Need Help with Strange ACCII Problem

[daimlerdude]

Hey Guys,

I was wondering if any of you could give me some help troubleshooting a problem I'm having with the crappy ACCII system in an 81 380sl I own.

For some reason, the compressor started running constantly whenever I push DEFROST button, even though the compressor button is OFF, and hot air is coming out the correct vents.

The compressor is also running constantly when I push the BILEVEL button, and the temperature is changing when I move the temperature wheel. So I know the wheel is working correctly.

I also know that the compressor button is working because the compressor is coming on when I push it and the AUTO HI or AUTO LOW buttons, and the center flaps are opening as they should. But in this case, pushing the compressor OFF button turns the compressor off, as it obviously should.

I've replaced the servo and the amp (twice) with units I know are good. But I've still got the problem, and I don't know where to look next. So any help that anyone could give me would be greatly appreciated.

Thanks very much in advance.

[meltedpanda]

Is the system fully charged? If so I would check the low and high pressure switches

[daimlerdude]

Thanks for the advice, MP. Could you tell me (1) where these switches are (2) how to check them and (3) why they could be causing the compressor to come on when I push DEFROST or BILEVEL.

I'm particularly confused now because I also posted what I thought was a problem in the TECH forum, and people over there said that the compressor is supposed to come on in BILEVEL to cool the air, and in DEFROST to dehumidify it. Here's a link to that thread:

Neede Help with Strange ACCII Problem

[rowdie]

I believe they are correct but I know nothing of your system. Here is a link on BW that may offer some help.
1978 450 ACCII Investigation/Repairs - Page 3 - Benzworld.org - Mercedes-Benz Discussion Forum

[97 SL320]

For most cars ( any make ) , the AC compressor will run in defrost to remove moisure. Sometimes makers haqve a override but that is maker specific.

[meltedpanda]

well it will come on in defrost but not sure about bilevel, I am under the impression that is was running all the time, which would indicate a pressure problem. best bet it to take it to someone familiar with older AC systems, not a DIY job without the right tools

[rowdie]

The 560SL (different system) indicates the bi-level setting is used for demisting. This indicates the compressor would come on to remove moisture from the cabin.

[UT_Tech]

The system is designed to operate the compressor in DEF mode. In this mode the ON/OFF switch is bypassed. The purpose is to dehumidify the air and thus clear the windshield as quickly as possible.

All of the modes run the compressor all the time, but only Bi-level and DEF bypass the dash switch. This behavior is by design. Also, the compressor is not part of the temperature regulation loop in any mode. The system is designed to regulate by adding a bit of heat through the heater core to "balance" the output of the evaporator. The system in your SL should cool and heat very well, the heat and AC components are the same as in much larger cars like the wagons.

[daimlerdude]

Thanks very much for supplying the definitive word on this, Steve.

I thought I knew this car extremely well. So I was surprised to hear the compressor running one morning while the defroster was operating, evidently because the compressor's starting to get noisy. Hopefully, though, it will last another 30+ years.

Take care.