W115 heater blower knob - level 3 not functioning correctly

[j-galap]

Hi folks,

The heater blower knob on my '74 240D is a little temperamental. It's not really a big issue for me but now I'm working down the list of 'minor' problems to sort out and this is one of them.

Here's a short clip showing what happens:

Heater blower knob acting strange

Any thoughts? Seems like it's the knob that needs some tinkering. I'm hoping it's not a dash out job. If it is then I'll likely not bother

Thanks!

[jay_bob]

The blower control on these oldies is a simple setup.

There is a 4 position switch, off, 1, 2, 3.
And a two segment resistor.
Position 1 is all the resistor in line with the motor.
Position 2 is half the resistor bypassed.
Position 3 is all the resistor bypassed.

I suspect the contact in position 3 is burnt. The current is highest in position 3 so that is the most likely one to get burnt.

I know on the 123 you can access the switch without pulling the dash. Not sure about a 115.

[gmog220d]

You can remove the ventilation/heater control panel without removing the entire dash, but is a PITA. It's been a while and I don't remember all the details.

- Remove the dash speaker or whatever's in that hole up there so you can remove the rubber fresh air boot from the plastic vent. Once the boot is pulled back disconnect the fresh air valve linkage.
- Remove the knobs from the sliders and fan switch, and two screws in the upper corners behind the fresh air vent, to get the front panel off. That will reveal two 10mm bolts holding the frame with the levers and fan switch on it to the dash. Remove those and pull the assembly back out of the hole.
- If you intend pulling the entire mechanism out of the car you will need to disconnect the cables and electrical connector.

I've probably missed something. I'll check the FSM later and if there's something important there I'll report back.

[resago2000]

some contact cleaner may help

[gmog220d]

I took a look at the procedure for removing the controls and it's pretty much what I laid out above. I think there is probably a nut on the fan speed switch that holds the plastic front cover/vent along with the two screws up in the corners. And I also remember there being a connection for the fiber optic illumination for the panel.

Here's what it looks like:






The switch is riveted to the frame. The contacts are exposed, and might be easy to work with.

[j-galap]

Oh great, thanks guys and thanks for the images/step-by-step, Greg! Looks like a weekend project coming up then. Hopefully it's a simple fix