'80 300D A/C - PeachParts Mercedes-Benz Forum

dwninit2001 · #1 04-02-2009, 02:34 PM

Currently not working, is it worth looking into fixing looks like a lot of parts involved plus pricey...Yea or Nay?

compu_85 · #2 04-02-2009, 02:39 PM

The AC itself is probably easy. What is hard is the automatic climate system that drives it.

-Jason

derburger · #3 04-02-2009, 02:40 PM

I went from a dead servo and dead compressor to cold r12 A/C, working servo for under $700. How many hours do you want to spend fixing this, or if you take it to a shop, how many thousands do you want to spend? Figure 1k for a digital A/C system and 2k for new compressor, etc. Figure more in if they want to convert to R-134a. (Do not go with r-134a.)

winmutt · #4 04-02-2009, 03:41 PM

R134 is just fine as long as you upgrade the condensor and compressor.

tbomachines · #5 04-02-2009, 04:23 PM

What's not working? Is it just not blowing cold? Is air even coming out of the vents? Could make the difference between a DIY rebuilding/cleaning CCU and extensive HVAC work....

ah-kay · #6 04-02-2009, 06:06 PM

It is not that difficult to fix A/C. If you take it to the shop then you do not see too much change for $1k. A lot of time it is no freon. Check the pressure switch at the receiver/drier. One is to detect pressure, if it is open circuit then you may be just out of freon. I think the other switch is on temperature to operate the cooling fan. The other test is to short the wires while the A/C is on, the compressor should come on. Don't let it run for too long! If the compressor does not come on then the compressor is shot or your Climate control is faulty.

Good luck.

leathermang · #7 04-02-2009, 09:12 PM

First we need to know for sure what your dash climate controls look like...
if you have a type one AC system ( they have one,two and three) it will be much easier..
I have a 1980 240 123 with the type one.. but I do not know when they started putting the type two and three into the other models...
Can you take a picture of the dash and the area at the front passenger side of the engine ?

mdlwolf · #8 04-02-2009, 11:41 PM

One of the best things I've ever done is invest in a decent vacuum pump, manifold gauges and UV leak detection kit. Once you get a handle on how the AC system works fixing and maintaining it yourself isn't all that complicated. A decent digital scale is nice too, for weighing in the charge from partial cans.

Also, about 134. It's bone dry here in the summer but it does get into triple digits. It took me allot of fine tuning to get my benz to produce cold air with 134 but it does now. for me the biggest improvements after weighing in a proper charge was to adjust the high pressure cutout point and the evaporator temperature sensor to make the compressor run longer therefore producing colder air.

Before having all the tools myself I took my car to a shop twice, spending over $1000 each time only to have it cool marginally, about 44-46 at best. Now I routinely see vent temps between 34-42 depending on driving conditions.

I've also converted my F250 and a friends 300TD both producing very cold air. The only thing I haven't had great results with is another friends 240D because I can't find any adjustment for the evaporator temp switch and am only able to get that car to blow 42-46 at the coldest.

leathermang · #9 04-03-2009, 09:09 AM

MDLWolf,
UV leak detection introduces some problems of its own.. we have seen many reports of leak detection fluid gumming up small parts like the TXvalve ( thermostatic block valve in our cases)...
Thus it is recommended that electronic refrigerant detectors be used ( I think they can be rented ) to find leaks...UV detectors do not work well with leaks in hidden places like evaporators ... whereas the electronic detectors will whelp loudly just by getting in the dash vents in those cases...
" Weighing in a proper charge" by using scales are is not possible when dealing with a leak and just maintaining the system... and if you are installing a new charge after vacuuming then it is recommended that you assess the correct amount by reading the high and low pressures in the system as you fill ... being very careful about not overfilling... as these systems will work with a pretty wide variance in terms of ' low' but any above the proper amount will really mess up your cooling big time.
I am really skeptical as to this ' evaporator temperature switch' .... are you referring to what the FSM calls the ' pilot operated ' switch ? The one the driver adjusts on the dash to determine what temperature he desires in the car ? I suspect that is not the one you are talking about ... so I am asking where this ' evaporator temperature switch' you are adjusting is located ? Are you talking about the Thermostatic block valve located at the end of the evaporator ?

mdlwolf · #10 04-03-2009, 12:10 PM

Some good points Leathemang. Will report back if I find plugged TXV's from using the dye, although my F250 uses an orifice tube, does this occur with tubes too? You are correct about weighing in a charge for top off. It's just not possible. I find the scale useful for weighing in a partial can when starting with an empty system. This way I know exactly how much refrigerant is in the system. And of course the gauges tell all. When I did my friends euro 300td the sticker said the same that my 300D says, 2.2# r12. I was thinking it would take somewhere around 30 oz of 134A. when I began charging by the end of the second can, 24oz, I had excellent low/high readings and great dash temps so I stopped there and watched the gauges for awhile. I decided to let it go and he had perfect ice cold air all summer.

