Hi all, I wanted to share the story of my AC rebuild process, and hopefully prevent others from making the same mistakes I did. After quite a few false starts and failures I have worked my way progressively through to a 100% rebuild of my AC refrigerant circuit. I am not a professional mechanic, nor do I have any formal training. I do have a good bit of wrenching experience, as well as a history with electronics. Please do not take my word as definitive, I am not an expert, please do your own research and consult a professional if needed.

Part the first: the story so far:

I bought this car in the fall, it had no service history but had a fairly new paint job and almost entirely new interior. I did not do as thorough of an inspection as I should have, however given the chance I would still buy the car today.

It was a chilly morning when I bought the car, so my test of the AC was not very definitive, but it did seem to get cold. That winter I was running into problems with the heater so I ended up replacing the radiator, hoses, thermostat, temp sensors, and monovalve.

Part Deux: The electrical and vacuum systems.

In the course of some other work to the car, I pulled the dash. I knew I had some vacuum pods that were leaking, as well as a broken cabin temp sensor. I sourced some repair diaphragms for the vacuum pods from a forum member and rebuild all of my pods, as well as two faulty vacuum switchover valves, and all of the rubber hoses. With the help of a friend, I determined the correct part to rebuild the cabin temp sensor and repaired that. (PM me for info if you are interested). During the disassembly process I discovered that my CCU (control switches) had previously suffered an electrical fire - there was a hole burnt through the housing. It had been repaired previously with a jumper wire, but I decided to order a rebuilt unit instead, as the PCB was pretty well ruined. I also installed a new auxiliary water pump with a 2amp fuse in series.

Part III: Failed attempts:

The following spring I discovered that the AC only worked on cool days. It had previously been converted to r134a. I discovered that it was leaking at the TXV. I tried some leak stop (bad idea) and added some r134a. Neither of which resolved the problem. Next I decided to try a minimally invasive fix: I had the r134a recovered by a shop, replaced the txv, drier, pressure switch and tried charging it again.

This time the system would not develop suction enough to charge the system, also the compressor was making a horrible noise. Again I had a shop recover the refrigerant I tried to add (only about half a pound made it in). I dismantled the entire system, and ran four cans of flush through it (most of which was used in the evap and condenser. I found several clogs, I believe those were formed by the leak stop product I had used earlier, but I can not confirm that was the cause. I also purchased a new r4 compressor with clutch, and installed it. I had purchased a vacuum pump in the meantime (previously I was using one at a DIY mechanics shop) and tried to pull a vacuum, but was unable to hold a good vacuum (and yes, I am aware a vacuum test is not definitive). I had noticed that the high side line to the condenser was corroded, as well as the fitting on the condenser, so I decided to upgrade to a parallel flow condenser. I ordered a 16x24 condenser with custom mounting hardware and new hoses, and installed it along with a new drier. I finally tried to charge the system again, and was getting somewhat out of proportion high side pressures in comparison with the low side, and was not getting cold air. I then purchased a leak detector and had found a potential leak somewhere inside the evaporator housing when the system suddenly discharged. It turns out that the compressor bolts were never torqued fully, which allowed it to split in half. I never definitively determined if I had a leak other than that.

Part IV: A new system:

I was sick of dealing with troubleshooting a 30 year old system. I decided to replace or rebuild every component possible with new parts, upgraded if possible. This includes:

A Sanden compressor with new mounting brackets.
All new liquid lines.
The compressor kit included a new compressor->condenser hose.
Suction hoses. I will be using the original metal ends that connect to the TXV and the piece that runs across the engine. I will be rebuilding them with new barrier hoses and using crimp ferrules.
A new condenser (Remember that I had previously installed a 16x24 PF condenser).
A new evap. There are two evaps for the w123, one of which is NLA. I have the other style on order and hope it will fit without modifications to the housing. If it is not capable of fitting at all, I may have to have my original one professionally flushed and repaired.
New receiver drier
New pressure switch
New TXV
Possibly a new temperature controlled fan switch
I may install a high pressure switch in the extra port at the TXV, to switch the fan if I am not satisfies with the temperature controlled switch.
A universal fit 16" auxiliary fan.

I will edit or reply to this post with further detailed information and pictures as the rebuild progress continues. I may post detailed DIY guides on individual steps. Please feel free to PM me any questions.

