Here is a short DIY that you can follow to update the fan that the climate control unit (CCU) uses in your '87 300D Turbo (W124). This DIY may also apply to other models that use this same fan.
In the W123 models, the CCU uses the intake side of the blower to pull a little air past the cabin temp sensor, to increase its sensitivity to changes. The 123 models had problems: the temp sensor (located in the middle of the dash) was too sensitive to radiant heat, the airflow technique worked only when the blower was on, and sensitivity changed as a function of blower speed.
For the "new" 124 models, Mercedes installed a dedicated fan that was turned on in position 2 of the ignition switch whether the climate control was being used or not. It pulled air at a constant speed past the sensor so the temperature measurement was more consistent.
The fan in my car was noisy. It could be clearly heard, buzzing away, during that moment of silence (in honor of Rudolph Diesel). I discovered that, not only were the fan's bronze bushings dry and worn, but the fan itself was mounted to the inside of the passenger-side lower dash panel. This acted as a resonator (sounding board), amplifying the noise.
While exploring a 1989 300E at Pick and Pull, I discovered that this later model had an update: the fan (same kind) was now held in a block of rubber and attached to the cowl crossmember, a piece of tubing that is designed to stiffen the chassis against crushing from the side.
The 1987 fan is part number 000 830 61 08. EPC shows that this was replaced twice, first by 000 830 96 08 and then by 000 830 19 08. The latter part is the one that is held in a rubber block. This update suggests that Mercedes might have updated the part due to customer complaints. [Incidentally, should you be one of those rare individuals who has a 124 model without a sunroof, you will not have this fan in your car.]
I followed the FSM instructions for removal of the lower dash panel on the passenger side (these have also been posted as part of a dash removal DIY by babymog HERE
. I then removed and set aside the "old" fan. Next, I attached the "new" fan with its metal bracket to the cowl crossmember with duct tape. I held the lower dash panel up and made sure it would clear the fan. Then I removed the fan from the bracket and marked the hole for drilling.
I drilled a small hole in the crossmember, just big enough for a self-tapping machine screw. Once the bracket was in place, I re-installed the fan and plugged in the air hose and the electrical cord.
If you don't want to bother getting a new fan, you could probably attach the existing fan to the crossmember with tape or cable ties. The important thing is to keep it from touching the dash panels.
The new fan is so quiet it is almost inaudible.
Discuss this DIY here.