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  #1  
Old 05-09-2014, 07:33 PM
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MOD- Condenser Fan & Compressor working together by adding a relay

I have been having overload/fuse burnout problems on my 123 on fuse #8. Since I am running 134a and a Sanden compressor, I have made up a wire harness & relay to bypass the factory fuse for the compressor clutch, and run it and the condenser fan together on it's own separate fused power. The first thing I did was harvest some wiring and relays from a parts car:

I used the relay/wiring with the big red wire, but most any relay and wiring will work. I wanted to use a factory MB relay so I could place it in the same enclosure as the other two:


I made up the harness and wired it up like this:


The big red wire (you can use either of the larger wires if one of them is not red, it does not matter) goes to the terminal in the glow plug relay with a fuse holder wired in:


The large black wire gets wired into the black wire of the condenser fan and the + of the compressor clutch. First photo shows the harvested connector plugged into the factory fan wiring connector, second shows the wiring to the compressor= brown from ground, blue from + fan connection:




The brown wire from the fan and the brown wire to the compressor clutch get connected together and screwed to ground (body):


The existing two wires that went to the compressor clutch gets wired into the smaller of the wires from the harvested wiring and relay:


It is actually quite simple, and seems to work great. No more blown fuse #8, as it is only running the blower fan and a relay coil. I may not have explained it good enough and have no idea how to create a wiring diagram, so please ask any questions....Rich

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  #2  
Old 05-09-2014, 08:30 PM
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JMHO, but it would have been a lot simpler to simply switch back to R12.
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Old 05-09-2014, 11:41 PM
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Hmmm......

This must be something (fuse #8) that is particular to the W123.

On the W126, fuse #8 is for LH low beam headlights. Fuses #10 & #11 are for Climate Control.
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  #4  
Old 05-10-2014, 12:04 AM
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One the W123 fuse #8 is climate control - blower motor, AC compressor, etc. It's the sole 20 (or 25?) amp fuse in the box unless you have optional heated seats I believe.
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  #5  
Old 05-10-2014, 12:34 AM
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Quote:
Originally Posted by SD Blue View Post
Hmmm......

This must be something (fuse #8) that is particular to the W123.

On the W126, fuse #8 is for LH low beam headlights. Fuses #10 & #11 are for Climate Control.
I did the same mod to my 126 as well. I heard about the same overloaded circuit problems, and decided to avoid it. The fan on while the compressor is running also helps cool the cab down sooner.
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Old 05-10-2014, 12:38 AM
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Quote:
Originally Posted by uberwasser View Post
One the W123 fuse #8 is climate control - blower motor, AC compressor, etc. It's the sole 20 (or 25?) amp fuse in the box unless you have optional heated seats I believe.
Practically every 123 I have owned had/has a melted fuse panel around fuse #8. The plastic fuses will also give off an odor that is very noxious, so this mod keeps that from happening.
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Old 05-10-2014, 12:54 AM
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Quote:
Originally Posted by ROLLGUY View Post
I did the same mod to my 126 as well. I heard about the same overloaded circuit problems, and decided to avoid it. The fan on while the compressor is running also helps cool the cab down sooner.
Is this something you recommend when using the Sanden Compressor? Could it be that the clutch on this compressor has a heavier current draw?

I was not aware that the R4 had this issue.
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Old 05-10-2014, 10:46 AM
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Quote:
Originally Posted by SD Blue View Post
Is this something you recommend when using the Sanden Compressor? Could it be that the clutch on this compressor has a heavier current draw?

I was not aware that the R4 had this issue.
I am not sure if the currant draw from the R4 is any different than the Sanden, but this mod definitely helps keep the condenser temps down by giving it a head start in stripping heat. About every couple weeks or so the fuse would blow, and I smelled it getting hot. I even used the ceramic bodied brass element fuses, and still got the smell. I recommend it for the current draw fix as well as to the cooling benefits. I find that the cabin cools down faster than without the condenser fan running from the start. Without this mod, the fan would come on much later, and have a hard time keeping up if the car was not at speed creating air flow across the condenser.
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Old 05-10-2014, 11:03 AM
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The current drawn is small in compressor. It should be similar between R4 and Sanden. It is used to activate the coil only. Info from Internet.

.The A/C clutch coil is acceptable if the current draw is 2.0 to 3.7 amperes at 11.5 -12.5 volts
at the clutch coil.
If the current is more than 4 amps, the coil is shorted and should be replaced.
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Old 05-10-2014, 11:06 AM
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I wonder if the melted box comes from the aux fuel pump locking up? It takes the CCU and the fuse with it? Neither of mine, and none of my friends W123's, have this fuse box issue but also none of us have dealt with a seized up aux pump.

Definitely agree on not using a plastic fuse especially in this location. I stick with the ceramic fuses.
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  #11  
Old 05-11-2014, 01:09 AM
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Vent temp

I got a photo of my thermometer in the center vent today. I was traveling on the freeway with the fan on low:

Most of the time it was hovering around 38-40, but it got so cold in the cab that I had to push the EC button (CCU puts out either hot or cold, nothing in between). This is with the stock condenser and 134a running the condenser fan all the time the compressor is engaged.....Rich
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  #12  
Old 05-11-2014, 08:41 AM
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Quote:
Originally Posted by ROLLGUY View Post
I got a photo of my thermometer in the center vent today.
Have you checked your thermometer calibration lately? A properly operating ETR would preclude a reading that low. Or did you wire around the ETR?
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Old 05-11-2014, 10:03 AM
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Originally Posted by tangofox007 View Post
Have you checked your thermometer calibration lately? A properly operating ETR would preclude a reading that low. Or did you wire around the ETR?
No and no.
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Old 05-11-2014, 11:20 AM
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On my late W126, which has dual speed fans, I have connected the fan to run on low speed when the compressor is running and kick onto high speed when the high pressure switch closes. Stock they would go on low speed when the pressure switch closed.

-J
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  #15  
Old 05-12-2014, 12:29 AM
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Quote:
Originally Posted by compu_85 View Post
On my late W126, which has dual speed fans, I have connected the fan to run on low speed when the compressor is running and kick onto high speed when the high pressure switch closes. Stock they would go on low speed when the pressure switch closed.

-J
Yes, anything to give the condenser a head start in cooling will cool the cab sooner.

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