300 SDL/ R12/ AC with 60F at vent
 

I have redone the entire AC of the SDL 3 years back (including evaporator!), all parts new basically.
I experienced high vent temperatures lately and thought it must be low charge, bought some R12 and tested, but couldn't find a low charge. So I have no idea what might be wrong. Maybe someone can help:
testing:
aux fan on high, fan inside on high, temp dial on low, recirculation on, at 90-95F ambient i can read low/ high at following rpms
idle: 40/180 or 38/ 180
at 1500rpm: 30/200 or 28/205
vent temperatures: devastating 60 F
I have no clue why there are not 45F at the vent. charge might be slightly low but.....I don't think it makes such a difference.

Hi Martin,
 

I'll have a look at my training CD...these numbers are ringing a bell for some reason and I can't recall...PM me in a couple days.

what is the temp of the low pressure line?

I saw condensate on the low pressure line though.
How and where do I best measure the temp of low pressure line? I have no temperature scanner. Martin

It might not make any difference..
but you should have your windows open during these tests...I mean the car windows... and if you have any way to put a squirrel cage fan so it blows into the condensor that can help a lot..
otherwise you have the possibility that the bottleneck in the AC system ( all ac systems ).. air flow across the condensor is combining with whatever other problem you have when driving around... you do not want extra variables thrown in to the diagnosing process if you can avoid them...

My aux fan was blowing on high during the pressure testing (I jumpered it) ...but for the next test in addition I can spray water on the condenser to make sure there is no problem with the heat exchange at the condenser.
I can open the car window...however I think this doesn't make the cooling work better or do I miss something?
I mean according the pressures this system should work but it doesn't.

I have a similar problem in my w126 300se, When I recharged, I introduced some air in to the system. (on accident)

Results in high pressures, blah cooling.

Did you look at your sight glass?

Flowing water over the condensor could work just fine....depending on accuracy of placement.

The object is not to cool the inside of the car... it is to check the situation inside the AC system.. thus you want all its factors facilitating its functioning at full working load. Not stop or step down half way through the test because the closed up car is cool and comfortable inside..
Anyway, open windows is what the MB AC FSM says to do...

How did you put air into the system. I mean charging 'by hand' with cans might have the problem of getting some air in but does this make the method so bad?
What are the symptoms if there is air in the system? Are there clear indicators other than bad vent temperatures?

How long are your lines from your charging valve body ?
Did you purge them before starting the procedure ?


The MB AC FSM warns against using the site glass as a definitive test on most things.. and I think one mentioned which can give false readings it in the event you do not have enough forced air across the condensor...

One of the main problems with air in the system.. is that it is not likely to be really really dry air..
Thus the potential for immediate problems with regards to ice crystals occurs..
This is in addition to the acid forming effect of the mixture of water and the AC refrigerant and oils in the system... which eats at the inside of the metal tubes and flakes can come off and lodge in places like the TXvalve...

a sticking monovalve will cause warm vent temperatures with a properly operating ac system. ask me how I know


'87 300sdl 286,000
'94 Sl 500 55,000
'89 300te 176,000

How can I rule out the monovalve? What are the symptoms if the monovalve ist not working properly? Shouldn't I have the heat on all the time?

Charging hoses at gauge set are very typical and I tried to purge before charging.
The pressure readings are very stable- no fluctuation on pressures. Also I think the pressures are in a decent range.
I think any humidity in the system should be eaten up by the desiccant as long as it is available....as my dryer was new it should have some capacity left.

So here is what I found so far:

After some testing I found out that the compressor gets cut off after maybe 5-10 minutes of driving and once cut off it won't come back....unless I restart the engine.
With a clamped heater hose at least temperatures went down to 55F.

Anyone who knows about the possible reasons why the compressor stops working?

The was one comment I wanted to share. Comment was that the pressures I was reporting were too low, at least the 30psi low pressure was commented that this could lead to icing of the evaporator in an R12 system.....If true can this be a reason for the compressor to quite work?

What year is the car? Does it have a compressor speed sensor?