I didn't want to hijack the other AC thread since my questions are a bit more basic...

The AC in my 300SD has never worked since I got it 2 years ago, and I've finally gotten around to wanting to fix it. Where do I start?

From what I've gathered here, the issue is probably in one of three places:
1) Bad compressor
2) Clutch not engaging
3) AC system has leaks/is empty

Ignoring #1 for the moment, how to I test the clutch? What about the wiring to the clutch - can someone tell me when it will be +12v so I can see if that's working properly?

For #3, is there a standard set of gauges I can get at Kragen or something that'll let me see if there's anything in the system? Where do I put them?

It's probably obvious I've never really messed with an AC system before. If there are any major issues I'll be taking it in to the shop, but before I do that I'd like to at least diagnose the issue if possible.

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'81 300TD Wagon 355k (Miss Diesel)
'83 300SD 180k
'84 500SEL 190k (Parting it out)

Being new to AC repair you should be careful as refrigerant can be harmful. I would start with checking the fuses and the system for refrigerant. there are two ports, one in front of the radiator on the drivers side has a cap on it with a flat screwdriver slot. The other is about a foot in front of the brake master cylinder. follow the lines and you will see them. this is for an R12 set up, 134a will have different caps with a quick attach port. get the right service gauges for your system. once you have found if the system has any gas check the compressor. A quick test to see if it is any good is turn the end of the compressor in the proper direction. You will feel a slight resistance smoothe, not chunky and skippy. check for oil stains around all fittings and components. This is where the true Mercedes guy's step in as to getting power to the compressor boyond the climate control switches and checking fuses.

first step is to see if pressure is in the system, then find out how much. if you are unfamiliar with your car's a/c take a pic and upload it here, and we'll tell you what hoses you will need.
ebay is a good place to get a decent set of gauges.
most any auto parts house will have them too, but you'll pay 100+ for a set.
best to post a pic for us first to point you in the direction you need to go.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

You should be able to go on craigslist and pick up a set of used gauges for R12 systems. Then measure pressure in the system. If there is none, then you obviously have a leak.

I hate to say it because i'm all for DIY work but to detect leaks and work on A/C systems, you will need to buy specialized equipment like leak detectors, gauges and a vacuum pump which are quite expensive.

Maybe you could look around for forum members with the knowledge and the equipment to help you. If it was me and having the right equipment, I would pull a vac on the system for an hour, then shut the gauges and the pump off then leave the gauges on for another hour to see if it holds a vac before charging it.

If the system is empty then the compressor will not engage (fail safe), it will probably run once you have enough gas in the system.

It's not good to over charge these systems as will put an unnecessary load on your compressor so look up the correct pressures. There is a great a/c manual by Haynes that lists all the pressures for most MB's from the 70's and 80's. It's probably out of print but well worth finding.

__________________
With best regards

Al

omg! how many times must I stress it!
DO NOT TEST FOR LEAKS WITH VACUUM!!!! IT WILL PULL MOISTURE IN IF IT LEAKS, AND IT WILL NOT SHOW A LEAK UNLESS THE SYSTEM IS TOTALLY BUSTED OPEN! seriously, testing for leaks with vacuum is a waste of time. vacuum will cause a small leak that would show with pressure, to seal up and hold. it reverses the conditions of the system, and will seriously mask a small leak. also it's such a small differential of pressure that it's going to take weeks under vacuum to appreciably show any issues.
just get some co2 or nitrogen and fill it to 150psi or less, and start spraying joints with bubbles, or just wait a day, and see if the pressure drops. if it does, there is a leak! if not, then you are safe pulling it into a vacuum and recharging!

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

So what are you trying to say here? Seriously, you learn something new every day. Thanks for the tip.

__________________
With best regards

Al

I'll agree with you, since my system passed a 30 minute vacuum test (Cool, no leaks!), but start dripping oil under pressure (WTF? A big leak?). The pressure test more closely replicates actual system operation.

But...the vacuum leak test is a standard practice and is programmed into some modern diagnostic machines. It's simply what they do. The tech simply runs the steps programmed into the computer. Having bottles of co2 or n2 around would be great, but maybe not realistic.

