Does the refrigerant eyesight look different at high or lower pressures?

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1992 400E current
1988 300E sold
1973 280C totalled
1973 280SEL never got it running
1971 250C my first love but rusted out

Low refrig would be indicated by foam/bubbles.full charge would be clear ..
That is if the unit was not converted to R134...many were. R134 will show some bubbles even when fully charged,,that's why you would need gauges and sw pressure cut-in spec to make that sw diagnosis if R134.

Usually you can get the high pressure sw to trigger if you try the a/c at a high idle on a hot day. Can even block the radiator a little to getthngs up there. Just use your head.

On you coment of a manual toggle in the cabin..this is an excellent idea used by many on the older Benzs..you put a toggle in the cabin wired across the a/c pressure sw. that it gives you control of low fan anytime you see the temps creeping up..this is excellent in traffic or towing, etc...the niceity of it is that it does not interfer with the stock system, but lets you over-ride the sensors to get a jump on it...best of both worlds..I have used one on all my Benzs over many years and they are just a nice saftey feature to have when that one time comes into being..like stuck in traffic at the Beach, ya know??????
The newer units have fan default if a sensor goes, so they do not need it, but the older units don't , so you just add this and you are in control..

Refrigerant is a colorless gas or liquid depending on temp or pressure. The sight glass will appear clear when the system is empty or when it full. When it is part full the liquid is foamed by the turbulence of flow with gaseous refrigerant. This will look milky.

To really get an idea of what you are looking at, have someone inside the car shut off the AC compressor while you watch. Even a clear/full system will slow down and show some foam and then the liquid surface level will appear and recede. The only true way to know the state of charge is by weighing in the proper amount. Arthur is sure right about that fan not coming on if the pressures are low.

I get a few cars every year that start to run hot because the AC has lost enough refrigerant that the fan stops coming on and the cooling system gets noticably hotter as a result.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

SB.
Thanks for the compliment......Do I get the job??? I will start at the bottom , sweeping the floors

TSH24.

Here is a schematic that will help ..your chassis uses *footnote #1..
You can see that the #2 circuit went with the later temp trigger sensor using a thermistor type [ that had the default fan activation feature].

http://members.aol.com/ajdalton7/auxfan.jpg

PS . I see that schematic did not come out too clear, so S/32 a/c pressure sw specs. are: Closes/20 bar---open/15/bar...

Thanks to all of you for the insight. I appreciate the help.

The car was converted to 134A some time ago. It was low, so I added approx. one 12 oz can. I can see lots of bubbles in the sight glass. the system didn't seem to want to accept the whole 12 oz can (I did not evacuate the system before hand) and the pressure gauge was reading about 50-55 PSI (in the yellow area of my cheap little gauge). I assumed that was a sufficient amount. It does seem to blow fairly cool--not what I would describe as really cold, but ok. Maybe I should evacuate the system and recharge?

Thanks again for the info. I've just had the car 2 wks now, but it wasn't driven much the last few years (owned by an elderly man) so I'm planning on having to work out some bugs. It's a pretty nice car though--91k miles, nice paint/clear coat, smooth ride, runs well, etc. I'm liking it so far. I know it's cliche at this point, but it does seem really well built--pretty cool.

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Ty H.
'87 300E 135K Miles

As we can see you are inclined , Go to ebay and get a 124 chassis service manual cd ..I see them for $5/$10 now ..worth it .... if not just for the schematics

What actually causes high pressure in the AC system? thus triggering the sw? Do higher temps cause the pressure to rise? If this is the case, then the ac sw is actually triggered indirectly by heat (directly by pressure)?

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Ty H.
'87 300E 135K Miles

I've seen the service manuals on cd on ebay. I wasn't sure if they would be legitimate and worth it or what. They are pretty cheap though, so probably worth it like you say. Thanks for the suggestion.

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Ty H.
'87 300E 135K Miles

Yes...Kind of.
The system has two sides .The Low side [evaporator] and the high side
[ condenser] . The main cause of high side pressure is the output of the compressor...but the proper refrigerant level effects this capacity.. the compressor can not compress something that is not there. And the higher the temp, the higher the pressure becomes..that is what High Termal Load refers to. So, if this high side presure gets too high b/c of high ambient temps, an added amout of air-flow across the condensor helps keep it in check. This air flow reduces the condensers temp and that directly reduces the high pressure..if the pressure get too high for even the aux fans to keep it in check, then you will see a compressor saftey cut-out sw ..These vary on different systems..
Your sw is pressure triggered, but some condenser fans are in fact temp sensed/triggered.
The refrigerant just has a higher pressure at higher temps, regardless if the a/c is on or not. Same as your tire pressure increases w/temp. It is a contained gas and a law of physics..
Just a SB mentions cars coming in that are running hot coolant temps b/c they are low on refrigerant [ low fan never coming on, but enough passive heat from condensor to effect the engines cooling ability], the same applies to the a/c fan where the complaint is the fan never comes on when a/c is on. You wind up with a hot running engine temp b/c you have a hot condenser in front of the cooling radiator, thereby restricting the radiators ability to tranfer coolant heat. .......BAD
Another reason for a no low fan can be [ and often is] simply b/c there is low ambient temps when the a/c system is on, so the condenser never needs assistance from the fan . [ never reaches high pressure cut-in]. This condition seldom cause high engine temps and is Normal.
That is why the correct diagnosis should be made with gauges to compare the sw specs to the actual high side readings...and evap temps.
Same problem as with a unit that is low on refrigerant, but different diagnosis b/c of ambient temp coming into the equation..that is why we referred you to eyesight..MOST common NO FAN on a/c system is low refrigerant, where you still have cooling, but not enough refrig to trip sw.
You were corrct in jumping the high side sw before anything else b/c you verified the circuits electrical integrity before getting into the refrigeration system itself. I see many guys just change the sw [ which means total evacuation. etc] when they really never needed one in the first place.
Clear as mud.........
In you newly aquired car, I would evacuate the system and recharge the syatem with the correct capacity charge ..that will probably do it...