Need AC Help... - PeachParts Mercedes-Benz Forum

longston · #1 05-20-2001, 06:08 PM

I have searched the archives, but can't find a similar enough problem that was of any help for this 1982 300SD.

Here's the problem. The AC worked fine in Oct. when I bought the car. I put it in the shop back then to have the valves adjusted and some minor work done. Then I installed a new stereo/CD. That's all that was done to the car so far.

Since then, it got hot enough to need the AC, but now, I don't get any air at all from the center vents, and I get no cold air anywhere in the car. And, the side vents don't blow as much air as they used to.

I don't have a manual, or a CD, but decided to take out the stereo this weekend to see if I has disconnected something by mistake, and to remove the center air vents to check the function of the vent control. Well, no luck so far. There is nothing that seems out of place or disconnected, and the center vent control is opening & closing the door just fine.

Also, with the engine running, and the climate controls set for maximum AC, I am getting no voltage to the wire(s) connected to the compressor.

Any suggestions or assistance would be appreciated. Where can I start to diagnose this problem?

engatwork · #2 05-20-2001, 06:26 PM

is not coming on longston. My troubleshooting procedure went like this on my project 300D. (This procedure applies to the vehicles with the Klima relay).

1) Make sure the system is charged. This can keep the compressor from coming on. Get it charged if it is low.

2) Check voltage on both sides of the pressure switch at the dryer. If you do not have 12 volts on either side then you will need to get a little deeper into the control circuit. After installing a new compressor I could make mine come on by supplying 12 v power (through a toggle switch) on one side of the EVT switch. This provided power through the freon pressure switch circuit which pulls in the Klima relay suppling power to the compressor. This led me to replace the pushbutton unit (purchased from Fastlane.com). Seems on the 300D the power source for the pressure switch is influenced by this unit. Also, underneath to the right is a relay with a fuse. There was no problem with it.
hopes this helps some with your troubleshooting
Jim
'85 300D
'95 E320
'97 CRV

longston · #3 05-20-2001, 08:15 PM

I do have 12v at one side of the pressure switch at the dryer. But no voltage at the other side. I assume that this is normal. Can I safely short the leads bypassing the switch, and will that possibly make the compressor come on?

As for the center vents, I found a vacuum pot under the dash on the driver's side just to the right of the accelerator pedal that seems to control the air to those vents. When I disconnected the vacuum line, and used my mity mite to provide suction, it opened the valve, and I got air through the center vents. The vacuum line going to it is green, but doesn't seem to be producing any vacuum.

So, what supplies vacuum to that line, and why am I not getting any there?

engatwork · #4 05-20-2001, 08:42 PM

have power on both sides of that pressure switch for the compressor to come on. The switch will make up at the proper pressure. You can safely short these leads just long enough to see if the compressor comes on. If the compressor comes on when you bridge it then get the system evacuated and recharged with freon/dye. Find a good independent a/c shop where you can go in and talk with the guy doing the work. After a couple weeks running with the dye any leaks will show up and then you will need to resolve the leakage issues.

The vacuum can get a little more cumbersome because of the limited spaces underneath the dash. You will want to try to get a copy of "Electrical Troubleshooting Manual" for your model if it is available (call the 800 MB number to see if they are available). This manual has really good schematics showing the vacuum and electrical systems on the 300D.

When you say you put a vacuum on the pot to the right of the accelerator pedal I suspect that you are "closing" the defrost vacuum pot which forces air through the dash vents when it is closed off. I got in there and just dug around and repaired leaks/problems as I could get to them. After getting everything working as well as it could I finally just tie-wrapped the defrost vacuum pot in the almost closed position. I want all of my cold air coming through the dash vents.
Jim
'85 300D
'95 E320
'97 CRV

longston · #5 05-20-2001, 09:11 PM

While I was waiting for further inspiration, I went ahead and shorted those two wires running to the dryer. To be safe, I used a 15A fuse to do the job. Lo and behold, the compressor kicked on, the vacuum pot opened, and I had air flow through the center vents.

I am assuming, baring further enlightenment, that the switch closes when there is sufficient pressure in the AC system, and in so doing, supplies voltage to the other lead going to (from?) the pessure switch.

I guess all I need is a recharge of the refrigerant, and to see where the leak (if there is one) is located. Correct?

engatwork · #6 05-20-2001, 09:14 PM

make sure you get it evacuated properly and recharged. There was a pretty long thread on here a couple weeks ago concerning the evacuation of the system prior to recharging.
Jim

#7 05-20-2001, 10:40 PM

Scott,and others,
The switch on the side of the receiver/dryer is a low pressure cutoff switch. It's a safety feature that shuts the compessor off when the refrigerant gets low. It's supposed to prevent oil from being sucked out of the compressor. The early MBs with AC did not have this switch. After MB started adding this switch, I had quite a few people come to me and say their 'mechanic' told them that they needed a new compessor because they could not get the compressor to run so they could add refrigerant.
I have a short jumper wire with spade connectors on each end, that I use to bypass this switch. After about 12 ounces of refrigerant has been added to a completly dry system, the jumper wire can be removed and the switch reconnected...
Bill Lewallen; Home of The Rolex Three Day Event...

stevebfl · #8 05-20-2001, 10:44 PM

Yes, you will need some pressure to make it work.

