Last week my '80 300SD's climate control seemed to have died - just cool
air, no fan no heat. I've owned the car since new and have learned a bit
about this climate control system, so I tried the DEF (override) mode -
a brief spurt of fan and a bit of heat and it died for good.
Fuse #2 and fuse #9 were blown. I also found some coolant seepage on the
auxiliary water pump. I unplugged the electric pump's power line and
replaced both fuses. The climate control resumed its regular operation -
heat, fan etc. - all works fine.
Today I used a jumper lead to test the auxiliary pump - only a very
brief hum then silence. Apparently coolant seeped into its motor and
The heater works well without the auxiliary water pump so if you
encounter this problem simply unplug the pump's power lead and leave the
pump in place or remove it from the car and substitute a piece of pipe.
This climate control is plenty complicated so a bit of simplification
was the solution.
Another example of: "Why make it simple if we can make it complicated."