The horizontally mounted switch is bad. It has no contact with charge.

__________________
Make Biodiesel
Not War

'85 300TDT

"So the next move is to replace the pressure switch," BigRichard

I am just going by your description... and if it is the pressure switch then it is connected to the refrigerant... I don't want to assume that all R/Dryers are built and connected the same... and have someone experience an accident...

NO, No, no! You don't understand. That switch will NOT make contact with a normal charge. That switch will make contact WHEN THE HIGH SIDE PRESSURE GETS EXCESSIVE, thus adding airflow across the condensor resulting in the lowering of high side pressure.

In all likelihood the switch is fine, you just haven't gotten into an EXCESSIVE high side pressure situation. The ONLY way to check that switch would be with manifold gauges connected and monitor the high side pressure. This, however, would be risky because if you CAUSE the high side pressure to increase excessively and then the switch is not working you might cause damage.

That warning said, you can hook up your gauges and stabilize a/c for about five minutes on max. To raise high side pressure, you can block air flow to the condensor with cardboard if necessary. I don't remember the exact pressure at which it should make contact, but it seems like I remember 270 something pounds. If it reaches 290 without the fan kicking on then shut it down and consider the switch bad.

I REALLY think that you are trying to fix something that is not broken.

I hope you understand the concept now.

Good luck,

If it's the switch that the fan is connected to, there is no contact with the refridgerant, so you can remove it just fine without worry of evacuating the R12. The one that operates the fan (when you D/C the wires from that switch and short them, the fan comes on - looks like the pic below) is a thermistor that is threaded into a blind hole in the R/D, just senses temp. That's probably the one that's bad. I've changed a lot of these.

All you have to do is order one (NOT from the dealer - if you're paying more that $30, LMK, I get them for around $25), unscrew yours, screw this one in (preferably with some thermal paste), conn to the fan wires and you're in biz.

__________________
1984 300D Turbo - 4-speed manual conversion, mid-level resto

1983 300D - parts car

1979 300TD Auto - Parts car.

1985 300D Auto - Wrecked/Parts.


=========================

"If you don't know where you are going, any road will get you there". Lewis Carrol

You can't say that whatever switch is hooked to the fan is safe to open up...

Any IDIOT who had access to the car in the past , be it owner or mechanic or helpful neighbor, could have changed how it came from the factory...

PEOPLE MUST BE CAREFUL TO IDENTIFY THESE CORRECTLY ... SINCE THE PRESSURE SWITCH IS CONNECTED TO THE REFRIGERANT CAVITY. EYE DAMAGE , SKIN FREEZING... ETC... COULD BE THE RESULT OF NOT BEING CAREFUL ON THIS.

Ive been reading this thread for sometime now and still quite confused. I have the same problem. My '85 300DT condenser fan wont turn on. Replaced the thermo switch, still no go. Bypassed the thermo switch, fan engages. I have cool air inside cabin so pressure switch is OK. RU saying that there is something inside the RD that actuates on pressure so that it touches the thermo switch? Or is the pressure high enough to engage the compressor but not high enough to engage the fan? From what I read, the thermo switch closes at around 56 deg.C and is not controlled by pressure. Just a friendly question. Im thinking about moving that thermo switch to where it reads the coolant temp instead.

I will post this again except without all the other explanations.

It is very possible that if you live in a mild climate, the aux fan might NEVER come on! Even in hot climates it will not be on all the time. It is to reduce EXCESSIVE high side pressure, it is not there to add to the cooling capacity of the a/c.

If it ain't broke, don't fix it.

Have a great day,

Re-read the guys posts.

__________________
1984 300D Turbo - 4-speed manual conversion, mid-level resto

1983 300D - parts car

1979 300TD Auto - Parts car.

1985 300D Auto - Wrecked/Parts.


=========================

"If you don't know where you are going, any road will get you there". Lewis Carrol

"If it's the switch that the fan is connected to, there is no contact with the refridgerant," -- TomJ

I am referring to this blanket statement... which might allow someone to misinterpret which switch can be taken out without evacuating the system...
You did not say the one which operates the fan...just the one connected to it.... and if people have been working on a system which still does not work... the wires could be attached to the wrong switch...

How do you get an excessive pressure on the high side? A blocked RD or EV?Since the condenser fan doesnt work, is it right to say that system is OK? Or could it be that the compressor isnt working right(low compression)? High pressure = high temperature?

I think it's time to throw in the towel on explaining this. You need to get a book on refrigeration and learn the basic principles of refrigeration. There are several things that can happen under normal use that can cause excessive high side pressure.

The most common thing that would lead to excessive high side pressure is a hot condensor. This could be caused by stop and go driving on an extremely hot day, or condensor air flow being blocked by a buildup of bugs.

Heat and pressure are proportional. When the refrigerant temperature goes up, the pressure goes up.

Once you understand the basic priniciples of refrigeration it will become obvious what is going on.

Good luck,

I ran the system prior to starting this thread with guages hooked up. The hi pressure side reached 300 psi without the fan coming on. The switch is bad.

__________________
Make Biodiesel
Not War

'85 300TDT

bigrichard,

Now that you have some precise data to work with I did some looking in my Haynes manual. They show and '84, they don't have an '85, as having a 144 degree F switch.

I went out to the shop and looked at the temp scale on my gauges and 144 degrees corresponded with about 340PSI. That didn't sound right, so I looked in a Motor Manual which is all that I had in the house(too lazy to go back to the shop) and it only went to 134 degrees and indicated that to be 190 PSI.

I think that I must have looked on the R22 scale on my gauges rather than the R12. Neither one of those numbers sound right, but the Motor Manual is probably right.

Since I'm not getting conclusive data here, I would recommend that you try to find your particular models docs and see exactly what switch you have. The 270 PSI or so that I remember, might have been from a converted system in which case the pressure would be different.

It's possible that your switch doesn't come on until a higher pressure than what I remember. Let's try to get more data and be precise before we condemn the switch. I will try to remember to check further tomorrow.

The good news is that it is indeed a temp switch rather than a pressure switch, so you can change it without losing charge.

Good luck,

OK, re-read my post. There's a picture posted of what the fan should be hooked up to and this statement: "(when you D/C the wires from that switch and short them, the fan comes on - looks like the pic below) "

As Larry has stated, it's a temp switch, so the fan still might not come on because the temp sensed at the R/D isn't high enough to switch the thermistor and this is NORMAL operation.

If you must test the switch, just take it out, hook the wires up as normal and use a bic lighter or propane torch on the threads and see if it switches (you could also hook up a meter to it to see when you get conductivity.)

__________________
1984 300D Turbo - 4-speed manual conversion, mid-level resto

1983 300D - parts car

1979 300TD Auto - Parts car.

1985 300D Auto - Wrecked/Parts.


=========================

"If you don't know where you are going, any road will get you there". Lewis Carrol

Sorry you got irritated at my questions. I have the mercedes book on climate control for my car. It doesnt really tell you anything about the fans' operation, only how to test the thermo switch. Anyway, got a can of refrigerant today, proceeded to load it and the fan started running.