chasing electrical gremlin with AC - Mercedes-Benz Forum
 
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post #1 of 9 (permalink) Old 06-25-2019, 01:34 PM Thread Starter
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Date registered: Oct 2016
Vehicle: 1973 w114 280, m110 engine
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chasing electrical gremlin with AC

So recently i noticed that my aux fan (the one in front of the condenser) doesn't kick in when the AC was on. I connected the fan directly to the battery and it is working fine.
Today i cleaned the fuse connectors (the ones on the firewall that are for AC only) and replacing the fuses the AC doesn't turn on at all (was working just minutes before i started the cleaning). I put the vent fan on and it blows but the moment i turn the AC knob, the vent fan turns off and the clutch doesn't engage either.

Anyone can help me figure out where to start diagnosing this electrical gremlin?
The AC was working last summer no problem.
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post #2 of 9 (permalink) Old 06-25-2019, 04:31 PM
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Check the change-over micro switch on the side of the center console. It shouldn't affect the compressor operation but many of the A/C systems in the earlier years were dealer installed units, the wiring can be a bit different.

It's been a while since I dealt with my W114's but I seem to recall the A/C compressor and condensor fans are powered from a separate relay which was mounted on the fender well. Check the fan by grounding the 100C sensor. The fan should engage with the ignition in the "run" position. The sensor is mounted in the cylinder head of your M110 and looks identical to the temperature gauge sending unit.

Last edited by Mike D; 06-25-2019 at 04:42 PM.
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post #3 of 9 (permalink) Old 06-25-2019, 07:02 PM Thread Starter
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Here is the AC diagram from the manual: http://patrykrebisz.com/pics/AC_diagram.jpg
And AC wiring diagram: http://patrykrebisz.com/pics/AC_wiring.jpg

What i'm trying to understand is how is it possible that clutch, nor aux fan, nor cage fan inside cabin is getting any power (none are working). It seems they are all controlled by different components. As i said earlier the heater fan is working until the AC knob is turned. Anyone who knows electricity could point me in right direction?
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post #4 of 9 (permalink) Old 06-26-2019, 03:16 AM
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Per your diagram, the W114 aux fan is just like my W108 fan. It only comes on if either the coolant temperature is over 212F or the refrigerant temperature leaving the condenser is over 52C (125F). My aux fan never runs unless I'm stopped in traffic for more than ten minutes in hot weather because my manual fan keeps both of those temperatures below the setpoint. I think your aux fan may be ok if it runs when you connect it direct.

As for the clutch and blower not working, the common link in that chain is the changeover switch.
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post #5 of 9 (permalink) Old 06-26-2019, 10:53 AM
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First things first. Don't jump to suppositions when dealing with electrical systems. Start at the beginning of the circuit and follow it to the end.

Check the fuse which supplies power to the blower systems. I believe it is the #8 fuse in the fuse box. It is probably fine but check it anyway. With the key in the "run" position check for power on both sides of the fuse. Rotate it and check for corrosion. If it is one of the aluminum strip fuses now is a good time to change to a copper one. Power to both sides?

As per your diagram 83.2, locate the relay labeled #9. This is/was a metal canister and is usually mounted on the fender well. I don't have my /8 here at the moment so I can't post a picture. This is an "old school" type of relay and doesn't look like the newer style. If you feel up to it, now is a good time to replace that old can with the newer type.

This is the relay which is activated by the temperature control knob. With the key on to run position, temperature switch to off,check for voltage to pin/terminal #30. The numbers are stamped/marked on the relay. Power?

With the key still on, turn the temperature switch to "colder", check terminal #30 again. Power? Now check terminals #86 and 87.

Here's a link to help you understand relay circuits.

https://www.hagerty.com/articles-vid...-relays-part-3

That's enough to get you started. I'll monitor your progress and try to guide you in the proper direction.
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post #6 of 9 (permalink) Old 06-26-2019, 11:20 AM Thread Starter
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Mike, as always thank you for thorough response!
I ended up finding out that my changeover switch (7 in diagram) doesn't work properly not feeding power to plug 6 when switched (thus my AC fan not kicking in).
I'll probably wait the few days it will take for replacement switch to get here before ripping further into the dash.
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post #7 of 9 (permalink) Old 07-01-2019, 05:21 PM Thread Starter
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Just a quick update.
Replaced the changeover switch and the AC fan + clutch kicks in.
I grounded the wire going to the temp sensors on the right side of the engine bay and the aux fan kicked in (after i replaced the connectors that looked corroded). This meant that the relay controlling aux fan worked. When testing the changeover switch, the aux fan kicked in after the engine got warm - so changing the connectors was the answer.

Bottom line: old cars and simple things start going wrong due to old age.

I put up the wiring diagrams with electricity trajectory on my blog for those who might need it in the future:
w114 Mercedes - AC wiring diagram explained
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post #8 of 9 (permalink) Old 07-03-2019, 07:55 AM
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Thanks for the update. It helps other people when you let them know what worked.
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post #9 of 9 (permalink) Old 07-03-2019, 10:30 AM Thread Starter
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Also, out of curiosity i opened up the old changeover switch. The arching at the contacts going to the clutch and the AC fan produced tons of carbon build up so there was no electricity flow. Again - old age...
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