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1983 500SL
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Discussion Starter #1
1983 500sl how can I test to see if the BCU is bad? I can put a jumper wire on the back of the BCU and get the fan to turn on, but I don't know if that means its bad or if it could be the control unit. Any help or suggestions would be great, I'm sure where to go from here or what to test next.
 

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1983 380SL, 2000 S430, 1991 420SEL (retired) - RHD
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5,507 Posts
Blower control.

1983 500sl how can I test to see if the BCU is bad? I can put a jumper wire on the back of the BCU and get the fan to turn on, but I don't know if that means its bad or if it could be the control unit. Any help or suggestions would be great, I'm sure where to go from here or what to test next.
You must have auto climate control? Try searching in the"google" search box above. T here are threads descibing fixing broken solder joints in the ACC control unit on the dash. If you are getting fan by applying power directly then it's probably not the little box on the blower unit. Many of the posts are about sedans but I believe it's the same unit.It's often poor/broken grounding that's the problem (brown wires) Good luck
 

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1983 500SL
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11 Posts
Discussion Starter #3
I heated the solder on some of the conductors on the control unit and I am now able to get the high fan button and the defrost button to work. But air is only coming out of the defrost vents under the window and side vents. I am going to heat up all the points on the control unit should that allow me to get air coming out of the front vents or is there something else I need to check.
 

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1983 380SL, 2000 S430, 1991 420SEL (retired) - RHD
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5,507 Posts
You're making progress! Once you have determined that the control unit is working ok, if you still have a problem then it begins to get trickier.
The heater box flaps are controlled by vacuum pods.Several things can be wrong.
Vac pods don't work, perhaps perished or maybe the vac line is cracked or disconnected.
The heater box could be cracked and jamming the flaps.I don't have this system so I'm no expert on it.My flaps are the same but lever controlled.
I recall Roncallo did a post on these problems . Search heater box vacuum repairs. I'd link it but don't know how.
BTW, new pods are not expensive but fitting them is kind of tricky- some are inside the heater box.
 

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1983 500SL
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11 Posts
Discussion Starter #6
Thanks for all of your help guys. I pulled the acc control unit and soldered all of the contact points. I plugged the unit back in and everything thing works. However it was cutting out intermittently, I wiggle the 2 plugs on the sides of the unit and everything will comeback on. I put some dieolectric grease into all of the conductors on the plug, now the unit is working perfect.


I went through the and tested the vacuum system that link helped Rowdie thanks. After searching around and tracing vacuum lines I found that there is two lines going to elements 38 but but of them stop there and don't go anywhere else. I looked all around the center vent and inside of the vent , I found a little arm inside of the vent that had come unscrewed from the vent flap. I was able to get it all connected and I now have air coming out of the center vent, I can open and close the vent with the little slide lever under the vent. So after going through everything I discovered that the 83 500sl doesn't have a vacuum element for the center vent at least mine doesn't.
 

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1983 380SL, 2000 S430, 1991 420SEL (retired) - RHD
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5,507 Posts
If you are referring to the face vents, mine operates by a slide lever on the dash.
 
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