Stay tuned. Up next: dismantling the old system (currently underway)

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'85 300D - federal spec, built in late 84. 85 300D Complete AC System Rebuild

Good luck. All new everything should do the trick.

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Current fleet
2006 E320 CDI
1992 300D - 5speed manual swapped

former members
1984 300D "Blues Mobile"

1978 300CD "El Toro"

If it doesnt, I am going to give up and change hobbies to something less stressful. Maybe tight rope walking?

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'85 300D - federal spec, built in late 84. 85 300D Complete AC System Rebuild

It sure helped to completely rebuild my 98 E300's AC (all new except the hoses).

take your time and do your steps right and you won't be disappointed

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Since yours is an 85. How is the klima unit? Open it up and make sure nothing looks burnt.

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1985 300d Turbo Diesel

Good question. I will look!

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'85 300D - federal spec, built in late 84. 85 300D Complete AC System Rebuild

The Klima unit is a large relay that's located inside a housing near the brake booster, right? If so, mine looks to have been replaced at some point in the car's history. It reads something like "AC Clutch Relay" from a different manufacturer, and the bottom half of the case has been sealed with silicone or something similar, so I can't open it. It does seem to have been replaced at some point though, as it's stamped with "20.06" - which I'd take to mean week 20 of year 2006?

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'85 300D - federal spec, built in late 84. 85 300D Complete AC System Rebuild

How hard is it to replace the lower hose on an R4? After I get the system discharged?

Not terrible. You do need to drop the compressor though. Its got a clamp near the alternator, one or two brackets then 3 huge bolts that hold the compressor to the engine. Be sure to keep track of which bolt goes where with what spacers.

For me it's a 2 person job. I am not comfortable holding the compressor with one hand and unbolting it with the other

You will need new sealing washers for the compressor. Or a new compressor would be smart if yours is old

Read the fsm for oil requirements

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'85 300D - federal spec, built in late 84. 85 300D Complete AC System Rebuild

Please take voluminous photos since I will have to do this on two models. AC is an area where I never dabbled.

Thanks!

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Strelnik
Invest in America: Buy a Congressman!

1950 170SD
1951 Citroen 11BN
1953 Citroen 11BNF limo
1953 220a project
1959 180D
1960 190D
1960 Borgward Isabella TS 2dr
1983 240D daily driver
1983 380SL
1990 350SDL daily driver alt
3 x Citroen DS21M, down from 5
3 x Citroen 2CV, down from 6

Assume yours is a 82-85 300D or at least OM617 engine.
Can leave the R4 installed, just unbolt the hose manifold. Look for my post on PN's for new seal washers.
You will need to disconnect the suction nut above the compressor (replace O-ring).
Unbolt the triangle bracket on passenger side and the bracket behind the alternator. At least one of my 300D's (1985 CA?) had a fitting there and a 2nd tube that ran to the condenser via still another bracket on the frame.

Might do like many here did (me) and toss that contraption and run a hose straight to the condenser. Cut the ferrules w/ a hacksaw and peel off. Slide new hose (-10 barrier) on the two outer fittings and secure w/ Oeticker stepless ear clamps. All available on ebay. Will be much easier to get at the alternator bolts in the future.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Best thing you can do if your 123/gen 1 126/earlier has an R4...

Evacuate the system
Remove the R4
Heave it into the trash can
Install a Sanden

I am not a special pleader for Roll Guy, but I have thrown good money after bad replacing an R4.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

What was the problem with your R4? In Demothen's case, it was the installer's fault.
There's millions of R4's still running in GM cars. Are you saying they're all trash?

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

A quick data point. I installed Roll Guy's Sanden bracket in my 1985 300D a few years ago. This weekend, I found a big crack. The part of the bracket with the round tubes that envelops the axial hole in the block had broken off. I had wondered what the rattling sound I had been hearing just off idle for a month was, and finally noted it was louder with AC on. For now, I just removed the AC belt and will suffer the rest of the summer (higher priorities). Maybe do a weld repair next spring and beef up the bracket. I am using the smaller SD7H13 on this car, so maybe others will crack sooner, though metal fatigue failure is statistical.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Sorry to hear that. What gauge steel is used on those tubes and brackets?

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked