The pressure switch is for the compressor clutch. The aux fan is controlled by a temperature switch, also on the receiver/drier. When operating as designed, the aux fan won't operate until the refrigerant reaches a specified temperature, which requires that the A/C compressor operate for some period of time before the fan would be activated.

The aux fan relay would be at the top of my list. (That might not be THE way forward, but it is a step in that direction.) Is it possible that your aux fan has been "re-engineered" to operate differently than the original design?

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

The monovalve is powered whenever the key is on, not just when A/C is selected.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Terminal 86 is the 12V side of any relay and is connected to 12V. In this case both the aux fan and the power window relay terminal 86s are powered from fuse 12.

Don't know if you have this, but section 24 has the wiring diagrams and shows the wiring for the various components powered by fuse 12.
Model 123 - OM616 Maintenance Manuals

If everything is working with a new fuse, it wil be hard to troubleshoot until the fuse blows again!

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

I hope you get to the bottom of this soon, electrical problems can cause serious damage .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Certainly nothing I've done, though in the past, I have noticed the fan seems to activate almost instantly.

Thank you for this. Pardon my inexperience, but as I have manual windows, you are referring simply to the Aux fan relay? Would unplugging the Aux fan be the same as removing the relay or is that too far downstream?

A five-pack of fuses just arrived from Amazon so I suppose I have about four shots at getting this right.

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1981 240D 4sp manual. Ivory White.

Looking at diagram, it looks like aux fan should only activate if refrigerant temperature is above 126DegF.

I would pull the aux fan relay and see if that helps. But if fuse has not yet blown, how would you then know if this helps? Disconnecting fan may help, but won't tell whetehr the problem is in the rec/dryer switch or the fan itself.

I didn't know you had manual windows. You said that fuse 12 powered the power windows relay, but if you don't have one, ignore that!

Sorry I can't be of more help.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Thanks for this. I guess I’ll know because the AC will be working and no fuse will have blown. That’s the confusing part, the AC compressor is not on fuse 12.

Apologies for the windows -I guess the fuse diagram on the box is w123 generic.

I have no idea how the AC system communicates. I assume there’s something in drier/fan relay system that talks to the compressor… ? When I’m idling, I can feel the compressor switch on, and then the fuse blows and my idle returns to pre-AC rpm.

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1981 240D 4sp manual. Ivory White.

Duplicate

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1981 240D 4sp manual. Ivory White.

Okay -got around to a little trouble shooting with a friend today. We began by removing the fan. Everything worked for about 20 seconds before the fuse totally melted. The AC compressor kicked on, cold air blew, etc.

So it isn’t the fan.

FYI, the AC compressor is on fuse #8

I’ve just removed the fuse box to have a look at that wiring.
Thought perhaps something from fuse 8 was making contact with 12, blowing 8 when the AC kicks on. Not sure that makes logical sense though, as the blower motor still kicks on, albeit sans compressor.

I admit I’m stumped.

What’s upstream of the AC switch on my climate control panel?
Is there a switch somewhere around the drier that can short?
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1981 240D 4sp manual. Ivory White.

Can a faulty relay blow a fuse? Not my area...
Will removing this relay prevent the AC compressor from engaging?

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1981 240D 4sp manual. Ivory White.

Anyone?

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1981 240D 4sp manual. Ivory White.

Wow! You may be making this more complicated than it is. You may have done so already but it doesn’t hurt to take a few steps back. Firstly there are countless places in the circuit that can cause a fuse to blow but only one reason(current exceeds capacity of the fuse). An overload is possible if you add loads to the circuit. Every part of the circuit has resistance including wires, connectors, relays, and loads. if a load has a rated resistance that requires 12 volts to operate, it may still operate at a lower voltage but it will draw more current. However, loads are typically not the weak link in a circuit. They function, don’t function, or function poorly. The weak links are all the connections in the circuit susceptible to physical damage. eg. pinched/broken wires, corrosion, or loose terminals. You don’t need a wiring diagram at this point but you should have a voltmeter, small wire brush, flash light, wrench, and screwdriver. Keep in mind the age of the wires, insulation, and number of times the harness has been pushed, poked, and prodded. All areas have been subject to corrosion and some more than others. The prerequisite for corrosion is moisture. All of these cars have been subject to humidity and many have had leaks. Multi stranded wires soak up any and all moisture like a sponge so areas that have been exposed to it are likely to have been exposed to water damage are places to focus. Short of rebuilding and replacing every part of the circuit, focus on the most likely places for resistance to occur. Always start at the power source to make sure you have 12.5-12.7 volts(loads off/engine off). Then observe exposed areas looking for signs of corrosion, loose connections, and broken wires, measuring voltage at every point that’s accessible. Don’t cut into or pierce insulation to get a measurement. If nothing’s obvious extend your view starting at the fuse block and inspect connection points, connectors, terminals, and wires, front to back. Next pull the instrument cluster and inspect the primary ground point on the column mount or chassis(many brown wires soldered and crimped into a few large ring terminals, fastened to the ground point). While you are there clean up the terminals and ground point with a small wire brush until they’re shiny. You’ll need to remove these terminals and clean both the hard point on the chassis and the terminal surface in contact with it. It does no good to leave it in place and clean only the top. If terminals are stacked, then it’s important to clean both sides. Finish up with a VERY light coat of your choice of electrical/corrosion coating or silicone dielectric then torque the fastener. Next check the connector to the instrument cluster for corrosion and the dimmer switch resistor. If the female connector is very corroded, cleaning it up is damn near impossible but electrical contact cleaner and compressed air might help. The male pins on the board can be cleaned up but be very gentle. A pencil eraser or small piece of 800 grit on a toothpick… it’s a tedious job.
Now you can remove kick panels and inspect the harness anywhere it contacts a hard point ( places wires are susceptible to damage from tears of vibration). Look for areas where the harness has been disturbed or altered by yourself or PO’s). If a problem presents after any job no matter how small or seemingly unrelated, 9 times out of ten it was caused or compounded during that process. If you’ve done a thorough job of the previous steps it’s time to roll up your sleeves a bit higher but 75% of the time you’ve already encountered the culprit before diving into more complex or harder to reach points in the circuit.

Thank you for such a thorough and thoughtful response!

Pardon my ignorance, but would you be able to explain how I’m to use my mulitmeter to check voltage in this case? I certainly understand cleaning ground points -that’ll be my first port of call. Though I don’t understand how to measure voltage or (load), other than placing probes on the battery terminals… for instance, is it useful to know just how much current is passing through the #12 circuit? It being a 6amp fuse/circuit?

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1981 240D 4sp manual. Ivory White.

Sherm ;

I'm not aware of any 6 ampere circuits in a W123..... might it be this simple ? .

I just had the devil of a time finding proper copper 8 & 16 ampere fuses, my brother's '87 300SDL's aluminum fuses are al corroded and he won't touch it so I had t go to a Porsche 911 partshaus to get the correct yellow fuses .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

My mistake, 8amp. My recall my not be, but all of the fuses are correct

Basically, that circuit appears to be fine or potentially on the brink using all components other than the aux fan/relay.
So either the fan relay takes it right over 8amps, or it’s near 8 amps because of some issue elsewhere in the system.

Is it possible to test the amount of current running through the 12 fuse before running the fan, and potentially after it blows?

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1981 240D 4sp manual. Ivory White.