged into a vacuum line; the other end is open to allow air to enter the vacuum system at a very slow rate.

For later model 201, 124 and 126 cars, the inside air sensor is located next to the overhead courtesy light behind a small grill. A small blower located behind the dash below the right hand dash vent induces air flow into this grill. If no air flows into the grill check this blower. Remove the glove compartment or under dash panel to reach it.