Original poster here.

My pressure and temperature switch have been tested and are working fine. My auxiliary fan is operating when I attach 12v to it, and both relays have been tested (and swapped out with 4 different relays each). The system has a new compressor and has been vacuumed. It has had the proper oil and refrigerant added by a professional a/c tech who works on these vintage a/c systems. The air will blow cold if I am driving highway speeds. When I slow down and sit in traffic in the 90+ heat we have been having, the system and condenser heat up and cannot be cooled because the aux fan is not coming on.

I have determined that there is a problem in my a/c wiring that is not allowing the auxiliary fan to operate as designed. Everything else tests fine.

I have ordered the switch from Rollguy that will bypass some of my a/c wiring and turns the aux fan anytime the compressor comes on.

I'll report back.

That’s all wonderful, but exactly what refrigerant and what are the pressures?

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution

I was wondering the same thing.

Hopefully they went back with r12.

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Jim

"That’s all wonderful, but exactly what refrigerant and what are the pressures?"

Like I said in post #3; The system has been charged with R12. That's wonderful.

I do not know the pressures; I did not do the charging.
I watched it being done and was told the pressures were correct, but I do not remember the numbers. I guess that is not Wonderful.

We are here to help. We WANT to help, but without knowing the pressures, we don’t have the necessary data to draw a proper conclusion. When you go to the doctor he will listen to your heart and take your blood pressure before he can start a diagnosis. That’s where we’re at with this.

Just trying to help.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution

I had the same symptoms as the OP. I realized that my aux fan wasn't working at all. You don't have to perform a complex test. When the A/C is on and warmed up, check to see if the noisy fan is on. I had to wire mine to work at all times when the A/C was switched on as opposed to being switched on by the relay that senses the pressure in the system. Also, the parallel flow condenser made a huge difference!

Greg this entirely depends on your climate and usage. On a summer day here in SK the air is fairly cool, but the powerful sun heats the cab terribly. The aux fans are not required at all when cruising on the highway, and in my case they will not come on unless I sit and idle the car with the AC on for quite a long time.

Running condenser fans constantly can result in too much subcooling, frosting up your evaporator or even slugging liquid refrigerant to the compressor in extreme conditions.

I just set up a tractor this week that has a jumper on the condenser fan, however I charged it with this in mind (charge set according to actual subcooling). With a factory system charged by weight, you are changing the parameters by enabling constant condenser fan.

Note that yes it's probably fine in a hot climate like the original poster's and definitely fine for a test. And it's far better than them not running at all due to failed controls. Just something to keep in mind that those fans are switched for a reason.

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1993 300D 2.5L Turbo

How can I get one of these and where does it go?

Check out this link, dig around for his contact info. He mailed me mine today and I will report back. Hope this helps.

http://www.peachparts.com/shopforum/diesel-discussion/351863-1985-ca-emission-617-owners-you-need.html

My fan relay mod came in today.
It was quite easy to install. My aux fan now comes on any time the ac compressor is engaged and stays on until the ac is turned off. Not sure if there are any long-term complications not having the fan cycle on and off while the ac is in use, but the good news is, my air is now blowing cold at slow speeds.....and I am happy with that!
Thank you all for your help.

Interesting. I may wire something up to do this. I pulled into my garage yesterday and I heard the telltale whine of the auxiliary fan. I’ll have to check the circuit on the Gen 1 W126 to see exactly what turns it on. My AC was not on when the auxiliary fan was running which I found odd. Clearly I need to look into how it functions.

ETA - I kind of recall doing something like this in my w123. I’ll have to go out to the garage to check it. Perhaps I’ll try to replicate it on the w126.

I got the idea from a video by Jarod on YouTube.

https://youtu.be/-ydCsCSxiiA?si=xcfJd7ljwwl1ADq6

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Rich sent me the same kit and I'm happy. My indie said it would cost an arm and a leg to do such but I frequently find other ways around problems.

I replaced my compressor last year after the original one was suspected of being faulty, and the ac has never blown colder (at least in the last 15 years that I owned the car). It's made a big difference.

1991 300d, 256k