1972 250 A/C problem - Mercedes-Benz Forum

 
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post #1 of 4 (permalink) Old 08-28-2010, 01:47 PM Thread Starter
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Date registered: Jun 2010
Vehicle: '72 - 250
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1972 250 A/C problem

Does anybody have any experience with the Temperature/AC switch and the blower switch? When I turn on just the blower switch, it blows fine and has the 3 different speeds. Then when I turn on the Temp/AC switch the blower shuts off and nothing happens.
I used contact cleaner on switch contacts and that helped the switch action move easier, but, otherwise made no difference.
I checked all the fuses that I could find and didn't see anything unusual.
Anybody have any ideas?
Thanks.
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post #2 of 4 (permalink) Old 08-30-2010, 10:17 AM
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Originally Posted by Miss Daisy View Post
Does anybody have any experience with the Temperature/AC switch and the blower switch? When I turn on just the blower switch, it blows fine and has the 3 different speeds. Then when I turn on the Temp/AC switch the blower shuts off and nothing happens.
I used contact cleaner on switch contacts and that helped the switch action move easier, but, otherwise made no difference.
I checked all the fuses that I could find and didn't see anything unusual.
Anybody have any ideas?
Thanks.
Yes. The 250 C (all W114 and W115 cars with air conditioning) have a separate fan for the heating system and a separate fan for the cooling system. The fan switch that you are turning controls both of them. WHICH fan it controls is based on the position of the temperature/AC switch. If you were to take off the temperature switch you see that it contains (in addition to the temp stuff) a vacuum switch. The vacuum switch activates in one direction when the knob is COMPLETELY to the left, and the vacuum switch activates in the other direction when the knob is NOT COMPLETELY to the left.

Result: temp knob completely to left, vacuum switch causes a vacuum actuator to move a rod that directs air a different direction AND it causes the fan switch to control the other fan. When knob is to the left, the HEATER fan should be in operation, which is the top fan and the one that is extremely difficult to get to. Does your heater work? Meaning when you set the car to heat and turn on the fan does it blast out hot air? If it does, this is a good thing.

When the temp knob is NOT all the way to the left, the vacuum actuator moves a rod that changes an air flap and also causes the fan to operate the AC fan. The ac fan is down low, with air intakes on either side of the center console and on the front (under the ashtray?). Is that the fan that doesn't come on, or is it the heater fan that doesn't come on?

As far as I know, the AC fan is much easier to get at than the heater fan.

So it has nothing to do with the fan switch itself. It has either to do with the ac fan or the vacuum controlled switch that moves the fan control to the lower fan. If you were to stick your head by your gas pedal and look up with a flashlight and have someone move the temp knob from off to not off, you should hear/see a vacuum actuator move. The switch is at one end of the rod that is moving I think...

Scott

Currently: 1967 250 SL, 1963 220Sb, 1965 300SE Lang, 1971 280SE parts car, 1972 Alfa GTV, 1965 Alfa Duetto, 1993 BMW R100R
Past: 1971 250 C, 1985 300 TD, 1967 250 S, 1968 280 S,1981 300 D, 1982 280CE, 3 Facel Vegas, Borgward Isabella Coupe, Alfa 2600 Sprint, Volvo P1900 (yes), numerous less interesting Volvos, ...
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post #3 of 4 (permalink) Old 08-31-2010, 10:23 AM Thread Starter
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Thanks for your response

Thanks for your response Scott, I'll check it out and let you know the out come..
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post #4 of 4 (permalink) Old 09-05-2010, 10:48 AM Thread Starter
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Okay, Scott, I checked it all out. Everything that you mentioned seems to be working fine. I took out both the temp/AC switch and the blower switch and cleaned them up and made sure they were functioning right. They seem to be working fine. The temp/AC switch does actuate the vacuum switch, but then nothing happens. Finally gave up and took it to the local "A/C guru" and he said it was in the dryer high pressure switch. So he bypassed the switch and it all worked just fine. Got two blocks from the shop and it all shut down and went back to where it was....when you turn on temp/AC switch, everything shuts down. So-o-o back to the shop it will go on Tuesday. I'll let you know what the outcome is.
Thanks again for your help.
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