Hello!

At least on some 123 cars, the electric radiator fan's set up to operate when the pressure in the a/c line is above a certain point. What point is this? Also, what side's it measured on (I'm guessing the high-pressure side)?

In addition, what pressure does the high-side (or whichever side the fan's switched from) "normally" run at?

I'm asking because a group of us over on the Ford site are talking about trying to rig an electric fan in front of the condenser to do the same job on our trucks that the factory fan does on the 123 (and other cars), and we're trying to figure out what criteria we should be using to cycle the fan.

Thanks!

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2001 VW Jetta TDI, 5 speed, daily driver
1991 Ford F-350, work in progress
1984 Ford F-250 4x4, 6.9l turbo diesel, 5 speed manual
Previous oilburners: 1980 IH Scout, 1984 E-350, 1985 M-B 300D, 1979 M-B 300SD, 1983 M-B 300D
Spark-free since 1999

In the old days I heard the pressure switch on the Reciever/Dryer referred to as a low pressure cut out switch.... meaning a sensor designed to stop cycling the compressor when the refrigerant has leaked out... and no lubricant would be making its way back to the compressor...
So there is a chance that the fan is operated by the temp switch on the Reciever/dryer and the one on the block... since it is a dual function fan... a check of the wiring schematic should prove this one way or another....

i have had my aux fan go on when the car was idling in hot ambient temperatures and the a/c was not turned on (or even working for that matter).

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1984 300D Turbo - 231k....totalled 11/30/07 RIP

On the particular 123 cars I've been exposed to the high side pressure switch is the ONLY condition that turns on the aux fan. It is not there to cool the engine, it is there to bring down high side pressure when it gets dangerously high.

Some 123's have a dedicated high side pressure switch that connects to the fan. Others have what is called a trinary switch that uses high side pressure to turn on fan and low side to protect compressor from low charge condition.

Hope this helps,

I'm trying to remember way back to when I owned a 123... Mine was the very common 1984 300d - 123.133.

It has a low pressure cut-out switch to disable the compressor at low ambient temperatures or in the case of refrigerant loss.

The electric fan was actually run by a freon temperature switch on the dryer, rather than a pressure switch. It activated whenever the temperature of the liquid freon in the dryer reached about 70C, as I recall. Really unusual way to do things, but I guess measuring the temperature is just as good as measuring the pressure, pv=nrt and all.

- JimY

There are two switches located on the receiver dryer. One is a low pressure switch (which can leak when old, replace this when doing a conversion or rebuilding your A/C system). Its cheap insurance against leaking $$ R12. The other is a high temperature switch whick kicks on the Aux Fan when the temp gets to a certain point. The temp switch won't leak.

In 1985 MB started putting a 212f sensor in the water inlet yoke right near the thermostat. This was on the CA 123's. Don't know about Federal cars. It may have been there all along on the 126's??????? Once the engine temp hit 212, the aux fan would also come on. Extra safety device?

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84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)

It seems like it might be easier to use the power going to the compresser to also run a relay for the fan. That way the fan automatically comes on whenever the compresser is running.

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FAA Certified A&P Tech
Still lookin' for my diesel! <><

So, the fan's activated by a temp switch, not a pressure switch? Also, would it be possible to confirm the temp that the switch turns the fan on at?

I thought about just making it so the fan would be on at all times, but at higher vehicle speeds, I don't think it's really necessary, and why put unnecessary wear on the fan?

Going off somewhat on a tangent I just took a look at my car for the 212° sensor that was just mentioned. Where, exactly, is it located? I've got a sensor with a black wire leading to a black circular plug attached to it in the area that was described, right by the hose connection on the block for the upper radiator hose, maybe 1/2" from the valve cover. I've got 12 volts at the plug with the key on (car not running), and with a cold engine, the sensor does not complete a ground. I attempted to jump the plug to ground, and from the best I can tell, nothing happened. The fan didn't come on, I didn't hear anything click or anything, and the temp gauge stayed at the lower peg.

FWIW, a friend of mine has an '84 300D (federal car) and i believe he has the same plug/sensor setup; when I first saw it, I thought it was the sensor for the water temp gauge.

My fan doesn't come on at 100°C, although I would like to set it up so it does...

Thanks!

...anyone?

I ground the black wire on my 1984 300D, the one behind the upper hose and my fan does not come on. When I jumper the two wires going to the temperature switch on the dryer, the aux
fan runs.

The single wire on my 1980 300SD DOES make the aux fan run when grounded.

I hope someone can answer as to whether the black wire connecting to some kind of temp switch on the thermostat housing is indeed to trigger the aux fan to run. If not what is the purpose of this sensor and wire?

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1980 300SD Manganese Brown Metallic Clearcoat 248k
1979 300SD White 148 k
1984 300D Silver project car for daughter
1982 300D wrecked
1980 300SD 310K not running

WHOA this fell down fast...

C'mon, anyone? It'd be really nice to hear a confirmation on what freon temp the fan's activated at, not ot mention finding out for sure what that sensor by the upper hose does.

Any replies would be greatly appreciated...thank you!

i also have 2 wires / sensors going into the top hose above the t-stat housing.
haven't had a chance yet to trace the wires, but i'm of the mind that one goes to the aux. fan. as said, my aux fan went on when the car was idling on a very hot day and the a/c was not on or even working then. if memory serves it activated as the temp gauge went above 100deg C, and then shut off when it went below 100.

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1984 300D Turbo - 231k....totalled 11/30/07 RIP

I don't have A/C or a condensor so I moved the aux. fan over some to cover the oil cooler. Looking at the front of the grille, it looks like the oil cooler won't get very good air flow at any speed. I just rigged the fan with a relay and manual switch that I use whenever I feel like it.

Since the engine oil if the first means of cooling, might as well keep it nice and cool.