I just bought a 1980 450SL 2 weeks ago as a birthday present for my wife.
The AC worked during the test drive and for about 1 week later and then quit. Not knowing any better I was told by the previous owner to activiate the AC click the AC compressor switch on and push the DEF button in. This worked well to cool the car on hot days. Now that I know more about the car I determined that the Auto Climate Control System never did work--buttons don't do anything (maybe a later repair topic). Only the DEF button activated the blower and AC.

So my question is what should I look at to diagnose the AC to at least get it
working in DEF mode again? I will tackle the more complex Auto Climate Control System at a later date. Here is some helpful data:
- After starting car and turning on AC with DEF button, I get cold air for about the first 60 seconds and then it turns hot and stays hot
- The compressor turns on, runs, and cycles but I only get hot air.
- Yesterday I took it to a shop to check the freon level. They said it was low and added some along with some dye. No signs so far of leaks. That's all they could do since they were not familar with the Auto Climate Control II System and were honest and said that they were sure that I didn't want to pay them by the hour to learn it.

So for now I would just like to get the AC working again in DEF mode and then later when I have the time and patience I might tackle the Climate Control System.

Any suggestions on what to check? I would even consider suggestions on what to bypass, unplug, or jumper to get it to work temporarily.

Thanks,

Jim

btt.....

This is the same problem I'm having. The advise I'm getting is that the servo is shot and probably also the amplifier or at least if replacing the servo replace the amplifier.
Some of what I've learned:
1) Vacuum in mine seems fine through the system except there does not appear to be a vacuum tank. Vacuum is supplied directly from the pump.
2) I can actually make a few jumpers across the vaccum activated switch after pulling out the dash controls (after plugging a few lines) and get the a/c and fan to come on and be activated by the temp dial. The a/c compressor never turns off, however.
3) I'm now figuring where to look to see if the servo gets a current. If it does then my guess is the servo is shot.
4) I've been looking for one and rebuilt they run about $500--steep. Plus the amp is another $100 or so. Performance has a solid state one for about $700. Not cheap but I've already replaced the compressor, dryer, expansion valve, temp switch, pressure switch. In for a penny, in for a pound?

Anyone know of a rebuilt one that is cheaper than $500????

GDEMOSS: very interesting symptoms and diagnosis. keep me updated on your continued diagnosis and fix.

I got my AC working again through DEF mode. It turns out that there were 2 issues that stopped it from working:
1. The AC coolant (freon or 134) was low and finally tripped the pressure sensor.
2. Before I took it to a shop to test and add freon I was looking around and notice one of the 2 connectors on the regulator being unplugged so I plugged it in. Nothing changed so I forgot about it.

I woke up thinking about this plug yesterday and went out and unplugged it and the AC began working thru the DEF mode again. Apparently the original owner (who passed away) must of been troubleshooting this issue and learned that this would bypass the issue allowing the AC to work in DEF mode while he was continueing to troubleshoot the Auto Climate Control System.

On the Auto Climate Control System, the original (Deceased owner) must of changed a number of parts since in the boxes of stuff I got with the car were a number of used AC component parts in original boxes.

Now that I have AC again in DEF mode the pressure is off me now and I can get to a number of non-mercedes projects and trouble shoot the CLimate control system later when I get time.

If you need any information let me know. I'm not an expert (only beginning to learn about mercedes since my purchase of car 2 weeks ago) but a full factory service manual set came with the car including the "Heating, Air Conditioning Auto Climate Control" service manual.

First check the fuses in the fuse box and there is a seperate inline fuse in the fuse box that needs to be checked. There is a second inline fuse that is for the radio. When the a/c fuse(s) blow the system is put into a safety mode. You really need to get your hands on the ACC manual. The system is not easy to work on without it.

I've recently retired my 450 and I'm selling all of my manuals and support material on EBay. http://motors.search.ebay.com/_W0QQsassZjrb18

Good luck, the 450SL has very classic lines.

__________________
S, J.R. Brown
2000 G500 LWB Obsedian Black
2005 Toyota Tacoma Access Cab Off Road Sport
1993 Volvo 240 Sedan Anthracite
1980 450SEL Champange (owned it for 15 years. Great car)
1986 280GE LWB Anthracite (Sold it and kinda wish I hadn't)

JRBROWN: I already have all the manuals you are selling including the Heating and Air Conditioner ACC Manual. You said when the inline fuse blows the system goes into safety mode. What is safety mode and what works/doesn't work in safety mode?