No, on my list of 'to do' things tomorrow...although I am having trouble tracking down where the fan connects where...

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RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

The easiest way to test the fan is to jumper the fan relay connector. Assuming, of course, that you have power to that point and the circuit downstream is complete.

The fan plug is near the bottom of the oil cooler. It's secured by a spring metal clip with sharp edges.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Doing some double-checking this morning and the only 'switch' that I can find for my 1984 W123 A/C system is this pressure switch located on my drier. A temperature switch comes up frequently - with 2 pigtails on it - but I can not find that part on my car and several online retailers do indicate that is does not fit my application.

Can anyone confirm that there is only the one pressure switch?

Cam I change this out w/o evacuating the system (probably not...)?

ryan

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RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

I can confirm that the original configuration featured one pressure switch. The purpose of said switch (aka low pressure cut-out switch) is to prevent the compressor clutch from engaging when there is inadequate refrigerant in the system. That switch is unrelated to the aux fan. The fan was actuated by a temp switch, which this website shows as being applicable to your vehicle.

That said, some R-134a conversions involve changing the original design, in which case all bets are off. Could you post a picture of your receiver/drier?

If you were jumping the pressure switch, any relay that was clicking would have been for the compressor.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Ok, you're right - the relay c,licking was the compressor relay. So, after isolating the fan relay, I jumped connections 30 and 87 as previously suggested and my fan runs - even with the key off.

I am trying to attach a pic - clearly someone has rigged up some kind of connection here - the black electrical tape is a giveaway...

Attached Thumbnails

 

__________________
RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

Here is a 2nd pic - you can see that two wires connect to the silver box...resistor?? These two wires are connected at the yellow wire connector...

Attached Thumbnails

 

__________________
RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

See:

MOD- Condenser Fan & Compressor working together by adding a relay

Rollguy did a nice writeup on a mod to make the aux fan come on whenever the compressor is engaged. That would fix your problem and it looks pretty easy to do.

Is there, perhaps, an unaccounted-for switch somewhere in the cabin?

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Good news that your fan works. As mentioned, the silver box that you call a "resistor" is not factory (not in my 1984 or 1985 300D). It looks like a relay for an older U.S. car, perhaps a horn relay. Most likely, it's contacts are wired in parallel with the temperature switch, so either it or the temp switch can turn on the fan relay (other side of engine bay). Most likely, its coil is actuated by the refrigerant pressure switch, with intent to run the fan whenever the AC clutch is engaged (its relay coil energized). A relay would have at least 3 terminals, but one can be the case to ground. I can't quite make out in your photo.

Since you likely have twice the chance of turning on the fan, most likely the problem is in the hacked-up wiring. Did they solder and heat shrink wires, or just twist them together and wrap w/ electical tape (gomer fix)? The later might work OK for a few years. How long since the relay was added? Try connecting the relay's case to ground with a jumper wire. If the fan comes on then, the rusty sheet metal mounting screw of the hacked-in relay is the problem.

Per the pictures, the temp switch is not present. And, if the fan was wired parallel to the compressor clutch, the temp switch would be useless.

As designed, the temp switch would never close until the compressor was operating and the refrigerant reached the necessary temperature to close the switch. With a properly operating system, that only occurs in more demanding conditions, such a high ambient temps and stop & go driving.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Good catch. I missed that the temp switch is missing in the photo.

I did not suggest the fan itself was wired in parallel with the clutch. Both still have separate relay drivers (driver's side inner fender, fwd is "fan", aft is "AC clutch" exc. 1985 replaced by KLIMA box). I was discussing the possible "relay logic" the hacker used, which involves only the relay coils. The hacked-in relay is probably actuated by the "pressure high" signal. The hacked-in relay's contacts probably replaces the temp switch.

That would make sense, if a binary pressure switch had been installed to replace the original low pressure cut-out switch. From the pictures, I would bet that did not happen.

As originally configured, there is no such "signal."

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

After you repair the fan problem, switch it back to R12 if you expect any type of AC performance in Oklahoma. It's also easier on the compressor.

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'83 300D, 126K miles.

No extra switch in the cabin...I did that mod on a 1978 300D I used to have - put a toggle switch on the dash so I could manually control the aux fan. Worked great, but I can't remember what/how I did it. Think it was from a post on here...

I looked closer at the silver relay in my second picture - stamped Made in USA on the top. 3 wires - the black one is grounded to the chassis just to the right of the relay. Under the black tape is a rough solder job with a spade connector, the yellow crimp connector was just replaced by me after finding the original was not connected all that well.

I'm not sure where to go here now...would running a toggle switch in to the cabin work? Anyone provide a basic how to...?

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RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

I recommend just buying a factory temp switch (the kind the screws into the dryer) and rewiring the thing back to stock.

When my A/C aux fan wasn't working, I spent an hour trying to figure out how the thing had been rewired. I realized after wasting that time that it didn't matter, and I just pulled all the wires out and wired it up per the factory service manual schematic. As the guys have said, it's just turned on by the temp switch on the dryer.

I do recommend a mod to save the temp switch from premature wear: Put an additional fused relay in to off-load the factory temp switch from carrying all the current for the auxiliary fan. The factory temp switch would control the additional relay, which controls the fan. My relay gets fused power from the battery.

Good luck,

Packman

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83 240D - 4 speed manual - Manilla Beige
189K miles, Tachometer mod, cool wooden shift knob from PeachParts, CocoMats, Original factory paint, manual windows, manual sunroof. Starting to add AudioWrap to this car too!