OK, it is just getting too hot here to not have a properly operating air-conditioning system.

I have the original R12 refrigerant and here is my question: In 90*+ heat shouldn't the aux. fan, due to the reciever/dryer pressure switch, be on nearly all the time?

Mine only seems to come on when the coolant temp. reaches the 100*+ mark. Only then do I get cool air. I am trying to see if there may be a problem with the aux. fan relay, or wiring, before tracking down a set of guages.

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Sam

84 300SD 350K+ miles ( Blue Belle )

My W116 has the Aux fan kick on withing a a few minutes after starting the car...but then I understand my W116 and your W126 may behave differently.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

I don't know that I would look at it from that angle...
If it is not giving you cold air then it ought to be ON....
By 100 degree coolant temp... you mean your engine coolant temp ?
90 degree ambiant air temp with AC on and I would think it would be on all the time... unless you are traveling at enough speed to provide forced air without the need for the aux... but if it is not cooling it sure ought to be on...
Clean and straighten any fins you need to on condensor AND radiator first...
Then test again...

Your aux fan is tied to the refrigerant temp and engine coolant temp. So it really depends on the temps it's sensing.

using gauges is the best way to see what's going on with an AC system but there are other ways to get a basic feeling on how it's working.
How often does the compressor come on?
Are there lots of bubbles in the sight glass on the filter/dryer?

You can also check for leaks by looking for oil. When the refrigerant leaks out oil will leak with it. Make sure you check the expansion valve on the firewall next to where the steering wheel goes through.

Danny

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1984 300SD Turbo Diesel 150,000 miles

OBK member #23

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"Your aux fan is tied to the refrigerant temp and engine coolant temp. "

Are you saying for example... if you jump into his SD when it has been sitting in 105 degree Texas sun for hours.... and he cranks his AC down all the way on Temp and all the way up on Fan...it will not come on until the ENGINE coolant reaches some preset temperature ?
Or is that engine coolant a high temp cutoff ?
or Something else ?

At the risk of stepping into an area in which I am admittedly un-trained and not very experienced, I think on the 84 SD, the aux. fan is tied only to the AC temp to the switch on the receiver/drier. See attached.

Attached Thumbnails

 

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1984 300SD 326,997 miles and counting . . . No wait, my odometer is still dead

It's a simple mod to rewire the fan so it always runs with the AC is on. That's what I did with mine and it works great. There are detailed instructions in the archives (complete with drawings contributed by a helpful member from Down Under).

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'81 300TD turbo <<sold 12/05 to another diesel nut

Yes, what I am saying is the aux. fan only comes with 105* engine coolant temp. and never seems to operate as a function of the air conditioning. Sight glass is clear, radiator and condensor coils have been cleaned and the clutch cycles on the compressor. But the only time I get cold air is when the aux. fan cycles on if/when the engine ever reaches 105* engine temp.

I know I need to connect some guages to actually determine the refrigerant operation of the air conditioning but I get the feeling that I may have either an electrical problem or a failed pressure switch on the receiver dryer. From what I see on the schematic, I think I can jumper the press. switch plug and check the relay, fuse and wiring portions of the system. Since I would have to purchase or borrow some R12 guages, I would like to ensure the rest of the system seems to be in operating condition.

My question is, shouldn't the press. switch usually be activated and the aux fan on nearly all of the time when the air conditioning is on?

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Sam

84 300SD 350K+ miles ( Blue Belle )

Perhaps you could tell by my question that I think your system Should not be configured in a manner in which the engine temperature controls the AC coming on..
Given where you live... I think that WannaWagon's suggestion would be fastest and best solution.... until maybe November when the temperature was more conducive to messing with this stuff...

I answered that last question above...
"I would think it would be on all the time... unless you are traveling at enough speed to provide forced air without the need for the aux... but if it is not cooling it sure ought to be on...
"

Pay attention to Rockinwagon, he's got it correct. The switch on the R/D activates the fan, but it's a temperature switch, not a pressure switch. When the liquid R-12 on the high side of the a/c system reaches a set temperature (which seems to be on the wiring diagram provided, but I can't make it out) the aux fan is switched on.

