300TD Air cond puzzle
 

Well I am narrowing down my a/c problem on 1984 300 TD.
COmpressor clutch comes on when relay pins or jumpered.
Aux fan comes on when switch on receiver/drier is jumpered. Not otherwise.
Swapped out ice cube relays and ccu with known good units.
Compressor comes on itermittently, without apparent reason. Aux fan never comes on unless jumpered.
A/c runs ice cold with compressor clutch jumpered.
Any ideas? What’s left?

There is a temperature controller located behind the glove box mounted underneath the dash. Looks kinda like a cruise control amp. See if you can find a good used unit to try in it's location.

Right on! As Sherlock Holmes once put it, when you have eliminated all other possibilities, the one that is left, no matter how unlikely, is the correct solution.
Because I don't have a spare ECR module, I simply bypassed it. Voila! Air blows cold and compressor can be turned on and off from the ccu. All I lack is the auto climate control feature. But compared to sweltering in the heat, this is a small sacrifice.
To get the aux fan I will change out the pressure switch in the receiver-dryer. Since the ice cube relay is working, and the fan runs when the pigtail leads are jumpered, that remains as the only possibility.
I thank you for your quick comeback and solution to my problem.

Before you change out the pressure switch check to see if the pressure is getting high enough to pull the fan switch in. Do you have a/c gauges. I think the pressure has to get on up there (don't remember the number right now, it should be stamped on the switch) to pull it in. You will have to evacuate the system prior to removing that switch. Is the site glass on the dryer clear? When it is you have the correct charge, assuming you are running r12 freon.

Fan switch...
 

Hi there,
I believe that the switch to turn on the fan is actually a temperature sensing switch, at least that's what it is on my '82 300D - it senses whether the temperature of the coolant coming out of the condenser has gotten hot, and if it has, it turns on the fan to cool it. On my '82, the fan usually only comes on in slow traffic or if the vehicle is stopped. In very hot areas, it might be a good idea to switch on the fan all the time, just to make sure there's less load on the compressor.
Regards,
Richard Wooldridge
'82 300D/4.3L V6
Etc...

You are absolutely right Richard. I looked up in the 1983-1985 Electrical Troubleshooting Manual and yes the switch is called a receiver/dryer temperature switch which should close at 144 dF (62 dC). Now that you shook my memory cells awake the last time I pulled one of these out I seem to remember that you CAN replace it without evacuating the system. Thanks for helping jog the memory bank to life:). My brain is quite cluttered at 45 and being the typical engineer I have to look stuff up to confirm on occasion:).

Very useful info, thanks. Yes, the switch I ordered was a temprature switch that screws into the receiver-dryer and switches on the fan. One tech with whom I spoke advised wiring the fan directly into the compressor clutch circuit so it comes on when the a/c comes on. I have the switch jumpered so the fan runs all the time in summer.
He also advised that he simply bypasses the ecr unit on 123 cars to avoided the expense of replacing them. If the temperature gets too cold, I can switch off the a/c.
I have been poring over the schematics of the climate control system ( as opposed to pouring sweat while it was off) and have concluded that the engineer was exacting revenge for the Allied destruction of the M/B works in WWII.
When Dr. Porsche was captured by the French, he designed the Renault 4CV as a POW, which has been known ever since as “Porsche‘s Revenge.”
Thanks again for the info. Saved me much time and hair-tearing.