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Old 09-10-2003, 02:54 PM
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Heater Questions 83 300sd

Hello, and good day to all. I have just spent 3.5 hours searching the history of acc problems and now feel I can ask a few questions, with a reasonable understanding of what the system is supposed to do. The ac worked great all summer, thax Larry Bible, as fall is soon going to be on us, I decided to check my heating system. I found defrost works great,inside heat dos not, when I switch buttons to cab heat, all I get is cold air from center vents. I checked aux water pump, nothing. The hose from the main water pump has little or no pressure, is really not even warm. My question is, is the aux pump a good place to start, as fuse is good. Does the pump being bad mean the conrtol unit is cooked? I can get no air out of the floor vents,yet the rear vent works, just cold air. As I switch from defrost to cab heat the defrost gets cold also. Any and all suggestions will be greatfully recived thanks steve
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Old 09-10-2003, 05:31 PM
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If the defrost is warm but nothing else is, then that sounds like a pushbutton problem to me. Somebody else with W126 specific experience may know for sure.

Of this I am certain, you won't get warm air from the center vents. They are designed to provide cool air only. Close them off when you select heat, in order to remove the path of least resistance and force the air to go out the floor vents.
Rick Miley
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Old 09-10-2003, 06:39 PM
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thank you rick , as I play with the buttons, i"ve noticed that the ac compressor kicks on no matter which button I push. Also, should the aux water pump always be running? As I have said, it is not running, am wondering if it is burned out? I have sent
away for 3 mercedes manuals at books 4 as it is the only place I could find them. I hope they will help me to understand the acc system. any other help would be great steve
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Old 09-10-2003, 07:14 PM
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I don't know anything about the aux pump, but you definately have a prob if the AC is coming on no matter which button you hit. It should never come on when the furthest-right button, (next to the "Off" button) is pressed. So you may just be having a control problem. But beyond that, I'm out of help for ya. I haven't delved into the CCU controls for these cars that much.
Best of luck. Let us know if/when you figure it out.
past MB rides:
'68 220D
'68 220D(another one)
'67 230
'84 SD
Current rides:
'06 Lexus RX330
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'96 Corvette
'99 Polaris 700 RMK sled
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Old 09-10-2003, 08:14 PM
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Thank you RickG. I will post all findings steve
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