You can test the fan by unplugging both leads to the temp switch and connecting them. The fan should run.

If not, time to go troubleshooting. Check for +12v on one lead at the switch. Check for continuity to ground on the other. Check the aux fan relay - should be a standard Bosch ice-cube relay.

- JimY

There are two switches on the receiver/drier. One is the low pressure switch which controls the compressor and will cut power to the compressor when the refrigerant charge is low. Never jumper the pressure switch (except for short periods for testing purposes only). The other switch is the temp. switch that will send ground to the aux. fan relay if the temp in the receiver/drier reaches 126 degrees F or 52 degrees C, based on the diagram.

My aux. fan is not coming on either, so I will check the temp switch/fan and relay tonight and report. If the fan is OK, I plan to jumper the temp switch and bring 12- to make the fan run anytime the engine is running (at least until I have time to do the job properly).

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1984 300SD 326,997 miles and counting . . . No wait, my odometer is still dead

Thanks for the info guys. I was looking at the wiring to the receiver/dryer and noticed the two switches. I will need to check the color coding of the wiring unless someone knows off the top of their head which is which. Maybe I will get real lucky and find out that is is only corrosion and reseating the connector takes care of it.

Does anyone know which icecube relay it is that controls that?

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Sam

84 300SD 350K+ miles ( Blue Belle )

as you look down on the receiver/drier, the temp switch is on the fender side and the pressure switch is on the radiator side. Good luck. My aux. fan now runs full time but will not know if it makes any difference until tomorrow afternoon when I start home. BTW, the interior of my car was 120 deg. F, when I started home this afternoon. By the time I was home, the vent temp at the center vent was 50 degrees F. The interior was comfortable, but not cold. I think I will do better when I revert to R12.

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1984 300SD 326,997 miles and counting . . . No wait, my odometer is still dead

Yes, it is a temperature switch, but functionally it is measuring pressure. Pressure and temperature have a direct relationship. You need to monitor high side pressure and see at what high side pressure it comes on.

The purpose of the aux fan is NOT to increase air conditioning effectiveness. It is to LOWER high side pressure to keep it from getting too high in those situations where you don't have a lot of ram air (city traffic conditions.) SOooo, the fan is on when you have good high side pressure. The good high side pressure also causes more cooling. To me the aux fan being on is an indicator of cooling (good high side pressure) not the cause of the cooling.

When first starting to read this thread it sounded like the fact that it was only cooling when the aux fan was on was a low charge. Since you have a clear sight glass, it doesn't sound so much like that is the case. That said, however, you are flying blind. You need a thermometer in the vent and you need to monitor your pressures.

You can get a manifold set for R12 pretty cheap. The same set is used by every commercial/residential a/c service man in town because the same gauges are used for R12/R22/R502. You can get a dial thermometer at the auto supply for about $5.

Good luck,

"Yes, it is a temperature switch, but functionally it is measuring pressure. Pressure and temperature have a direct relationship." --Larry Bible

Pressure and temperature have a direct relationship when you are talking about a gas in a steady state.
Here we are not talking about a gas at the point where the switches are located.
If the object were to measure pressure... why do it in an oblique manner... using a temperature guage instead of a pressure guage ?
The fact is that as the vapor goes through the compressor and into the condensor... it turns into a liquid....as a liquid it has the ability to be different temperatures at the same pressure moving it through the system.
Reading the temperature at that point in the system is a good way to determine if enough air is crossing the condensor to keep the system within its designed parameters. IE, cooling the vapors enough to make sure that only liquid leaves the condensor .
High side pressure is caused by the compressor being on. Heat is not needed in the condensor ... the condensor's purpose is to take into the airsteam as much heat as possible.