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  #1  
Old 12-26-2009, 01:48 AM
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Blower blues

In the past, when the AC blower wouldn't come on, I used to be able to jump start it by pressing the defrost button. Now that no longer works. The following have been tested and shown to be in working order:

Blower fuse
Blower
Blower regulator
I'm assuming CCU is working because I can hear the compressor click when pressing the buttons

Any ideas?

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  #2  
Old 12-26-2009, 10:52 AM
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Quote:
Originally Posted by WDBCB20 View Post
In the past, when the AC blower wouldn't come on, I used to be able to jump start it by pressing the defrost button. Now that no longer works. The following have been tested and shown to be in working order:

Blower fuse
Blower
Blower regulator
I'm assuming CCU is working because I can hear the compressor click when pressing the buttons

Any ideas?
The engine has a temperature switch that prevents blower operation until the coolant temperature gets to approx. 50C. The first thing to do is to ground the wire to this sensor to see if you get the blower.

If this does not work, and I suspect that it will not work, get a rebuilt CCU.
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  #3  
Old 12-26-2009, 11:35 AM
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I'd also suspect the brushes. The increased voltage in the defrost position may be overcoming the poor connection. There are threads on here describing how to replace the brushes.
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  #4  
Old 08-23-2010, 12:42 PM
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blower blues redux

I ended up not looking into the CCU (as recommended by B Carlton) at the time I started this thread because the blower came back on and this was the CCU that I had resoldered shortly after purchase of the car 4 years ago curing all the problems (blower was then not an issue) I had with it back then. While it worked, I unplugged the temp switch sensor (I assume its the plug located on top of the thermostat housing where radiator hose attaches) which did not stop blower function.

The history since my original post is that on one occasion I was able to revive it by replacing the melted fuse (#11). From then on it has worked for months, stops working for weeks, comes back on for 20 minutes, quits again. No correlation to going over bumps or anything else.

Before ripping the CCU out again, does this sound like CCU symptoms? If so, what part of the CCU governs blower function?
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  #5  
Old 08-23-2010, 12:50 PM
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I would recommend checking the blower motor. The symptoms in your first post sound very much like the problem I had with my '85 300D some years ago. The brushes and the commutator were badly worn; I ended up replacing the motor. I don't know how similar is the 300SD (or not) but the blower is the first item I would check.
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  #6  
Old 08-23-2010, 01:45 PM
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I understand short brushes will melt fuses as it will draw more amperage.
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  #7  
Old 08-23-2010, 02:24 PM
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As stated in my original 12-26-2009 post, the '83's blower had been tested good -swapped it with the known good from the '85. Result: '83's blower worked fine in the '85. '85's known good blower did not work in the '83.

Now you ask: why not ""simply""swap CCU's? Because that whole radio/ashtray/surrounding buttons CCU panel is an infernal rats nest of bad design, flimsy manufacture, brittle snap/push/bend/twist in removal and reinstall which never goes back in as it should and won't improve with repeated removal. I had done both cars, cured all AC issues ended up with both radio face plated not engaging solidly. So I don't want to go back in there unless all else exhausted. And then only pull the suspect '83 CCU and fix it, for which it would be nice to know which area governs blower function.
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  #8  
Old 08-23-2010, 05:31 PM
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Define "tested good." The 126 blower is mounted vertically which loads the bearings in a way not conducive to an infinite life. The blower can test positively while on its side or even vertically in a static environment. Add the vertical jarring of driving the car and it's possible the brushes shift enough because of bearing play to lose contact.

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  #9  
Old 08-23-2010, 09:29 PM
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Testing blower as mentioned in my penultimate post involved exchanging blowers (fully installed in normal operating position) between my '83 SD (the car we are troubleshooting in which the blower was not working at the time of test) and the '85 SD whose AC is fine.

Result: '83's blower worked fine in the '85. '85's blower did not work in the '83. Conclusion: blower is not the problem.
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  #10  
Old 08-23-2010, 10:29 PM
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Oops, I missed that. Carry on

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  #11  
Old 09-22-2010, 09:22 AM
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blower issue resolved

Brian Carlton's diagnosis got it right: The CCU was bad.

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