Climate Control Unit Fault or Blower Control Module Fault?
 

I have an '87 560SL with climate control issues. The blower does not work at all. When driving I can feel hot and cold from the vents corresponding to temp setting but I have no blower fan at all. I have eliminated the blower unit as it works fine with 12V directly applied to the terminals on the firewall socket. I have the Mercedes workshop manual CD for the car and yesterday I ran through the climate control check procedure and have concluded it is either the push button unit in the dash or the blower control unit behind the glovebox. The procedure says it could be either unit, which is not much use to me. Does anybody know of any checks I can carry out to determine which unit is at fault?

Best guess is push button but only a guess.

JR

Thanks. Doing a bit of research it would seem the push button units are unreliable on these cars so I would tend to agree. Is it unusual for the blower control unit to fail? Also, do I have to source a replacement push button from a 107 or will units from other models also work? The same looking unit is fitted on quite a few other models around that year.

It's going to be difficult to determine the fault without known good substitution replacements or a voltmeter and the schematic, which on the CD service manual.

As bizzare as this sounds, jiggle the key in the ignition (but read below about temperatures first). I have found a bad ignition switch to cause no power to get to the CCU (and therefore, blower control).

The blower control unit is basically a box of power transistors that switch various resistances in the "plug" end of the blower motor on and off and is not likely to entirely and completely fail.

Caution: Many of the climate control units LOOK the same but are quite different electrically. For example, the 85 CCU and 86 CCU look alike at first glance, but are electrically different and not interchangeable because the older design would tend to pop transistors if the auxilliary water pump failed. I think it provides +12 to the pump, the newer one switches ground.

There are a lot of interconnected parts here. I don't know where you live but be aware that the fan won't come on at all until a sensor on the engine reaches a minimum temperature. If this wire is missing or the sensor is bad, the fan won't come on, period (and the engine won't run well either).