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  #1  
Old 08-04-2016, 04:57 PM
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W123 Aux Fan pressure switch?

After reading up on ROLLGUY's custom harnesses for running the aux fan under the control of the compressor clutch, rather than the temp switch at the receiver drier, I contacted the company that sold me most of my AC rebuild parts. I'm going to be running a new auxiliary fan in addition to a Sanden and PF condenser. I asked their advice on performing this modification, and they suggested running the aux fan off of an additional pressure switch at the receiver drier, so that it would engage only when the pressure is high. Initially they mentioned a w126 pressure switch, but that doesn't look like it will fit the w123 receiver drier's additional port without drilling & tapping it out. Another option would be to replace the stock binary pressure switch (which cuts out the compressor at low pressures) with a trinary switch, which would then engage the fan at high pressure and the compressor at low pressure. My queston is, does anyone know the switching pressure for the stock binary switch? I thought I might be able to find a trinary switch that would work at the correct low pressure cutoff, and have a high pressure switch to engage the fan.

This is not really strictly necessary, the original temperature switch is probably fine, as is ROLLGUY's modification. I just think that I'd like to improve things as much as possible since I'm going to be replacing the entire system at this point.

Edit: Also, does anyone know the temperature that the original temperature switch is supposed to activate at? I'm still running the one that was in the car when I bought it, but I've never seen it activate. Would like to bench test it to see if it actually works.

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Old 08-04-2016, 09:28 PM
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Quote:
Originally Posted by Demothen View Post



Also, does anyone know the temperature that the original temperature switch is supposed to activate at? I'm still running the one that was in the car when I bought it, but I've never seen it activate. Would like to bench test it to see if it actually works.
144degF(62DegC)
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  #3  
Old 08-05-2016, 06:48 AM
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i did the ROLLGUY mod and LOVE it. I also put a fan from a 126. The only change I made to the ROLLGUY system was to add an ON/OFF toggle switch in the fan circuit. I turn the fan off if I'm going to on the interstate and driving at speed. Otherwise the switch stays on. I love the fact that even in the heat of Baton Rouge summers my coolant temperature gauge stays at about 85 degrees even when stopped for extended periods of time (long red lights, dash into a store, etc.) and the AC remains extremely cold all the while (R12 system).
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Old 08-05-2016, 09:29 AM
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Originally Posted by BayouFlyFisher View Post
i did the ROLLGUY mod and LOVE it. I also put a fan from a 126. The only change I made to the ROLLGUY system was to add an ON/OFF toggle switch in the fan circuit. I turn the fan off if I'm going to on the interstate and driving at speed. Otherwise the switch stays on. I love the fact that even in the heat of Baton Rouge summers my coolant temperature gauge stays at about 85 degrees even when stopped for extended periods of time (long red lights, dash into a store, etc.) and the AC remains extremely cold all the while (R12 system).
More info on your R12 system please. vent temp/ ambient, type of compressor/ condenser etc. I am making tooling to pressure test my evaporator by itself in situ, if good, I am converting back to R12.
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Old 08-05-2016, 10:28 AM
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I think the concern with running the fan with the compressor clutch was that it would cause premature wear of the fan due to the high frequency stop/start cycle of the compressor clutch when the AC is on low speeds. Whether that's true or not, I have no way of knowing.

I haven't been able to find a trinary switch or a high pressure switch that will fit in the extra port of the receiver drier though, so it's a moot point - will probably run off the stock circuit for now, if i'm having problems I might switch to a temperature probe at the condenser with a variable thermostat, or try ROLLGUY's mod.

I'm also curious if my ETR switch is working properly. I need to read the FSM some more, but I assume the ETR is what controls the cycling rate of the clutch?
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Old 08-05-2016, 12:50 PM
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The fan is very robust. Just make sure to open it up and clean up the bushings. You can even replace the brushes if needed. Do it every few years if you want peace of mind.

The 201 and 124 receivers have both a high/low pressure switch and a fan pressure switch. These switches may fit the 123 receiver, or maybe you could just use the 124/201 receiver.
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  #7  
Old 08-05-2016, 02:03 PM
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Quote:
Originally Posted by funola View Post
More info on your R12 system please. vent temp/ ambient, type of compressor/ condenser etc. I am making tooling to pressure test my evaporator by itself in situ, if good, I am converting back to R12.
Well, I stuck one of the digital kitchen thermometers in the discharge vent while running errands this morning. I found the coldest I observed was 45.6F and while sitting in traffic at a long red light the highest was 50.9F. Temp on outside ambient gauge was showing 99F while stopped at light. Ambient was about 92-93F.

I have a 7 cyl Sanden compressor that I got from ROLLGUY and i am using the stock condenser in my 77' 300D. I installed a new expansion valve, dryer and suction & discharge hoses. I had the local Thermo-King distributor make the hoses for me (about $65-75 each??). Prior to installation the 16" W126 auxiliary fan was dismantled and thoroughly cleaned, new brushes installed, new double sealed bearing installed on fan blade end of the shaft and new grease installed in/on the cleaned bushing on the outboard end of the shaft. BTW, I've read of folks concerns about these fan's life expectancy in this higher than normal use and while I think those concerns are probably not an issue, I do have 2 spares including new brushes and bearings lined up in the shop.

One last quick note, earlier this year I re-installed one of my rebuilt/reconditioned stock 12" fans (I have two of those too) just so I could get a feel for whether or not the 16" fan was actually making a real world difference. I changed nothing but the fan. During an errand trip I saw my red light engine coolant temperatures going up to 100C pretty quickly and the dash vent temperatures rising too (didn't measure it just "seat-of-the-pants gauge). I went home and put the 16" fan back in and, as said earlier, the engine coolant temperature doesn't seem to get past the 85-88C range on the gauge and vent temps remain satisfyingly cool. YMMV!!
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  #8  
Old 08-05-2016, 03:15 PM
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I believe the 126 pressure switch and 123 switch have the same thread size, so I can't see why the 126 fan switch could not be used on a 123 drier to switch the fan. A 126 drier is opposite, so the connections will not work in a 123. I am not sure if the system cares if the fan switch is on the outlet side if the drier, and not the inlet where the pressure switch is. I suppose as long as it sees pressure, it will close the contacts and power up the fan. There are two ports on the drier, so I can't see why you could not use a low pressure only switch (not binary, as you would not want the fan to be powered down in an over pressure situation) to turn on the fan. I have a NAPA part # (I have to look for it) for a switch that is low pressure only if you want to go that route......Rich
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  #9  
Old 08-05-2016, 04:37 PM
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ROLLGUY,
Is this the part you're thinking of?
A/C Pressure Switch on Receiver Drier 0048206810 - O.E.M. - Mercedes-Benz - 004-820-68-10 | Pelican Parts
It matches the description I was told, the thread size matches what the current pressure switch uses. I will have to pull the extra plug out of an old receiver drier to make sure it uses the same threads as the pressure switch does. I think it would make a lot of sense to go ahead and install that in the new drier, even if it ends up not working well I can always go back to the temperature controlled switch instead without draining the system.
Thanks for your help!
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  #10  
Old 08-05-2016, 04:48 PM
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Thats the same switch used on 201/124 chasis. It's NO, comes on when pressure hits 300. Its non polarized, and usolated from ground.
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  #11  
Old 08-05-2016, 05:09 PM
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Found it: NAPA Temp Products # 207068

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