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  #16  
Old 11-15-2007, 10:26 PM
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Fried PBU

Thanks for the input, all. Time to bite the bullet and get a rebuilt ccu. Ouch. Does it make sense to check any other parts of the system to make sure I don't fry the new ccu, or does the aux water pump drawing .8 to .9 amps explain the death of the ccu? Also, I must admit that I remember a particularly stupid move I did a while ago while disassembling the center console area to get at the footwell pods. The plugs attaching to the pbu were hard to remove, and I put a screwdriver under the edge of the plug on one side to pry it up. I don't remember any arcing or anything, and see no signs of arcing looking in the area, but if there was power to one of the pins and I shorted it to one of the others I suppose that could kill the ccu. One last thing: the consensus is that the programa rebuilt ccu is the best thing to get?

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  #17  
Old 11-18-2007, 10:35 PM
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Blower regulator?

I have just looked up the ACC on my new CD manual, and found a great 35 pg. discussion of the function of the ACC, as well as some diagnostic tests to run with a multimeter. I'm just diving in now, but I have reason for hope that my PBU is not dead. Here's what I found:
1. The PBU will protect itself from short circuits in its outputs by shutting the ACC system off completely. The PBU manufactured by Kammerer will immediately turn the ACC on again once the short circuit stops. The PBU manufactured by Bosch will wait 30 seconds.
2. The Blower Regulator (BR) will not allow the blower to power on when the engine is cold, the outside air is cold, and the inside temp is lower than the temp selected on the rheostat of the PBU
3. The BR will shut off the blower motor if there is a short circuit or excessive temperature.
Here's what my system has done:
1. When I have gotten into the car on a cold morning and turned it on, the blower motor has at times gone on and off rapidly right after I start the car. This would indicate a faulty blower regulator or temp sensor, because the blower should not be allowed to come on until the car warms up.
2. Other times, the blower/ACC system has not turned on and off rapidly until a few minutes into my ride, long enough for the engine to reach the required temp to allow the BR to turn on the blower. So sometimes the blower regulator waits until the car warms up, functioning properly in that respect.
Here's the theory:
1. The blower regulator is faulty, creating a short-circuit condition. The PBU's response to the short-circuit is to shut the system down. The blower regulator's response to the short-circuit is to cut power to the blower motor, thereby eliminating the short circuit. This then allows the PBU to turn the ACC back on, and the whole thing starts again from the beginning.
2. Why does it only happen sometimes? Maybe the BR has something loose inside that causes an intermittent short? Maybe the wires to the BR are shorting intermittently? I will study the manual and report back...
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  #18  
Old 11-23-2007, 06:05 PM
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Bad blower regulator

I just conducted a test of the voltage from the PBU at the firewall, where the blower regulator is plugged in. I found the black to be a good ground, the red to be 14.5v with the engine running and the acc on, but the yellow had no voltage most of the time and short bursts of a volt or two. I unplugged the blower regulator and tested the connector on the firewall, then ran the acc through different cycles as specified in the cd manual. I got all the right voltages at the different acc settings. So the PBU appears to be OK. Since the blower motor is new, I suspect the blower regulator. I found one at a junkyard and it would have been cheap 'cause this guy sells parts cheap, but some bonehead had used a cutting torch to cut out the engine and tranny, and in the process melted the blower fan and regulator. Oh, well. Another local yard may have it or I'll get it on ebay. I will post the results.
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  #19  
Old 11-23-2007, 06:53 PM
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Could still be the CCU (PBU). If it has the voltage output, but no current, it is bad. Try putting a test light on the output to see if it will light (if you haven't already). A test light is an excellent diagnostic tool.
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  #20  
Old 11-23-2007, 06:57 PM
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ramjesen,

I believe you said the system worked fine on A/C. That would indicate the blower controler and blower are fine. I believe I would look into the blower lockout temperature sensor. The one that keeps the system off if the engine is cold and heat is called for. Also, since unpluging the aux pump seemed to help for a while, I would unplug the monovalve for a few cold days and see if that makes a difference. I did have a blower problem where the fan would start and stop that was a bad solder connection in the CCU. But, it was Heat or A/C operation. I think you may have a bad connection on the lockout temp sensor causing the system to think the engine is warm/cold/warm/cold/etc.
Just my $.00 worth.
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  #21  
Old 12-05-2007, 10:04 PM
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Wasn't the regulator

Got a used regulator from socal mercedes parts on ebay, still have the same problem so that wasn't it. Also have the same on/off symptoms with the blower regulator unplugged, and with the monovalve unplugged. So I think it's the PBU. I sure hope it's not a bad connection to the engine temp sensor, cause I have already laid out some cash for the PBU, it arrives tomorrow, a rebuilt unit for $200. I tried to rule out the temp sensor by testing with an ohmeter and it tested ok against ground but not against the wire on the harness which I believe corresponds to the jack specified by the manual on the magical "socket box". I am somewhat mystified by that, especially since the other sensors check fine against this wire, and the wire connected to the sensor itself tests fine also (see earlier in this thread). I guess another way to test it would have been to run my own wire from the engine temp sensor to the corresponding pin on the pbu. I think I could do this with alligator clips on the sensor's pin and by taking the cover off the pbu harness and removing the single metal socket and hooking an alligator clip to the pbu's correct pin. I will post my results from replacing the pbu. I pray it works even if just to keep me from feeling like a fool for buying a pbu.
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  #22  
Old 12-05-2007, 10:41 PM
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Quote:
Originally Posted by ramjensen View Post
I pray it works even if just to keep me from feeling like a fool for buying a pbu.
Nah.........no issue. A new CCU is a welcomed addition to these vehicles. Makes the whole system function properly without the ongoing questions like "My defroster doesn't work when the third pushbutton is pressed in........what do you think it could be".

I'm done with old CCU's..........a rebuilt with a 3 year warranty is the way to go.
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  #23  
Old 12-06-2007, 12:59 PM
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Location: Memphis, TN
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My 87 300TD does EXACTLY the same thing. I don't think its the temp sensor because even before the car reaches operating temps, pushing the defrost button is supposed to blow air to the windshield on full blower.. and when mine is acting up I don't even get that. Please keep us posted on your findings. I was going to go through all of the fuses next, but I'll wait and see what your results are.
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1987 300TD
1988 300E (sold, sniff)
1999 S320
1981 Volkswagen Rabbit Truck (Diesel)
2001 Ford Expedition
1988 Suzuki Samurai
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  #24  
Old 12-08-2007, 11:21 PM
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New CCU does the trick

Eureka! I now have fully functional ACC! The programa rebuilt unit works flawlessly. My only issue with it is that the pushbuttons themselves have decals on them that don't block the light like the originals. You know how each button has a round black decal with a symbol in the middle that lights up? Well the decals they put on the rebuilt units look real good until you look at them at nite with the lites on, and the lite shows through the black background of the decal. Not a big deal, my car is no showpiece. Can't beat a 3-yr warranty on something electrical like this. It's a good deal overall. Thanks to all who participated in my struggle!
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  #25  
Old 12-09-2007, 10:57 AM
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Yep, no surprise there.

And, I'll bet that you would have preferred to avoid the struggle and just replace the damn thing.

Many of us do exactly the same thing..............it's just not worth the time.

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