When the refridgerant or coolant gets hot. The numbers are something like 57C for the refridgerant and 105C for the coolant.

If the car is idle in 90F ambient air with the AC on, the fan should start fairly quickly. A few minutes at most.

Could it be possible that the fan doesn't come on because there isnt enough refrigerant in the system and that it doesnt get compressed to hot enough temps to trigger the condenser fan?

Very possible. Feel the receiver to see if it's hot.

I will do that tomorrow... What is the sure way to test operation of the condenser fan - Wire its plug directly to the battery? Where is the temp sensor that makes the fan come on?

The sensor is on the receiver/dryer. One starts the compressor the other starts the fan.

Unplug the lines at the dryer and power it up there. If it doesn't start try jumping right to the fan. If it starts then you possibly have a bad relay.

Danny

Jumper the pigtail leads that go to the red switch on the receiver/dryer.

Edit: dannym beat me by seconds! ;)

fan opperation
 

On my 91 350 SD ....they come on with the A/C.....You do need air flow across the condenser at idle...

Ok, the receiver/dryer, as well as the high-pressure line going into the condenser, get pretty hot. Not so hot that I'd burn my hand by touching, but I wouldn't keep my hand on it for more than about 5 seconds, maybe.

Still, the condenser fan doesn't come on at idle.

I jumpered these leads, and voila - the fan started blowing!

So my question now is this. I am leaving on vacation down south tomorrow, and I don't have time to investigate this or replace the relay... Do I need this condenser fan on only when the car is idling? If so, then I probably don't need to worry because I will be doing mostly highway driving - it will probably get a good measure of fresh air without the fan. But I was also wondering if leaving those leads jumpered (fan always on) during this trip is a good or bad idea... Advice???

The fan on the SD only comes on when the refrigerant reaches a certain pressure. Since jumpering the connector ran the fan, I would guess it is your pressure switch. These are moderate failure items but I have replaced them. If I am remembering correctly there is a schrader valve that will allow you to change it out without losing refrigerant. The only consequence of it not coming on is higher evaporator temps at low speed and idle. I do not recommend wiring it on all of the time.

I'm sure you are trying to help, but the fan is engaged by the TEMPERATURE sensing switch on the top of the receiver/dryer. If you have the original 6mm thread style switch it can be removed from the R/D without loss of R12. If this is an aftermarket type, it may have the 9.5mm threaded switch that is in direct physical contact with the R12. BTW, the original switch that I have is stamped 52C, and it did indeed switch right near that temperature when I tested it.

how the uninformed do it, not a suggested guide by any means

the '82 SD, compressor would run but minimal cooling - down from poor cooling last summer (Houston)
R 12 per the connections so I bought some 2oz oil and 12oz cans; added most of the oil through the high side with difficulty, and then blew up a can of freon in 45°C water (impressive) - then added 2 cans in 45°C water through the low side (duh) w/o gages
- now cools like a champ

the '84 SD went from slight cooling last summer in So CA to zip here in Houston, but the compressor would run
has apparently been converted to 134A but there are no stickers to indicate such and only the low side has a 134A 'adapter'; added 2 cans of 134A with oil using a cheapo low side (only) pressure gage, corrected for temperature
- now cools, but nothing like the R 12 system - thinking I need to change it back but TX seems very anti-R 12

I was told one cannot get an insp sticker with R 12, but this seems to be untrue as I just got one for the '82

I stand corrected as it is a temperture switch that is on top of the receiver/dryer which turns the fan on. As per the manual it is suppose to kick in at 52C. I am not sure that it really matters as it is most likely still the problem. As I stated before, I can't remember if you lose refrigerant if change out. I don't think mine did. The manual also shows a temp switch for Aux fan to come on when coolant temp reaches 100C. In future I will leave this subject to the "experts"....

Hm... Maybe my receiver doesn't reach 52C because the system isn't all the way filled with the refrigerant. I see refrigerant flowing turbulently through the viewglass, but there's lots of bubbles in the viewglass that are obvious during compressor operation. Someone said that when the bubbles go away, the system is full... I am no expert on that... But perhaps my condenser fan isn't kicking on if the system isn't completely filled?

Pete Burton is correct. Odds are you WILL lose refrigerant since most all of our older MB's have had the R/D changed to the newer style by now. The 9.5mm. switch is in contact with refrigerant. Because the P/T curve is mostly linear for R12 and R134a in the range we're discussing, MB switched to pressure switch from temp. The end result is ~ the same.

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1UE0YGZY61UE0YH47S&year=1984&make=MB&model=300-SD-002&category=R&part=A%2FC+Temp.+Switch
Top picture is old style, bottom is new style (9.5mm.)

FWIW, the R/D I have now has the 9.5mm threads. Not wanting to replace a perfectly good temp switch, I drilled and tapped a blind M6 hole in one of the 9.5mm threaded plugs, then screwed my original switch into it. Works fine.