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From TXV to CCOT

Merc450 on Tue April 13, 2010 6:54 AM User is offline

Year: 1979
Make: Mercedes
Model: 450
Engine Size: 4.5
Refrigerant Type: 134a

I am just tired from operation ,reliability and location of the TXV type used on old cars like mine. I am looking into either changing the location of the expansion valve by refitting a new "block type" expansion valve in the firewall and re-do the fittings or change into CCOT system.

Assuming all other components are functioning well, my questions are:

1. What loss of performance would I get if a New TXV or Orifice tube is fitted about 15 inches from evaporator ?

If I go with CCOT:

2. I know that I have to add an accumulator and fix it with a low pressure switch, but do I need to take the receiver/dryer out ?

I think some suggest to just to take it out, and others suggest to keep it and account for the extra oil and 134a. I like the idea of keeping it, and I have
the space for it.

3. What low pressure switch can I use on the accumulator ?

4. In CCOT, What controls the low pressure on the low side ? orifice tube size, suction of compressor and efficacy of condenser and fan ??


Edited: Wed April 14, 2010 at 5:11 AM by Merc450

Merc450 on Wed April 14, 2010 5:12 AM User is offline

Please Help

bohica2xo on Thu April 15, 2010 4:06 AM User is offline

Um, yeah.

a '79 euro car, retrofitted to 134a - with poor cooling. Yup.

Retrofitting to a CCOT? That would be like shooting yourself in the OTHER foot.

TXV's typically last for decades, and by the time an MB needs a TXV there is usually a bunch of other little things to fix while the dash & interior is all out on the shop floor.

If for some reason you can't keep the car R12, you will need to upgrade the condensor to a high effeciency type, and increase the airflow over the condensor.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

NickD on Thu April 15, 2010 7:50 AM User is offline

Though I was going to die of heat exhaustion riding in a brand new one of these while in Las Vegas in 1979.

Undersized evaporator, condenser, compressor, and a very poor blower motor.

I feel you have a challenge on your hands, but hey, its a Mercedes 450! And really worth something today. More than I can say for my 78 Fleetwood, but at least it kept me cool.

Merc450 on Sat April 17, 2010 5:18 AM User is offline

Have faith people., but with the help of the precious members in this forum, Job is almost done with success.
I don't know if its ok to plug our forum which is dedicated to this car model, but we go to really extreme measures to sustain and improve performance of these cars.

I already installed a parallel flow condenser, two electrical fans and a larger blower fan and made the move at that time to 134a. This was couple of years ago.

In the last couple of days I have reworked the pluming under the dash, installed an accumulator, reworked the electrical, got new hoses because the old ones are really old
flushed the system, added a GM .072 orifice, pulled a vacuum added oil and 134a.

Yesterday I tested the system and I am very happy about the results.

Ambient 83 F
RPM 1500
HP 160
LP 22
Windows open and Fan on high
center vent: 38-42 F
Idle at 800 RPM 46-52 F It was never this good before.

I went for a ride and center vent temp was ranging 44-47F. I got home and tested at 2500 RPM and guess what,, the outlet of the evaporator was little freezing.
I think I need to look at the Thermos switch in the console and adjust.
I really like the simplicity of the CCOTsystem, its much easier to troubleshoot.

That is one beautiful color on a W116.

Edited: Sat April 17, 2010 at 6:19 AM by Merc450

bohica2xo on Sun April 18, 2010 2:25 AM User is offline

With the low side @ 22psig, it will ice right up. It should cycle higher than that.

With the added condensor capacity & airflow you can get away with 134a.

You did not mention any non-OEM components in the original post. Your big limitation now is the evaporator size, and the weak blower - or have those been replaced too?

While you think the CCOT is an improvement, it may not be once you have it functioning properly. I can drop the cycle pressure on a CCOT to produce really low discharge temps for a short time, but icing will eventually block the evaporator.

You need to adjust the cycling switch for 26 to 28 psig. It must spend some time above 30 psig to de-ice. Usually the switch hysteresis takes care of that if you set it to 26 psig.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Merc450 on Sun April 18, 2010 4:38 AM User is offline

Thanks B for your input. The blower was also upgraded at that time but it is a major job to upgrade the evaporator. I would have to take the center console and dashboard apart which one day I will do. By the way I have a thermo switch with sensing end that go all the way to the evaporator and should prevent freezing, I believe its either defective, needs adjustment, or the sensing bulb is not in its correct place.

Her is my dilemma;

Mercedes manual states the flowing low side pressures when testing, thats for an A6 compressor R12 and expansion valve :
RPM 1500
Humidity 20%
At 68F ambient, 17.4 PSI
At 86F 20.3 PSI
At 95F 23.2 PSI
At 104F 27.5

at 2500 - 3000 RPM
and 40% humidity

At 77F 13 PSI
At 96F 18 PSI
At 104F 22 PSI

Does that make sense to anyone ?

Chick on Sun April 18, 2010 5:53 AM User is offlineView users profile

That is while using the TXV, that all changes when using a CCOT, as Bohica said, you will then need to cycle around 26/28psi, and the A6 isn't the best compressor for cycling, it's meant to run all the time....I would stick with the expansion valve if it were my car.....

Email: Chick


Freedoms just another word for nothing left to lose

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