Hey, I'm just learning. but to me it seems that if you charge in what you think should be the correct weight in refrigerant but you have a low side that won't drop below 35psi you're never going to get good dash temps. With proper airflow across condenser while you are watching the gauges at about 1800 rpm.

the switch I am referring to is the evaporator temperature switch, a small square box with wire leads and a capillary tube that is mounted to the evaporator box. It is covered in a felt insulation and on it's face is a small redish screw that changes in cut in/out point. On my benz it was cutting the compressor at the mid 40's dash temps so I adjusted it to cut out at a lower temp therefore running the compressor longer and producing nice cold air at the vents. On my friends 300TD his switch was failed open, not allowing the compressor to even come on. The replacement has an adjustment screw under the cover. On my 300 the switch at the receiver drier is also adjustable, allowing for a higher high side pressure before it cuts out, at least that's how it seems to function when I watch the gauges and adjust the switch. My F250 has a similar switch on the drier and it was necessary to adjust it in order to get the compressor t cycle correctly with 134A. a firnd of mine who does A/C professionally is the one who told me about these switches. He says many techs don;t touch them and this is part of the reason that people experience poor temps with 134A conversions. there are many other reasons for poor temp of course. Also, on my friends 240 it needed a new pressure switch and the new one had no adjustment but performed correctly for 134A. There was a choice between two switches when ordering, one for r12 and one for 134A.

Not claiming to be an expert here, but just stating what has worked for me without spending $1000 for some supposed "certified" tech to return the car with sub par performance.

leathermang · #11 04-03-2009, 01:11 PM

Quote:

Originally Posted by mdlwolf

Some good points Leathemang. Will report back if I find plugged TXV's from using the dye, although my F250 uses an orifice tube, does this occur with tubes too?..... so I stopped there and watched the gauges for awhile. I decided to let it go and he had perfect ice cold air all summer.

On my friends 300TD his switch was failed open, not allowing the compressor to even come on.

The replacement has an adjustment screw under the cover. On my 300 the switch at the receiver drier is also adjustable, allowing for a higher high side pressure before it cuts out, at least that's how it seems to function when I watch the gauges and adjust the switch. My F250 has a similar switch on the drier and it was necessary to adjust it in order to get the compressor t cycle correctly with 134A. a firnd of mine who does A/C professionally is the one who told me about these switches. He says many techs don;t touch them and this is part of the reason that people experience poor temps with 134A conversions. there are many other reasons for poor temp of course.

I am relieved that we are more on the same page than it looked like... I was afraid you were messing with the pressure relay on the receiver dryer... which could blow up the compressor, burn out the compressor clutch... unpleasant things like that...
The valve you are dealing with is correctly termed either a TXvalve or a Thermostatic Block Valve in MB language and our cars... it has both the TX valve and the siphon valve ( which is designed to keep the evaporator from getting below 32 degree F in order to keep humidity in the air from freezing on the evaporator fins and blocking air flow completely ) in one rectangular metal block...
Our AC systems are calling ' Cycling clutch' ... meaning it is supposed to turn on and off... your 250 and some other newer cars with orifice tubes are likely to be constant compressor on situations.... I do not know enough about them to comment here except that any small hole can be adversely affected by junk in the system.. which is anything except the correct oil mixed with the refrigerant being used. Lots of reports on this type stuff on Aircondition.com, ACKits, etc...
You mention another thing which is often overlooked when people have gauges hooked up to their AC... engine RPM.. and AIR FLOW ACROSS THE CONDENSOR. Either of those not being right can compromise the guage readings enough to mislead a person about the proper charge.
There are threads from the past where people have adjusted these valves..

http://www.peachparts.com/shopforum/showthread.php?t=163394

mdlwolf · #12 04-03-2009, 11:49 PM

I need to clarify that the switch I am talking about is not the expansion valve. It is the evaporator temperature resistor, pictured here:

http://www.allpartsexpress.com/ShopByVehicle.epc?q=1983-MERCEDES--BENZ-300dt--/--L5--998cc--3.0l--Diesel--Fi--Turbo-Climate--Control&yearid=1983%40%401983&makeid=MERCEDES+BENZ%40%40MERCEDES+BENZ%40%40X&engineid=1193778%40%40300DT++%2F+L5%2D2998cc+3%2E0L+DIESEL+FI++Turbo%40%40300DT&catid=Climate+Control%40%40Climate+Control&subcatid=E+T+R++Switch@@E+T+R++Switch&mode=PA

leathermang · #13 04-03-2009, 11:59 PM

Still a tricky thing to set by an owner... not recommended by the FSM...
Often the other items in the system have degraded...like bent and dirty fins on the condenser AND radiator which are limiting the ability of the AC to function correctly...
AND we have talked about how seldom the fins on the evaporator get cleaned ...
D Morrison had a great how to thread on cleaning it in place... and how dirty it was..

unkl300d · #14 04-04-2009, 01:08 AM

d-nit there is a 'relay' (climate control amplifier) behind the glove compartment. if its burnt, the a/c won't work.
you can get used ones or a new one at this peachtree website!

.

mdlwolf · #15 04-04-2009, 10:33 AM

Leathermang,

I think it can be a tricky thing for people to set themselves if they aren't using a thermometer to check vent temps and also watching the manifold gauges. In my experience the ETR switch was cutting the compressor at around the mid 40's. It is a capillary tube temperature switch. I don;t see how any other component in the system will change the temperature at which this switch cuts the compressor. The evaporator box gets to 44F and the switch opens cutting power to the compressor. Evap box warms up to 46 ans switch closes powering compressor again, cycle repeats. How is it possible to get lower vent temps in this situation? What am I missing here?