Ahh.........the benefits of a W116!

The switch on the W126 is fixed.........you'd need to add some type of resistor in the line.............

Hmmmm.....would a rheostat added in the circuit help????

__________________
Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

Probably would.

Dave did some experiments with various resistors to get the temp down on the W124.

However, in my climate (rarely above 90), 47 degree vent temps are just fine.

It has been well over 100°F here for most of the summer months, but coupled with rather high levels of humidity too.

Down in Hesperia, where there is very low levels of humidity, the A/C feels much, much colder than it is....

__________________
Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

Two things.

1. Could you give me a vector to find Dave's thread?

2. When measuring gas pressure on the low pressure side is that done at idle, 1000 RPM, etc?

Thanks.

.

The '85SD (with the very dark tinted glass) gets 45°F at the vent with 91°F ambient and the high Florida humidity (at least 50 % usually over 75 % and often higher than 85% humidity). But only after about 10 minutes at highway speeds and with the fan set on minimum. With fan on auto (which gives a higher fan speed sending more air through the evaporator) it goes up to 52°F. At idle it can go up to 65°F or worse the longer the idle -the aux fan is not currently working.

This is with duracool -a hydrocarbon refrigerant (propane/ isobutane blend, I believe)

__________________
'83 SD, 2x '85 SD
You are entitled to your own opinions, you are not entitled to your own facts.

I've never experimented with alternate refrigerants. All I use is R-12, so I can't comment on anything other than that.

__________________
Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

Also could be broken recirculate function. A 30 degree drop is typical of what you get when cooling outside air. Diagnosing requires removing the glovebox and using a handheld vacuum pump to evaluate the 7 vacuum lines to the various vacuum actuators. I'd be surprised if a '91 doesn't have some failed actuators.

I'm a bit skeptical of this point for the following reasons:

1) Without control of the ETS, the SDL can get a 50 degree drop relative to ambient.

2) Down there in TX, you haven't a prayer at 100F. in the sun if the discharge air is 70............the system is practically useless.

I'm quite sure most systems have greater capability than -30.

I've been reading this thread, searching using the PP search feature, etc and still am somewhat lost (that's a surprise!?).

I've got this (I think):

1. Proper charge, composition, pressure, oil, etc
2. Clean all heat exchangers
3. Vac pods, recirc, HVAC controller working correctly

I've been hearing that the aux fan can be set to come on earlier which I understand but have not done.

I understand the temp switch to protect the system from freezing can be modified. However my unit at the region of maximum temperature drop is far from freezing so I don't think that's an issue.

Is there some other temp limiter switch (I'm not talking about the thermostat controller loop in the car)? Is this what BC means by ETS (can't find that acronym)?

Also, are low side pressure readings done at idle or higher RPM?

Thanks.

Data: w124, '90, converted to 134a (not by me).

.

ETS = evaporator temperature switch. An anti-icing switch, if you will.

Sixto
87 300D

got it - understand - thanks

.

You're not accounting for the fact that climate control systems use recirculated air. The a/c system is not cooling outside air 50+ degrees, it is recirulating interior air and cooling it via multiple passes over the evaporator. Eventually the discharge air reaches 50 or more degrees below ambient, but the intake air at that point is well below ambient as well. The MB climate control manuals give detailed information on the parameters used to select fresh vs. reciruclated air, and in the case of the 124, whether to use 80% or 100% recirculated air.

Agreed that 70 degree discharge would never cut it in 100 degree weather. BTDT, as a matter of fact.

My 30 degree observation is not an opinion, it's based on empirical data I gathered over the course of ownership of my 124.193. During which time I replaced the evaporator, all vacuum actuators, the condensor, the compressor, the manifold assembly, and ran it on 3 different refrigerants, collecting data all the way through.

On my 123, I designed a modification to take it from the factory maximum of 80% recirculated air to 100%. On a 100+ degree humid day, this change resulted in a 5 degree drop in discharge temperatures, taking it from approximately 55 degrees to 50 degrees. I do have a little bit of experience in this area...

Great information.

Thanks.

On the w124.128 does the ECC shift from fresh air, 80% recirc, 100% recirc automatically? I think that is what you are saying.

You say you get a ΔT of 50 with the mod to your w123.

Given a w124 in the same condition would you also get a ΔT of 50?

If not would modifying the recirc to 100%, as you did with the w123 do so?

Great info. Very helpful.

Thanks.

.

Agreed. I have not researched the amount of recirculated air on the W126 and suspect that it cannot be of any significant quantity if one needs to get -50 degrees at the vent.