It may be that way based on use of the tech's time: if I have to evacuate the system anyway in order to get an accurate charge, then I'll do that first. If it leaks, I'm done. If it holds, then refill and hope for the best; or two steps (simplifying things) and you're out of my bay and I get another customer in there. If I put another gas in there, first I evacuate, then charge with inert, then evacuate again, then charge with refrigerant; or four steps - and you're taking up time in my bay for the same operation so I have to charge you more. I'm not saying it's right; but it's what happens if you take it to a shop.

Me? I had my system evacuated first because I knew I needed some new seals (it didn't get low without a leak). Then after I changed some O-rings, the expansion valve and the receiver dryer, I had it evacuated again (theoretically to boil out the ambient moisture and get rid of the air) and it held vacuum. The shop didn't offer a pressure test and I didn't think to ask. Had I, it would have shown my leak and I would have been back to square one. Instead, I lost some R-134, found some leaks and was back there anyway.

For a R-134a, I don't see vacuum as a huge sin; for R-12, you may want to ask for the pressure test since losing it costs more.

I hope I say nothing controversial.

In the old days, before automatic recovery/recycle machines, it was standard procedure to do a QUICK gross leak check as a step in evacuation and charging. The idea was that after you pulled a vacuum, close up the manifold and give it a couple of minutes to make sure it would hold a vacuum for that amount of time before proceeding with the charging. We called this a "gross leak check."

From this, and I might be as guilty as anyone, people began using this as a leak check procedure where no other method was available due to lack of equipment. I agree with John that using a vacuum as a general leak test procedure is not a good idea. When we did the "gross leak procedure" it was more of a last minute, double check to make sure we weren't charging into a known leaking system.

My $0.02,

for a shop to use vacuum as a test is a bad idea too. unless you are using a digital micron gauge.
if you want to eliminate steps, use refrigerant to pressure test, and use the recovery machine to pull the vacuum. pull out charge, replace bad parts, except the dryer, add gas to a certain pressure, and get out the sniffer, find the leaks, evacuate again, fix em, put more gas in, time test, sniff, recover, change the dryer, and final evac, then weigh in the correct charge!

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Okay, following some advice here and from PM's I've tried these things:

* Run +12v to the clutch to see if it engages (it does)
* See if there's power to the switch at the receiver/drier (there is)
* Then see if there's power from the switch (there isn't - either bad switch or low pressure, yes?)
* Turn the compressor to see if it turns smoothly (if I'm correct that turning the front plate of the clutch turns the compressor even when the clutch isn't engaged, it does turn smoothly).

Additionally, the two fittings I could find seem to be R-12 style (they're original with no adapters or anything, they look like tire valves) and there's no sticker anywhere saying the system has been converted to r134 like my wagon has.

So I've gotten probably as far as is worth going before taking it in to the shop. Am I correct in assuming my compressor and everything else is probably fine, and that all the shop (probably) will have to do is replace O-rings and the receiver/drier, track down any leaks, and recharge the system? Anyone know what that generally costs?

Thanks for your help everyone!

__________________
'81 300TD Wagon 355k (Miss Diesel)
'83 300SD 180k
'84 500SEL 190k (Parting it out)

Is there pressure at the schrader valve (push with pen)? If so, maybe low on freon only.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

You need to put gauges on it and check the charge. Odds are high that it is low on refrigerant. A properly equipped shop will have a refrigerant identifier and can verify if it contains R12. If you have not had personal control of the vehicle since the a/c was last operational then the only way to know what refrigerant is contained therein is with a refrigerant identifier.

>>unless you are using a digital micron gauge.

I think it's well worth using a vacuum gauge that can measure down into the hudreds of microns range. It's quite sensitive even to small vacuum leaks, and, you can also see if there's still moisture in the system.

If it was never modified, unless you can find a good indy shop they'll probably tell you it has to be converted to 134a - along with fixing any leaks. Done right, that should involve some sort of flush. R12 mineral oil is incompatible with 134a.

By all accounts, R12 is a better refrigerant for the 123. But it's harder to come by.

Listen the tech, but don't believe that you MUST convert. Read some more threads, maybe do some local Craigslist searches for R12, eBay, make some local phone calls.

You can do a lot of the O-rings and the receiver dryer by yourself. Maybe add a new expansion valve - or maybe not on the SD - to the list.

It can be done, I just did it on Sunday to mine. However, I think I did invent some new curse words doing it

My experience was not near as difficult as some of the write ups I have seen. I only had to remove the lower dash panel. You do need some crows feet. (SAE, not metric...3/4" and 7/8" IIRC)

__________________
----Adam

1985 300SD 322,000