How to wire aux fan to always come on with A/C - PeachParts Mercedes-Benz Forum

braverichard · #1 07-13-2005, 08:16 AM

Hey everyone,

While my 300SD is being repaired at the body shop, I spoke to the guy over there and he said he could wire the auxillary fan to come on automatically whenever the A/C compressor starts, but he wants me to get instructions for him on how to do it in that particular system. So, I need two things.

Firstly, I need to get the instructions on how to do so from someone who has done it or simply knows how to do it so I can hand them to him.

Secondly, there is a tiny, screw on type sensor on the receiver-dryer that broke off during the accident. Is that the sensor that senses how much pressure has built up and then kicks on the auxillary fan? If so, do I still need to replace it even after we do the wiring? Thanks all once again for you help.

MarkM · #2 07-13-2005, 09:36 AM

there should be two sensors on the rec./drier...one is a pressure switch, which has a "female" threaded connector that screws onto a male threaded port...this obviously connects to the interior of the system...i.e. if you removed this sensor when the system was charged, all the refrigerant would drain out. This pressure switch connects to the compressor circuit.

The other sensor has a plastic collar around it, and it has a brass male screw connector that screws into a female threaded block that is NOT continuous with the interior of the system...i.e. if you removed this from a charged system, no refrigerant would leak out...this is the temperature sensor and is connected to the aux. fan circuit.

There are various ways to wire the aux. fan to go on with the compressor, but I would not bypass the temp. switch....there will be times when the compressor cycles off, but the aux fan should still run due to high refrigerant temp.

There is a clever way to do this by changing location of one of the wires at the "ice cube" relays (for compressor and aux. fan), but you need the wiring diagrams to do this. Another way is to install another relay.... locate the power source for the aux fan at the aux. fan relay and run this through the other relay to the hot lead to the aux fan on the other side of the ice cube relay. Then solder two additional leads onto the connector to the compressor. When the compressor goes on, the aux fan will go on. You still leave the temp. sensor circuit intact. In essence, you have two relay circuits in parallel on the same power source for the aux fan...one relay controlled by the compressor clutch, the other by the temp. sensor.

Good luck,

Mark

braverichard · #3 07-13-2005, 10:08 AM

Thanks for your reply...

On the issue of which sensor is broken, I have attached a picture of the receiver drier to this message. I believe the one that's broken is the one on top - that one isn't inside the system right? So that's the temperature switch that turns on the auxillary fan.

The reason why I want to wire the auxillary fan to come on every time the compressor is on is because prior to the accident, I hadn't seen my fan come on even in the hot summer months. Furthermore, the temperature of the air from my vents just creep up and up while idling like at a traffic light. I read from another forum member that the auxillary fan which wasn't coming on was the cause of that increase in temperature.

What you described as a "clever" way to do it seems like a much longer story than what I've read from people here before. Unless I guess, the simple way would involve by passing the temperature sensor on the receiver drier. Well I hope someone here who has done it replies so I can read how they did their specifically.

MarkM · #4 07-13-2005, 10:35 AM

I'm not sure if the SD is different from the TD, but I suspect they are very similar.

I would not bypass the temp. sensor. I would first find out why the aux fan did not come on. A very likely reason is that the aux. fan is NG. Follow the wire from the aux. fan to a plug connector, disconnect it, and connect it directly to 12 volt supply (use wire leads directly to battery) to see if the fan runs....if the fan doesnt run, its NG. If the fan runs, then there is a problem with either the aux. fan relay, or the temp. switch at the rec/drier.

There are two sensors/switches located at the rec./drier...a pressure switch and a temperature switch. The pressure switch has two blade connectors directly on the body of the switch; the temp. sensor has two wire leads with connectors on the ends.

Good luck

Mark

braverichard · #5 07-13-2005, 10:41 AM

I once tested the old fan as you just described and it worked very well. It blew on full blast. Right now I just purchased a brand new fan (as the old one was broken in the accident) so I know that the fan not working will be out of the question - for the most part, save for perhaps a manufacturing defect. So what do you suggest I do? Start by replacing the temperature and pressure switches on the receiver/drier?

Eric Eliel · #6 07-13-2005, 11:38 AM

I've got a hard copy of the electrical schematic. Check your email for my contact information.

jbaj007 · #7 07-13-2005, 12:50 PM

I put a standard auto relay next to the R/D and used "punch" connectors in the wires to the compressor for the input and "punched" into the wires to the temperature sensor for the output. The sensor is not bypassed (but what does that matter when the fan is on whenever the comp is) and the A/c relay does the work.

pawoSD · #8 07-13-2005, 01:58 PM

I was about to say that, a simple relay to the fan thats triggered by the A/C clutch wires would be a simple solution to making it come on whenever the A/C is...

