AC Compressor won't come on.. - PeachParts Mercedes-Benz Forum

agood · #1 04-18-2003, 03:05 PM

My AC compressor will not kick on when I turn on the AC. Is there a low pressure cutout switch on these AC systems like on GM. Something I can bypass to get the compressor going enough to put gauges on it to see if there is any freon? Or is there some type of common control failure in the controls or relays that causes this type of problem...
Thanks.

joe p · #2 04-18-2003, 03:23 PM

year and model please.

Joe

agood · #3 04-18-2003, 04:52 PM

ooooops!! Sorry, It is a 1983 300sd.

#4 04-18-2003, 09:34 PM

Yes, there is a low pressure cutout, but don't jump it to take pressure readings. You can jump it just for a second to see if the clutch and compressor will turn, but jumping it long enough to see if it will build pressure will operate the compressor without oil and will not be healthy for it.

Put on the gauges and check static pressure first. It should be in the area of 70 or 80 PSI depending on temperature.

Good luck,

agood · #5 04-20-2003, 11:30 AM

OK,
I had to jump the cutout just to get the compresor to come on. The readings led me to assume that I was low on Freon. SO I added some and the system started working again.....TILL today and now I am back to square one.....It won't kick on again....I must have a leak...I there a common place where these units tend to spring leaks? If not what is the best way to track down a freon leak?
Thanks
Brian

drbrandini · #6 04-20-2003, 12:28 PM

Larry can probably help you better, but you can purchase a leak detector which will run you a few hundred. You may want to take it to an indy if you can't get your hands on one. THe best place to check is the evaportator and the welds in the piping. Good Luck.

jbaj007 · #7 04-20-2003, 02:52 PM

Most COMMON leak location on a car with an R4 compressor (300SD) is the shaft seal, but it is usually slower than yours. The size of yours would indicate a large enough leak to find oil residue or the old soapy water trick should work. Heck, you can probably hear your leak. Look for a hole in the condensor from road debris (rock).

#8 04-21-2003, 07:40 AM

Yes, it sounds like you have a significant leak. Although compressor shaft seals are common leak sources, there are many other possibilities.

I prefer UV dye for tracing down leaks. Put in the UV dye and then use a black light to look for the leak.

In your case, however, the leak is significant enough that you may find an oil residue around the leak. Check all connections under the hood and lower the passenger side kick panel and look around the expansion valve behind the glove box.

Good luck,

agood · #9 04-21-2003, 02:19 PM

Thanks Larry,
I may need to clarify exactly what I did. I bypassed the low pressure cutout switch and connected my gauges. There was very little pressure on the high side....so I decided to add freon R134a. I only had one can so I put it in and the high side started coming up and the lines started feeling cool. I drove it for a few miles and it was cooling but not as cool as it should be. I just figured it needed another can.
The next day when I turned on the AC the compressor would not come on......SO,
If I only used one can, would that still constitute a "big" leak if it leaked back to triggering the cutout in 24 hours?

Thanks
Brian

#10 04-21-2003, 03:06 PM

Yes, that's a pretty significant leak. Since it is a converted 134 system, get a can of 134 with UV dye. Read the fine print and make SURE that the dye has Ester oil rather than PAG oil. PAG should not be used in converted systems.

Once the dye is in with enough refrigerant to cool for a little while, use a black light to look for the leak. If you don't find it at any of the joints or the normal places, look at the condensation drain water. If there is UV dye in the drain water, then the evaporator is leaking.

Good luck,

agood · #11 04-21-2003, 08:18 PM

Larry,
I got 3 more cans of R134a, one with the UV dye in it and put 2.5 cans in the car. It has been running for almost an hour now and I cannot find a leak using the UV light. I think I found the expansion valve under the driver's side dash and it shows no signs of leaking....all under the hood looks good to....so the only place left is the evaporator....and it too cool here tonight to generate any condensation......
How hard is the Evaporator to replace?

Also I noticed that the door the opens behind the glove box that allow inside are to be recirculated, was not open...when does this open? I have the controls sent to "min" and the blower on high.

Also what should the PSI be on the high side when it is about 68 degrees outside.....

Thanks
Brian

#12 04-22-2003, 08:02 AM

Run it a few more days, or until the refrigerant leaks out, then try again with your UV light. Check around the compressor pulley where it would sling out on the pulley. Check everywhere you can get to and wait 'til you can get some condensation on the ground, preferably at night to check the evaporator.

Patience my friend.

I don't know how hard the evaporator in a 126 is to do. The 124 is near impossible, the 123's are difficult and the 201's are a piece of cake. I don't know where your 126 falls. I'm sure someone here has done one and can offer a reply.

Keep on checking with that black light, preferably after dark, you will find the leak sooner or later.

Good luck,

agood · #13 04-22-2003, 09:19 AM

Thanks Larry,
I will keep checking.....I might just be chasing my tail though. Last night with 2.5 lbs of freon in the system it still did not seem to be cooling (only 60 degree air from the vents).....I had almost 200psi on the high side at idle but the los side did not seem to be coming down like I have seen before.....Wondering if I have something else wrong....
What do these readings usually mean???
The compressor looks like a GM compressor, is there an orifice tube like in a GM system....could these reading indicate a blocked tube....?
Thanks
Brian

#14 04-22-2003, 09:30 AM

A restriction will cause the low side to go down, even to a vacuum. This is an expansion valve system, no o-tube.

Since it has emptied, are you sure that there is no air in the system? You may need to evacuate thoroughly and recharge. But, for now, you have some dye in the system, so find your leak, repair your leak then evacuate and charge.

Good luck,

agood · #15 04-22-2003, 09:54 AM

OK will do....

What is the best way to flush the system if I have to open it.....I just want to clean it good and start fresh...
Thanks