Hello! My name is Nick. Thank you for taking the time to view this thread!

I have a 1983 Mercedes 300cd
When I bought it the AC did not work and the compressor was not hooked up.
The blower motor would not work at all.
The fuse box had a 25A blue fuse in the #8 spot feeding the blower motor.
This seemed to burn up one of the fuse housing clips, so i moved the #8 wires over to the fresh #5 location and installed the proper 16A fuse.
Still, I had no response from the AC controls!!
SO I opened up the AC control box to find that some of the copper connections were black and burnt.
At that point I purchased a new control box from a user here for the price of $75.
While it was coming in the mail i soldered some wires onto the burnt up control box.
I unhooked the blower motor and then tested the soldered up setup.
It promptly burnt up one of the soldered wires perhaps because it was simply too thin.

My Question:
Is there any advice you would suggest rather than just hooking up the new control box and trying it.
Such as: testing something in particular for a short?
It burnt up my soldered wire with the blower motor unhooked so I am thinking the blower motor could be ruled out as a source of the problem here.
Any type of checklist you would go through in my situation would be appreciated!





new

If anyone can give me an experienced detailed response that will ensure my new unit doesnt burn up i will paypal you $5!

Seized auxillary water pump is a known cause of frying CCU solder board traces.

Aux. Water Pump takes out CCU

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

As Alec300SD stated, it is your aux water pump that has seized. Your burnt trace goes to pin 10 which is the power out to the pump on a W123.
Replace the pump and add a fuse or as many have done, just leave it unplugged.

I read a posting on this forum a while back about someone who's car actually burnt to the ground from a fault in the aux pump cooking the CCU. Don't take this repair lightly! Delete the Aux pump, or if you intend to keep it, fuse it!

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Thanks for the replies guys that is very helpful!
I will get Alec300SD's paypal since he was first.

I had to install one of those aux pumps about a year ago so it did fail at one point. I will be sure to put the fuse in this time.

Rick76 i like how you know what pin goes to what. Is there a diagram that shows this?

This is the thread of the 300D that burnt to the ground due to a CCU fault...

I lost it all....electrical fire high up in the dash

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

I installed a mini harness with 1 amp glass Buss fuse for the auxilliary heater pump in my 83 W126 300SD.

I used a marine grade crimp-on glass fuse holder and some scavanged keyed plugs.
It's not the prettiest mod, but it seems to be holding up okay.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022