1983 380SEC climate control works perfectly - except that when I select low speed on the blower pushbutton it behaves as if the automatic speed control is selected. On automatic the speed varies as expected, fast initially to cool the car, then slower with the set temperature is attained. Max speed blasts air.
I thought the pushbutton unit was not latching at slow, so I installed a new one, with no change. So now I suspect something in the control unit, relays, resistors... I have done some searching but not found anything that matches my problem.
So please let me have suggestions for the source of the issue, and some logical troubleshooting steps. TIA.
The problem is the resistor under the passenger side duct work. Porcupine or blower motor resistor. Simple task 145 new for behr. Odd your ccu isn't working, did you try to get low speed via heat? Get the resistor if you buy, from a place you can return it. Maybe cheaper to buy new ccu or rebuilt
Also not sure of your aged gen 1 but if retro fitted with a strip fuse outside the fuse box In line, check that first it's the cheapest
Is it a true resistor, or a regulator? Seems this part goes by many names, with many variations in design and operation. If it's inside the cabin I expect is to be a regulator. Anyway, I'll get in there and look.
Not sure I understand your comment about the ccu. I think mine is working properly.
If the strip fuse is the problem, then the blower will not work at all - correct?
I'll study it more tonight, though my understanding in the GenI the ACC unit drives the resistor strip which is a row of 6? resistors, one for each speed. The buttons bypass the ACC using two of the resistors, I believe numbers 1 and 6. You can go in and apply 12 volts to the strip at each resistor to confirm that each speed is working. The ACC could be giving you multiple speeds but just not the speed that the low button uses. On the surface sounds like the ACC is working. Will follow up tonight after I study the schematic.
Okay, so in max button mode you get speed 6, in automatic you get speeds 2-5 and in low you get speed 1. So when you push low speed you are just getting the automatic which is the default mode. Basically the button is not doing what it should. Definitely looks like it is the button itself though you have eliminated that theory. That leaves the electronic unit for blower speed control (electronic switchgear for blower control). Seems unlikely that this unit is working fine except for controlling the low speed. It would not be the blower resistors since if the resistor for speed 1 failed then you would get no blower on speed one, not the automatic. You could test the unit in place by jumping a 12v source to pin 2 of the control unit which would simulate what the switch is suppose to being doing. The other way of thinking about the test would be to see if pin 1 of connector C1 is putting out 12v when low button is pushed. I don't know how incline you are to do that kind of testing. The other option is something in the ACC that the button connect into has failed. Another easier test would be to check continuity of the button connections. I am not too sure what the buttons unit off of the ACC looks like. Hope that is helpful - can talk it through on phone to if needed.
Basically the button is not doing what it should. Definitely looks like it is the button itself though you have eliminated that theory. That leaves the electronic unit for blower speed control (electronic switchgear for blower control). Seems unlikely that this unit is working fine except for controlling the low speed. It would not be the blower resistors since if the resistor for speed 1 failed then you would get no blower on speed one, not the automatic. You could test the unit in place by jumping a 12v source to pin 2 of the control unit which would simulate what the switch is suppose to being doing. The other way of thinking about the test would be to see if pin 1 of connector C1 is putting out 12v when low button is pushed. I don't know how incline you are to do that kind of testing. The other option is something in the ACC that the button connect into has failed. Another easier test would be to check continuity of the button connections. I am not too sure what the buttons unit off of the ACC looks like. Hope that is helpful - can talk it through on phone to if needed.
Liam, thanks for the thought and care you have put into helping me. First, I want to confirm that the low setting is indeed acting automatically, and not just at some 2 - 5 speed. So I will observe its behavior carefully over the next few days. I am reluctant to start jumpering connectors, because they are difficult to access and also I don't want to break anything more!
Are you sure a resistor failure would result in no blower? I assume there are two modes of failure - no continuity or dead short? Depends of the design of the "resistor".
I can test the continuity of the button connections on the one I removed - see attached pic. The connectors are in the six round holes, which mate up to the ccu.
To 281lxm, the blower runs when low is selected, just not at the lowest speed.
so yes if the strip fuse is out, you get nothing. The gen1 cars might have been retrofitted with a strip fuse, some have not. Porcupine is in the car, and if the CCU (push button) unit isn't giving you any fan on the low setting (you will see fan moving once you unscrew the access to the regulator). If it isn't moving, perhaps the button is shot and needs to be soldered or just replaced.
On the w124 E500, to check the voltage at the blade fuse (if you have one on your car) turn on the high fan with ac with key on or engine running. If you don't have volts or they are low volts at the strip fuse the ignition switch might need replacing as well. You can also test voltage from the CCU to the rest of the blower/regulator. I think the power wire off the CCU is yellow and it plugs into the harness going into the fan assembly. Check voltage there and if it is real low with fan controls on low and hi then it may be the CCU unit(or the offending button)
The more I think about it it must be the button or the route of the button through the ACC to the blower controller. The way the switch works you get automatic if the low button isnt pushed. Yes, test your old one to see if you are getting continuity on the low switch. I'm betting your old button works. Since you dont want to just buy parts as your test you want to do some sort of test. Either test continuity on the back of the ACC as a whole or it would be less work to pull the blower control unit and test if you are getting power to it where you should. It is under the kick panel and you would pull the unit and test the socket. A dead short on the speed 1 resistor would give you full speed blower when you push the low speed button.
When you select "auto" the switches don't make any connections. Selecting Low or High do make connections, so, if you select Low and get Auto the the Low button isn't making its connection or the wire it connects too is broken. If High works then it has to be the switch or the Low speed wire, which should be Yellow with Red stripes.
I've found controls start working correctly after a resolder of the AC circuit board. If you open it up and it the joins look milky, that could be the problem. Be aware, if you don't do it well, you can lose the AC functionality all together. Since the low setting is most likely used, it can get the most wear and tear. If previous owners hit it hard, the solder joint could possibly even be fractured.
I hate to leave a thread open, even if there is not resolution. My blower was becoming increasingly erratic. However, I found my problem and fixed it. alexstorm was on the right track.
This website, Mercedes w123 Maintenance: Automatic Climate Control Unit Repair was the final clue. I removed the climate control unit (Kent has a useful video at ) and removed the bottom cover. Contrary to first website, I did not need to remove the pushbuttons, temperature wheel or blower buttons. But if you need access to the top side of the unit, you would need to do so - I did not.
Carefully examining the the solder joints with a magnifying glass, I concluded that three had cracked where the blower pushbutton side circuit board connects to the main board. I reflowed all the connections with a soldering iron, including the ones that appeared ok. Just to be thorough, I did a few more on the main board that looked iffy, plus some additional side connections. I was a little nervous to do this, because I am firmly of the philosophy of "If it's not broken, don't fix it." However, all looked good.
Reinstalled the CCU, replaced a burnt-out bulb and closed everything up. Lo and behold, it now works perfectly! The cracked solder joints must have been making intermittent contact, with random results, making diagnosis difficult, but easy in retrospect.
Thanks once again to everyone who took time to help. I hope this will help someone else.
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