1985 W123 Defrost not Working
 

1985 300D Turbo
The defrost was taking longer and longer to come on, then it just stopped coming on at all.
There's been a vacuum issue, which I have yet to address, that prevents the different ducts from opening. The defrost, however, has always worked until recently.
I've made sure there was enough antifreeze and I've checked my fuses.
What's next to check?

Thanks in advance.
Jay

To clarify, are you not getting any air at all out of the vents in the dash for the defrost? Or is the air that is coming out not getting hot? Since you said you checked the antifreeze I kind of assume you mean it's not getting hot, but this isn't totally clear and the answer could be quite different depending.

No heat at all. Fan isn't kicking on and I don't feel any heat even creeping out of the vents.
Usually heat comes out of all vents except the foot vents.

OK, so the climate control blower isn't kicking on. There can be a lot of reasons on these automatic setups.

It could be a failed blower motor (due to worn brushes, usually).

Or a further failure of your push button climate control panel.

Or a failed temperature regulator.

Or a failed blower relay and/or resistor.

One easy way to begin is to verify whether your blower motor is good. To do this, simply take off the kick panel on the passenger side, unplug the wires, and remove the motor by taking out the 3 or 4 screws holding it in place. You can then run a 12v lead to the motor from your battery and see if it spins. If it's spins easily you can be fairly sure the problem rests elsewhere.

If your fan is good you'll need to diagnose the problem with your system. One fairly easy way to do this is to get your hands on a full spare set of parts (climate push button panel, temp regulator, blower relay, resistor) from a parts car. Then you can do some parts swapping to see if and how the system performance changes. I got lucky and this worked for me - I swapped in a spare temp regulator when my blower didn't work and viola, blower came back and works perfectly through all the speeds again.

If you don't have spare parts on hand, or want to do the more thorough job of testing the various components with a multimeter, see this guide.

http://www.benzworld.org/forums/attachments/w124-e-ce-d-td-class/512915d1367573783-middle-aged-mercedes-electrical-repairs-middle-aged-mercedes-electrical-repairs.pdf

The section on diagnosing the W123 climate control starts about halfway down on page 6.

Nice PDF! I'll be studying that one...

While we're on the subject, here is a higher resolution wiring diagram scan that I found a while back (the ones in the online FSM are basically useless).

http://i.imgur.com/9NYXnzk.jpg

Right click/save as and then you can use your pic viewer to zoom in.

This should help visualize what is described in that PDF.

Got it working- sure enough it was the fan motor.
Although..... I banged on it and it started working...so....
I pulled it out and put a spot of lubricant on it and re-installed. It worked fine until I let it sit off for 15 minutes. Then it went back to not switching on until I banged on it again. My friend says the bearing is probably going bad?

Intermittent operation like that is actually more likely a sign of worn brushes. Do a search for replacing blower motor brushes. Quite of bit of info out there on it.