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  #1  
Old 08-13-2007, 08:25 PM
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CCU or Blower Motor brushes?

Good Evening All;

Within the few days a problem has developed in my '85, Cali-spec 300TD that I'd appreciate the feedback from board members on.

The blower motor no longer spins up when the CCU selector is switched from Off to the Economy (EC) setting. Doesn't matter the setting on the fan speed switch (Auto, low, or high).

Select any other setting on the CCU however, and everything works just as M-B intended.

In fact, once the blower motor is running, select the Economy setting, and it will keep running without incident. The blower motor also responds just as it should, altering speed depending on the amount of airflow needed to achieve a temperature setting.

I can't imagine this is blower motor brushes, since it works perfectly on any setting apart from the Economy one. Is it time for me to open up the CCU and hit the connections with a soldering iron and/or look for bad wire runs?

Thanks in advance,
Dave

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Old 08-13-2007, 08:47 PM
Craig
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Sounds like a classic CCU failure to me. Mine eventually got to point where it only worked if I pushed defrost first. Either try to fix it (and risk losing your core) or buy a rebuilt CCU.
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Old 08-13-2007, 09:17 PM
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Quote:
Originally Posted by D_Ryan View Post

In fact, once the blower motor is running, select the Economy setting, and it will keep running without incident.
The CCU uses different relays to energize the blower depending on the mode selected. Once one of the relays is actuated, the blower will remain powered if the mode is changed. So the problem is most likely the relay or associated connections.
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Old 08-13-2007, 10:41 PM
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Hmmm.....did the 85 have the CCU with the circuit boards arranged in an "H", or did it change in 86? If you have the newer style, a search should turn up a really good test for that unit. If it's the older style, then soldering iron.
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  #5  
Old 08-13-2007, 10:56 PM
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A burned relay contact may have enough resistance that the motor will not start from a dead stop but will continue to run once started using another relay. That may be what you are seeing. Another possibility is that the other modes are starting the fan on a higher speed, which provides more voltage.

Try this experiment: With the ACC off, press the High speed fan button, the top one in the column of three. Then turn on the ACC by pressing EC on the CCU. Does the fan start?

Either way, it's probably time to pull the CCU and check all of the relay contacts. Carefully burnish with a relay tool or light emery cloth, then clean and reassemble. Check for fried traces on the pc board while you are in there. Not much else in that unit.

BTW, have you installed a fuse in the aux water pump circuit yet? About 2 amps should do, will save the CCU should the motor seize. See W123 aux water pump fuse install w/pix
for details.

Jeremy
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Old 08-14-2007, 02:20 PM
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Test recommended by Jeremy5848

All;

Thanks for the prompt and informative responses.

Jeremy5848, just attempted the test you recommended, setting the fan speed to high with the CCU "off" and then selecting the "EC" setting. Result: the blower motor remained off.

Looks like it is time to open the unit and visually inspect the runs and touch up the solder connections. One question, is it readily identifiable which relay corresponds to which function switch, or will I need to trace the wire runs?

regards,
Dave
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Old 08-14-2007, 03:10 PM
Craig
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Originally Posted by D_Ryan View Post
Looks like it is time to open the unit and visually inspect the runs and touch up the solder connections. One question, is it readily identifiable which relay corresponds to which function switch, or will I need to trace the wire runs?
Just one caution, if you chose to re-solder the unit, you will not be able to get core credit for it if you eventually decide to purchase a rebuilt unit, you will have to eat the core charge or find another unit to trade in.
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Old 08-14-2007, 03:22 PM
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Quote:
Originally Posted by D_Ryan View Post
All;


Looks like it is time to open the unit and visually inspect the runs and touch up the solder connections. One question, is it readily identifiable which relay corresponds to which function switch, or will I need to trace the wire runs?
I would clean the relays before breaking out the soldering iron.

If the fan kicks on when Defrost or Bi-level is selected, but not when normal a/c or EC is selected, the problem is the K-3 relay. The K-2 and K-3 relay are esentially wired in parallel. (The K-2 bypasses the cold engine lock-out switch (and powers the blower) in the Defrost and Bi-level modes only.)

Note: My reference is the 1982 300D wiring schematic.


Last edited by tangofox007; 08-14-2007 at 03:33 PM.
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