MarkM · #9 07-13-2005, 02:14 PM

What Jabaj007 did will work ok....in essense what i said previously, just tapping in at different locations. To answer Jabaj007's question, you don't want to bypass the temp sensor for the following reason:

If you wire the aux. fan to go on when the compressor goes on, the fan will cycle on and off with the compressor (obviously). The compressor normally cycles off when the refrigerant temp. reaches slightly above freezing (i forgot the exact temp)...this is done by the ETR Switch (electronic temp. relay??) located in the pass. side foot well with a temp probe just in front of the evaporator. This is done so that the evaporator will not accumulate ice and get blocked. So when this cycles the compressor off, your aux. fan will go off (if wired as we discussed). Now, another part of your AC system is trying to cool and condense refrigerant (the condenser). If refrigerant temps. at the rec/drier are high enough, the aux. fan will go on. When you are driving in stop and go traffic, or at idle, little or no air is moving across the condenser, so the temperature of refrigerant passing through the condensor will be too high...if high enough, the temp switch at the rec. drier closes thus turning on the aux. fan. There will be times when the compressor has cycled "OFF" due to cold temps at the evap. and there will be high temps at the rec. drier requiring auxilary cooling at the condensor. This is very important, because overall optimum functioning of the AC system is very heavily dependant on cooling and condensing refrigerant at the condensor (E.g. getting low side pressure as low as possible).

So, don't bypass the temp. switch.

With all that said, lets get back to the idea of having the aux. fan go on with the compressor. The only reason I did that (on my first 300D) was because I converted to R134 and wanted to optimize cooling/condensing at the condensor. Thus, I did not bipass the temp switch. (Note, I also installed a parallel flow condensor in place of the OE fin-tube type). On my current 300D I converted to R134, put in a parallel flow condenser, but decided not to wire the aux. fan to the compressor because I don't think there is any real benefit. The parallel flow condensor is a significant modification which enhances cooling/condensing sufficiently. Also, R134 gets "hotter sooner" than R-12, so with the aux. fan wired as designed to the temp switch at the rec. drier, it will cycle on more often with R134 than with R12.

Regarding the original line of discussion on this thread, the reason for wiring the aux. fan to the compressor, was because the temp switch apparently was not working....this is not really a valid reason...I recommend that the problem with the temp switch be troubleshooted (or troubleshot?) and fixed. Its a very simple circuit...if you jump the wires connected to the temp switch, the aux. fan should go on (if the ac system is on). If it doesnt go on, check the aux. fan relay. If it does go on, then check the temp. switch as previously described.

If the temp switch "broke off" due to the accident, there may be a little threaded stud left in the hole and will need to be removed.

Good luck,

Mark

Steve300 · #10 07-13-2005, 06:47 PM

For the auxillary fan, I by-passed the temperature switch and connected the two wires to a toggle on/off switch. During hot summer months I leave the switch to "on" position.

Smooooth · #11 07-13-2005, 07:01 PM

With us having a record temperature (and humidity) summer so far - I just jumpered mine so that is on all the time............. (Also because my pressure switch is bad and it was a cheap summer time fix - to keep SWMBO cool)

Returning to normal operation will probably be this fall - when I actually want to replace the R/D and associated parts)

Dang its hot up here...........................

Stephen

braverichard · #12 07-14-2005, 01:12 AM

Mark,

Thanks again for your insights. I'd like to extend that thanks to everyone else that has also posted here.

Perhaps to explain myself a little better (with more details), prior to the accident, everything was in place, but while idling the temperature of the air coming out of my vents would increase. Furthermore, I had never seen or heard the auxillary fan come on. I was beginning to suspect the fan and its sensors. So I tested the fan and it worked well. However I never got to test the sensors prior to the accident. Now that the car is being repaired and the body shop guy will wire it for me free of charge provided I give him instructions on what to do, I've decided to go ahead and wire the fan to stay on all the time even with the new fan I am installing because I converted the system to R134a which I understand isn't very efficient in that car as the system was originally designed to use R12. Indeed, I personally noticed this because even at high speeds, once the outside temperature exceeded 85 F, my A/C system was barely effective. So, no, the reason for wiring the aux fan to stay on all the time isn't just because it wasn't working. There are many factors that have lead to that decision, as are outlined above.

So, everyone seems to be saying they've by-passed the temperature switch but Mark discourages it due to reasons that make perfect sense. I think I'll just go with Mark's advise to not by pass the system. However I'll see if that guy can implement Mark's "clever" circuitry which will have the auxillary fan running whenever the compressor is on but without bypassing the temperature sensor such that if the compressor if off but refridgerant temperature is still too high, the auxillary fan would stay on until everything has cooled off. Right Mark?

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