Hey guys, the A/C on my 300SDL isn't working, the previous owner CLAIMED he replaced all the parts but the A/C compressor, but I believe he's a load of crap and take it with a grain of salt. How can I check the system to pinpoint what could be having the A/C not work?

Also, I don't know crap about crap about these cars, dump as much information and as you can if you like as if I don't know crap about cars (I really don't know much honestly). Instructions can be detail oriented, the more details the better.
Like I said, I don't know much about the car (You will not insult my intelligence!!!), so if you like, feel free to write to your heart's content a lot of information...if that's fine with you, because a crap ton of information laden with a crap ton of instructions is fine for me! Actually, it's a blessing!

Thanks in advance.

__________________
1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.

I've attached a link to our site's DIY tech articles on the W126 platform and also 2 links related to the A/C system. I'm sure other members will give you some more details and specifics on what to check over first; but these should give you some info on what to troubleshoot. Hope this helps a bit and best of luck!

Mercedes-Benz S-Class W126 (1981-1991) Technical Articles

Mercedes-Benz W126 A/C Belt Replacement | 1981-1991 S-Class | Pelican Parts DIY Maintenance Article

Mercedes-Benz W126 A/C Condensor Replacement | 1981-1991 S-Class | Pelican Parts DIY Maintenance Article

The mother of all a/c threads right here.

A/C recommendations for the MB vehicles

__________________
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

My suggestion: Unless you're familiar with an automotive refrigeration system and how it works, expect to become very frustrated with the Mercedes system. There are some good threads posted above, make some good bedtime reading.

You can also see the thread I had running earlier this year on "Resurrecting a Dead A/C". Mine had quite a bit more challenge to it than a lot of other systems that have been resurrected due to the half-wit that tried to "service" it, the electronic issues that caused the original failure, and the sheer amount of time that elapsed between the failure and when I rebuilt the system.

I will say though, once the system is operating as it was designed, it works very well. Mine's been up and running for over a month now and even on a muggy 90+ day, you can hang meat in that car. It's a very fast, and very comfortable system.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Additionally, keep in mind that rules and guidelines for the 123 vehicles and Gen I W126's will NOT WORK for your application. The SDL uses the Gen II's system which has a lot of electronics and safeties and a completely different compressor and condenser setup. Applicable information will come from the 300SDL, 350SDL, 300SE/SEL, 420SEL, and 560SEL. The W124 cars are very similar as well.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

The biggest reason for this is the fact that in 1986 the W126 changed from the OM617 5 cylinder to the OM603 6 cylinder. With the new engine design came the (new at that time) serpentine belt. The older engine has individual belts.

To prevent a seized compressor from taking out the entire serpentine belt and leaving you stranded, they provided a monitoring system that compares the RPMs of the engine and the compressor and de-energizes the magnetic clutch on the compressor if the compressor begins to drag.

The relay is known as the "Klima" relay and it is located in the electronics box in the rear right side of the engine bay. Note that if yours is bad you will have to replace it with one from an OM603 powered car. The relay from a gas car will not work because the tach pulses are completely different between gas (3 pulses/rev from the spark) and diesel (144 pulses/rev from a sensor on the ring gear).

The compressor has an rpm sensor inside that also feeds into the Klima. The Klima has an algorithm that compares the two rpms. There is also a time delay built into the relay so that the engine has a chance to stabilize before the compressor is applied, and also the kick down switch feeds into this relay to cut out the compressor on max acceleration.

The system also has a low pressure cutout switch so that if there is low refrigerant pressure the compressor cannot engage.

The remainder of the electronics on the 1986+ 126 and 124 system are actually easier to work on than the early 126 and 123. Everything else is contained in the push button unit instead of the older system that had the push button unit and a separate analog control box buried in the dash.

The complete troubleshooting procedure for the control system is in the 126 factory manual. This can be found online at startekinfo.com.

Here are a couple quick checks.
- Can you spin the compressor central hub by hand? With the engine stopped of course. If it moves then you don't have a locked up compressor. If not then the compressor has locked up and you're looking at major work.

- Does the system have pressure in it still? If so buy a lottery ticket.

There are lots of forum members who do this professionally so you have lots of expertise to draw on.

I would also recommend that if you have to refurbish the system keep it running R12. If you convert it to 134A and don't change the condenser you will be sorry. BTDT.

__________________
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

While I was working on replacing the tensioning devices, I decided to take off the belt and spin all the pulleys by hand. And the A/C compressor spins freely without suddenly "locking" up like a worn pulley would do.

How do I check if the system has pressure? Just use the vac device to test it? I think I'll have to do some homework on that...

__________________
1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.

The pulley should spin freely. It isn't engaged to anything. To check if the compressor is locked up, you'll have to grab the center section (which is part of the clutch) and turn it. If you can turn it, the compressor isn't locked. If it won't budge, expect to do major work on the A/C system.

To check for pressure, pull off one of the charging caps. One is next to the condenser, right next to the driver's side headlight, the other is right up next to where the A/C lines run through the false firewall just before the brake booster. Use a screwdriver or something and quickly push down the valve core. If gas shoots out, you have pressure. If it doesn't do anything, you have a leak.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Compressor is locked up.

I pressed the valve and it hardly made any noise, didn't feel the "pop" or pressure like when I press on Bicycle tire.
It supposed to loud and forceful like a bicycle innertube?


Looks like I have a ton of work ahead of me.

__________________
1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.

You're going to be in the same boat I was. New compressor, flush/purge lines, new O-rings, etc.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Spend the time to flush the system over and over again until it is clean. Do not be surprised to find 'stop leak' in there and metal filings all over. Drill that TXV out, clean, and reinstall to flush the evap from under the hood.

I bought a big ass Hecat MVAC flushing system as I have so many vehicles. It could be worth the investment for you if you have many older vehicles.

Pull the radiator and condenser to clean it all out.

You could have it operational in a weekend being a first time guy.

__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]