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  #1  
Old 11-04-2016, 07:46 PM
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PLZ help diagnose my 300d ACC

OK, I just recently bought a 1984 300d with 212k on it. When I bought it the previous owner described (briefly, possibly incorrectly) what the situation was with the climate control system. He said that the push button unit had broken and so a toggle switch had been installed to turn the system on. The AC does not blow cold at all. The compressor clutch will not engage at all when attempting to charge the system. I read that I might would have to jump 2 connections on the dryer to get it to engage. I did this while attempting to charge. Still no dice. The heater works fine as well as the temp control wheel. None of the other features on the push button head work. To use the heat I flip the bypass toggle switch and then adjust the temp wheel accordingly. I pulled the push button unit and inspected the internals and all the solder looks very intact so I am finding hard to believe it is the control box. Also, no matter how far down I set the wheel there is always slightly warm air coming from the vents. What has been bypassed and what part(s) need repaired to get the compressor ticking?

Also, I am a big proponent of searching forums for the answer and I have not seen any. So before you shoot back a "try a search, moron" response, please know that I have and to no avail.

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Old 11-04-2016, 07:55 PM
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If, when you toggle the switch you get 12 volts to the compressor and the compressor does not come on then you have a compressor issue. Check to see if you are getting 12 volts. The pushbutton unit can cause issues keeping you from getting 12 volts to it but with the toggle you should be able to make it happen if that is what it was installed to do.

Can you turn the center part of the compressor by hand without the engine running? If you can you have a compressor issue and possibly a system full of crap that needs to be blown out type situation.
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  #3  
Old 11-04-2016, 08:29 PM
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A few other things to check are the monovalve (a torn diaphragm will cause you to have constant heat - and sometimes the monovalve just doesnt want to work right). Theres also two boxes behind your glovebox that need to be working properly. Those are not particularly prone to failure though. There is also a cabin air temperature sensor that has a foam tube connecting it to the evaporator housing. That tube needs to be intact and so does the circuit board inside the sensor (though the sensor is overridden in max hot or cold settings i think)
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  #4  
Old 11-04-2016, 09:32 PM
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If you apply 12 volts directly to the compressor you should hear a clicking sound (engine off). If there is no clicking sound this means that the compressor clutch is bad. If you do get a clicking sound but the compressor will still not engage you have a bad compressor.
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  #5  
Old 11-06-2016, 11:32 PM
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Thanks for all your helpful replies, guys. I have not tested with a multimeter yet. But, I turned the key and bypass toggle on and there was no clicking. So, either I have to replace the compressor (not replacing the clutch, been there, done that. Too much time and crap to go wrong) or I need to know what to do to fix a loss of voltage. In the latter situation, how would I fix that? Would a secondary toggle to provide 12v to it do the trick?
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Originally Posted by Demothen View Post
A few other things to check are the monovalve (a torn diaphragm will cause you to have constant heat - and sometimes the monovalve just doesnt want to work right). Theres also two boxes behind your glovebox that need to be working properly. Those are not particularly prone to failure though. There is also a cabin air temperature sensor that has a foam tube connecting it to the evaporator housing. That tube needs to be intact and so does the circuit board inside the sensor (though the sensor is overridden in max hot or cold settings i think)
Ok, I am not sure if it is constant heat or just warm ambient air venting in from near the engine. I will find out when the air chills down here. Out of curiosity, where is the monovalve? I am a novice to these systems. Thanks again.
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  #6  
Old 11-07-2016, 01:03 AM
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Are you sure you have 12 volts at your toggle switch? The problem with providing 12v to the compressor via a toggle switch is two fold: does not allow the compressor to cycle off and on as designed; if the toggle switch is wired to have 12V constantly and you forget to turn the switch off when the engine is turned off the compressor clutch will overheat and eventually the clutch will become defective.
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  #7  
Old 11-07-2016, 11:33 AM
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Quote:
Originally Posted by BWhitmore View Post
Are you sure you have 12 volts at your toggle switch? The problem with providing 12v to the compressor via a toggle switch is two fold: does not allow the compressor to cycle off and on as designed; if the toggle switch is wired to have 12V constantly and you forget to turn the switch off when the engine is turned off the compressor clutch will overheat and eventually the clutch will become defective.
I think a compressor clutch coil is designed to be constant duty, it should not overheat and burn out if left on. The problem leaving the toggle switch on is you will run the battery down if the clutch coil is wired to a always hot 12V source.

Easy to check the compressor clutch: Engine off, use alligator clips to supply 12V directly to the clutch connector on the compressor, you should see and hear the clutch pull in.
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  #8  
Old 11-07-2016, 11:42 AM
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Quote:
Originally Posted by BWhitmore View Post
Are you sure you have 12 volts at your toggle switch? The problem with providing 12v to the compressor via a toggle switch is two fold: does not allow the compressor to cycle off and on as designed; if the toggle switch is wired to have 12V constantly and you forget to turn the switch off when the engine is turned off the compressor clutch will overheat and eventually the clutch will become defective.
If you're referring to the toggle switch already in place then, yes it is live because the fan kicks on when I flip it on. If I were to place a secondary toggle for the compressor it would be connected via the ignition to prevent messing up the clutch with the motor being off. However, how would I go about getting it to cycle off and on the way it is supposed to? I imagine I would need a thermostat of some type to place in the cab. Is there a way to pull this off without buying a new ACC control head? Anyway, it is all speculation until I can get down there and test for 12v at the compressor. I am hoping that it is just a very old compressor.

EDIT:
Quote:
Originally Posted by funola View Post
I think a compressor clutch coil is designed to be constant duty, it should not overheat and burn out if left on. The problem leaving the toggle switch on is you will run the battery down if the clutch coil is wired to a always hot 12V source.

Easy to check the compressor clutch: Engine off, use alligator clips to supply 12V directly to the clutch connector on the compressor, you should see and hear the clutch pull in.
So are you saying if it is a voltage issue I could place a toggle on it through an ignition switched source and be good to go?
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  #9  
Old 11-07-2016, 11:57 AM
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Quote:
Originally Posted by mthayes View Post
.......

So are you saying if it is a voltage issue I could place a toggle on it through an ignition switched source and be good to go?
Yes, but then the compressor will not be controlled by the temperature dial and other safety features.....not recommended. Better to find out what's wrong with the system and fix it right.

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