AC Climate Control II problem 1978 300cd diesel - PeachParts Mercedes-Benz Forum

bfowler71 · #1 07-22-2012, 07:21 PM

7/22/2012
I have a 1978 MB 300cd, diesel. I have type II climate control system. Several years ago it was converted to R134a by a professional MB shop. It was cooling ok for about 8 years. 2years ago then compressor went out. This week I installed a rebuilt York compressor, receiver/dry and expansion valve. I put in 4 4oz Ester oil in the compressor and 3oz oil in the dryer. The oil also had 1oz of Ice32. I pulled vacuum for 30 minutes. Vacuum held without for 1 hour without any leakage. The outside temperature is 88 degrees today. I put 12oz of R134a. The low side pressure shot up to 72 lbs and the high side was 145lbs. I raced the engine and the low side dropped back to 65lbs and high side went up to 180lbs, But no cold air. I shot another 12oz of 134a still no cold air. Vents were blowing but it was 85 degrees. I shot another 6oz of R134a and still no cold air. Total of 30oz of R134a. The low side was up to 75lbs and the high side was 150lbs.The aux fan was not running so I had a big blower fan from a home unit blowing on the condensor. The high side pipe coming out of the compressor was very hot to touch. Today, I have pulled vacuum again and researching what could be the causing the problem. Do you know anything that would assist in the troubleshooting of the system with the dreaded Servo. Thank you in advance for your help.

79Mercy · #2 07-22-2012, 10:06 PM

Where the low side hoses getting cold?

If so it could be that hot coolant was getting into the heater core

Phillytwotank · #3 07-23-2012, 10:34 AM

I assume that the compressor is engaged? What about your expansion valve? was it replaced?

hot water flowing in the heater could definitely be the culprit. try to feel the hot water lines and/or pinch the off to test that.

meltedpanda · #4 07-23-2012, 11:46 AM

1971 Chrysler Imperial Auto-Temp 2 Service Reference Guide - Session 281

this might help

bfowler71 · #5 07-23-2012, 03:37 PM

The low side line at the compressor was getting cold when I was putting in freon but It was not cold after it ran a couple of minutes.

bfowler71 · #6 07-23-2012, 03:39 PM

Yes the clutch was engaged.

bfowler71 · #7 07-23-2012, 03:42 PM

Yes the expansion was chaged. Sorry I orginally typed evaporator valve.

bfowler71 · #8 07-23-2012, 03:49 PM

Would hot water getting into the heater core create high pressure on the low side of the compressor?

bfowler71 · #9 07-24-2012, 06:35 PM

Ok here is the update, I pulled vacuum again yesterday. Pressure held.
Tried to push some R134a in today. Here are the results:
1. Normal Operating temp
2. Aux fan running (had blown fuse)
3. Ambient Temp is 100 degrees
4. System took 6oz then slowed down; I stopped at about 6oz
5. Clutch running
6. Low side pressure 90lbs (very high) should be around 45lbs
7. High side was 98 lbs (very Low) should be around 250-300lbs
8. Idle speed around 1500rpm (no gage just my ear)
9. Vend blowing 112 degrees
10. Temp of expansion valve 106 degree
11. System equalized ok.

Looks to me like the system is still calling for hot air. Do you think the Servo is stuck in heat mode? When I turn off car I heard the servo making a noise, I assume it is looked for parked position. I read somewhere that if the servo makes this noise it is not stuck, what do you think?
To the people that suggested I pinch the heater hose, I forgot to do this. I was very concerned about the low side having very high pressure.
My compressor belt will squeal vey loud at acceleration. The belt is new and very tight. Would high pressure on the low side do this? I had a new compressor installed in 1999 and had the same problem with the belt. I carried it back to the garage and the guy said he must have over charged it. He recharged and the compressor went out 3 weeks later and the garage was out of business.
If anyone can tell me what components should be test and how to make the test I would be very appreciative. l

Phillytwotank · #10 07-25-2012, 07:04 AM

I say... If the comp is charged and spinning there is nothing that the servo controls that would be preventing cold air from blowing. Unless the servo is crapped and its letting hot hatter flow at all times the that would overide Most of the cooling from the AC. I'm no "expert" on these systems but I sounds to me that you have a bad compressor. With the highland low side pressure being more or less equalized like that.

Isolate the servo. Do all the settings work? When you rotate the temp wheel from red to blue does it go from heat to fresh air? When you toggle the compressor switch does it cycle the compressor on/off?

bfowler71 · #11 07-27-2012, 12:34 PM

I agree I could have a bad compressor....Engine at normal oper temp. I rotated the temp wheel and yes it blows ambient temp air on 65 degree and hot air (about 114 degrees) on 85 degree setting. I have checked the temp of water houses going into the servo and going out of the servo then return from heater core back to servo. I don't have the actual temp in front of me but I am satisfied that hot water is not flowing to the heater when temp wheel is set to 65degrees. I let it cool off several hours with a shop fan blowing on the front of the car. I then tested the 65 degree setting and the vents blew very close to ambient temp. I pullled vacuum again and let it sit overnight, vacuum held. I pulled it agin this morning and tried to charge but the compressor will not accept but about 6oz of gas. I backed the low side Schrader vale core out a little to make sure the core was being pushing by the guage, still will not take gas. Clutch was turning because the low side had 90 lbs and the high side had 100 lbs. Set idel speed higher and the low side dropped about 5 lbs and high went up about 5 lbs.
I have about had it with this project it but am determined to not let it get me.

Tomguy · #12 07-27-2012, 12:47 PM

I could be wrong but on most cars the A/C evap is before the heater, not after (as far as air flow) and I imagine your MB is the same, so even if the heat were on full blast it wouldn't matter. Otherwise defrost would never work and still give you heat. It sounds like a defective expansion valve. I doubt it's on backwards, although I suppose it's possible.

meltedpanda · #13 07-27-2012, 04:00 PM

an expansion valve going south, will sometimes allow the system to "shut off" and then it blow hot air , then return to normal , perhaps yours is stuck?? or defective I'm with tomguy

bfowler71 · #14 07-28-2012, 12:31 PM

Yea expansion valvue will only mount correct. One of the three ports is much larger that the others and it would be really hard to mount it wrong. I pulled vacuum again yesterday and tried to charge again...Same results. I do have another question on the same problem. Will the vacuum switch for the refrigerant compressor create this problem. What is the function of this switch? If you have the official MB dvd, this is in section 83-628. The component number is #23. I wanted to ask this before I pull center console.Thank to all for you help.
Bob Fowler

P.S. The MB tech site for the above is ---> http://www.startekinfo.com/StarTek/outside/12265/disc_2/program/matrix6.htm

bfowler71 · #15 07-28-2012, 02:26 PM

I think I have answered my own question. When I looked at the vacuum diagram, it looks like #23 is the compressor off and on the the control button. This works.