A/C Advice 1983 300SD - PeachParts Mercedes-Benz Forum

VA-Merc · #1 06-10-2013, 05:55 PM

Hey guys, sorry to start another A/C thread. My relative inexperience with a/c systems has left me with the inability to know the best search terms for this thread...so I apologize in advance if a similar problem has already been discussed.

Today my a/c worked fine in my 300sd with the R4 compressor during my lunch break, but after work no cold air. When I hit any of the a/c positions on the ccu the triangle part on the front of the compressor engages and starts spinning. It does not make any odd noises, and if I hit the economy mode on the CCU it disengages properly. I checked the charge on the low port with the a/c off, and it shows approximately 80% of the air temperature outside (which tells me it has the correct amount of 134a in it). I know a 134a conversion isn't the best, but it was already like that when I bought it. What would be my next diagnostic step? Is it possible that the clutch is engaging but the compressor is still not pumping? Prior to this my a/c had been causing a bit of a drag on the engine when it was on, but I figured that happens in most cars so I didn't think twice about it.

mach4 · #2 06-10-2013, 06:55 PM

The fact that it was working fine a few hours previous would tend to rule out leaks. I think the best thing would be to get an A/C gauge set and check the pressure on the hi and low side under static conditions and again while running. That will tell definitively whether the compressor is doing it's job, and confirm the refrigerant level at the same time.

If it's got refrigerant it pretty much needs to be either compressor or expansion valve.

As a quick check in the interim, turn on the compressor and feel the hose on the high pressure site and the low pressure side. The high should be hot and the low should be cold.

I guess it could also be vacuum pods not opening, but the place to start is compressor and charge.

jay_bob · #3 06-10-2013, 08:10 PM

If you lost your vacuum feed to the CCU (green vacuum hose broke or fell off) the pods will default to the safe mode, which is all air directed to the base of the windshield.

I had this happen when I was replacing brittle vacuum hoses in the engine compartment and forgot to hook the green one back up.

VA-Merc · #4 06-10-2013, 08:18 PM

The air was still coming out of the vents as I it normally would when I selected the different modes on the ccu. Thanks for the tip though!

I read the pressures:

compressor off: 70psi low side, 70psi high side

Compressor on: 27psi low side, 125psi high side

my "compressor off" readings seem similar to what they were before, but the "compressor on" readings seem low on both ends. I had not noticed any symptoms of decreasing a/c performance or anything like that prior to today.

mach4 · #5 06-10-2013, 08:48 PM

Estimating an ambient of 75, according to the chart I have, you definitely low, but you should be seeing some cooling out the vents.

I think we can take the compressor out of the equation and start looking at the climate control side of things. You may still need to adjust the charge, but there are other problems.

Does the high pressure line feel hot and the low pressure line feel cold?

VA-Merc · #6 06-10-2013, 09:05 PM

I would say that the high pressure line feels "warm" and the low pressure line feels "slightly cool". Definitely not hot or cold. Checking it tonight...the air is slightly cooler coming out of the vents, but nowhere near where it was earlier today. I even tried turning the ccu dial to max cold and there is no difference.

ROLLGUY · #7 06-10-2013, 09:53 PM

If you decide to replace your compressor, check out:
http://www.peachparts.com/shopforum/diesel-discussion/339634-summer-here-time-sanden-ize-your-617-616-a.html

vstech · #8 06-10-2013, 10:05 PM

your compressor is working, but you are low on refrigerant.

VA-Merc · #9 06-10-2013, 10:59 PM

Thanks for the advice. In looking at the chart I was wondering the same thing. I guess I will try adding a bit and see if the pressures come back up where they are supposed to be. I did find that one of the fittings was a bit loose on the expansion valve when I was checking around this evening, so maybe it had a slow leak. I will let you guys know what I find out tomorrow.

Doktor Bert · #10 06-12-2013, 10:04 AM

Let me dwell on this for a bit before I respond.

VA-Merc · #11 06-13-2013, 10:10 PM

Hey guys,

Per VSTECH's advice, I was low on charge, and brought it up to spec based off the chart above with the ambient temp. I have acceptable cooling out of the vents, however, my R4 compressor is making quite a bit of noise. I did try adding a bit of oil to the system, but it didn't quiet it down any. I checked for loose bolts, etc...the noise is definitely coming from the compressor itself when it is engaged. So...thanks for the advice...I am getting great cooling for now, but now it is probably a ticking time-bomb before that R4 grenades haha. I guess I'll keep using it for now until that happens.

ROLLGUY · #12 06-13-2013, 10:23 PM

Quote:

Originally Posted by VA-Merc

I am getting great cooling for now, but now it is probably a ticking time-bomb before that R4 grenades haha. I guess I'll keep using it for now until that happens.

Eventually ALL R4's suffer that same fate. When it does go, contact me for the solution....Rich

mach4 · #13 06-13-2013, 11:27 PM

Quote:

Originally Posted by VA-Merc

Hey guys,

Per VSTECH's advice, I was low on charge, and brought it up to spec based off the chart above with the ambient temp. I have acceptable cooling out of the vents, however, my R4 compressor is making quite a bit of noise. I did try adding a bit of oil to the system, but it didn't quiet it down any. I checked for loose bolts, etc...the noise is definitely coming from the compressor itself when it is engaged. So...thanks for the advice...I am getting great cooling for now, but now it is probably a ticking time-bomb before that R4 grenades haha. I guess I'll keep using it for now until that happens.

Be aware that if your compressor grenades and pumps black death through your system, you could be looking at a complete flush as well as a replacement of your expansion valve, dryer and potentially other components. Whereas if you replace the compressor now, there is a great chance that your system is clean enough that you could put in a new Sanden, replace the oil and charge with your favorite refrigerant - stay with R12

, convert to 134a

or use one of the designer refrigerants (as Rollguy likes to call them)

.

In addition, if the compressor grenades and locks up, you could potentially throw a belt and take out a transmission line or an oil line - not a huge chance, but a potentially costly one.

I'm running a heavily modified Rollguy bracket, a Sanden and Envirosafe and this was my vent temp today (ambient 80).

(Please note: I'm not advocating any particular solution just stating what I chose as a datapoint for anyone doing their own research)

Brian Carlton · #14 06-14-2013, 10:16 AM

Quote:

Originally Posted by mach4

I'm running a heavily modified Rollguy bracket, a Sanden and Envirosafe and this was my vent temp today (ambient 80).

Fantastic result.

Good work.

I am curious as to why the evap is not freezing however? The evap temp must be around 26F. and it can't last long (till it freezes) at that temperature.

Usually the ETS prevents anyone from getting that cold.

ROLLGUY · #15 06-14-2013, 10:40 AM

Quote:

Originally Posted by Brian Carlton

Fantastic result.

Good work.

I am curious as to why the evap is not freezing however? The evap temp must be around 26F. and it can't last long (till it freezes) at that temperature.

Usually the ETS prevents anyone from getting that cold.

Evidently my Sandenized 'Benz has the same problem:

I believe that the fact that air (warmer) is passing through the evaporator, that is what is keeping the evaporator from freezing. If the compressor fails to cycle and stays running with no air flow across the evaporator, then yes, it would freeze. I have noticed that when idling after the cab is cooled down, the compressor cycles on and off. I believe the efficiency of the Sanden is what makes this possible. I am running 134a in my system with the stock condenser and a 5 cyl Sanden. I have a 7 cyl Sanden on my 300D, and will hopefully get it up and running soon so I can see how much more efficient it will be.