I just want to point out one thing. Jim's car is the RARE case. Most cars with Klima relays use a ground signal from the push button controller. When working properly the voltage coming from the compressor relay is pulled to ground to activate it.

If one were to apply 12v to this circuit it will take out the PBC PC board. There are way more models that use ground signals than use 12v signals. Jim has a one year situation with the 85 300D. Earlier cars (such as yours)don't have the klima relay and the power to the compressor actually passes through the low pressure switch.

Just be carefull if you want to jump this circuit. (This is a general warning not specific to this case)

longston · #9 05-20-2001, 11:26 PM

After Jim's explanation, I had figured that the switch on top of the dryer was a safety item added to prevent damage to the system once the refrigerant was depleted. I also figured that using a spade-type fuse was the safest way to jump that circuit. I do appreciate Bill's heads up on having that to make the compressor work when recharging, and Steve's clarification to others with regard to model differences.

So, if I have my problem correctly figured out, all I need to do to get my climate control to work properly is to evacuate the system, recharge the refrigerant, add a tracer dye when doing so, and then look to see if there are any leaks, then rectify the leakage issue to have a perfectly sealed and correctly functioning climate control system?

Beats the hell out of having to spend $500 to have someone at an AC shop diagnose the problem, and tell me the same thing.

My heartfelt gratitude will increase during the summer in proportion to the temperature...

Ron Johnstone · #10 05-21-2001, 05:15 PM

If you have to completely refill the AC, I'd give some thought to switching to R134. You can then do it yourself and it will be much cheaper to do so than working with R12. Quite a few threads on the subject.

longston · #11 05-22-2001, 01:23 AM

I spent a lot of time online and on the phone today about R12 vs R134. I read every thread about the conversion I could find, called my local independent MB mechanic, called my other independent mechanic, and even spoke to a guy at H&H who handles west coast distribution for Johnsen's Freeze 12.

I still don't have an answer. The R12 is about $60 a Lb., the R134 is about $5.99 a Lb, and the Freeze 12 is about as cheap as the R134 @ $6.99.

Problems seem to be that the R134 doesn't cool as well, and will, given sufficient time kill the compressor on older systems not equipped to deal with it's higher pressure. AutoZone has the conversion kit to R134 for about $35, but my other mechanic says that I will get about 6 Mos before the compressor will die. He suggests staying with R12.

The rep for Freeze 12 says that it is fully compatible with R12, that I could just add it to my system without a purge and evacuation of the system, and it would be likely to run for 2-3 years even with a minor leak before it needs replenishment. But Freeze 12 is 80% R134 & 20% 142b. I was told by him that the 20% R142b eliminates the corrosion factor, and makes it compatible with the R12. I have a can of unused R12, and he suggests just adding it to my system as is, and running the system through the summer or as long as it will last.

The Mercedes mechanic wants to do a diagnosis for $117, retrofit the system for R134 @ $200.00, plus the cost of repairing any leaks. The other mechanic says the R134 conversion is not as good as repairing the system, and filling it with R12 at a cost of about $180 for freon, plus about $80 labor.

So, barring any further research (unlikely in my case), do I add the R12 I already have to the system, or have the system inspected and pumped down, then add the R12 with a dye to look for leaks, or do the conversion over to R134 or to Freeze 12?

Bottom line, is the 1 can of R12 I have sufficient to get the system to work? Or am I better off using it to augment the recharge of R12 after inspection and sealing?

Oh, but what about the combination marker dye and AC system sealer you can add to the system? Does it work?

So many questions, so many options...

jackmac · #12 05-24-2001, 05:41 PM

I also bought my car in the winter and the a/c worked fine. Come the summer, no a/c from the center vents and hot air from the sides. My tech evacuated the system and everything worked well, although the system does leak. He said he found two leaks, but that it should hold out for a couple of summers if I didn't use it a lot (honest guy!!). It seems when the system is weak, it doesn't have enough behind it to blow through the center vents. I had this done last year and it still works, leaks and all. Keep in mind, these old MB's aren't going to freeze you out, even when working 100%.

Ron Johnstone · #13 05-24-2001, 06:23 PM

I've been advised that with any A/C, you should run it for at least a couple of minutes a month to keep the seals in good shape - winter or summer. It might eliminate those "summer surprises" on the first hot day. It's always worked for me.