I wouldn't have it run all the time. It's extra wear on the motor, extra load on the (too small) alternator, extra noise, etc.

If you just want it to run all the time just connect the two wires for the pressure switch. The fan will run anytime the key is on; no relay needed.

Who said anything about cutting wires? Just make a Y adapter for the plug on the compressor. Hmm, offhand I don't know what the connection for the clutch coil looks like.

-J

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

How competent are you with auto electric? There are simple hacks and there are better hacks.

1) Simple hack -Wire a toggle switch into the cabin to jump/connect the high pressure switch on the rec/drier. Toggle it when A/C is on. This will turn on the aux fan at slow speed thru the R14 ( may be R15, cannot remember ).

2) Another hack - Wire a toggle switch into the cabin to jump/connect 1 pin of the thermostat on the engine block to ground. Toggle it when A/C is on. This will turn on the aux fan at full speed.

3) Good hack - Splice into the compressor wiring or Kilma pin 7 to get a switched 12v. Then wire up what compu_85 drew. I also posted it earlier.

Good luck.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

On my car this kills the AC... but I know the late cars are wired a bit differently.

-J

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

It is possible if the Kilma relay also monitors the switch setting. This is to simulate the 110C ( or 115C depending on the color of the thermostat ) coolant temp. It is does then ignore it.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

I believe this would work best because all I need is just the aux fan to run when the car is in traffic basically..
and I am not super competent at soldering in tight spaces etc...

Thanks guys

__________________
2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

That's what I was going to do for A/C in the summer, even ran the wire into the cabin for the toggle switch....BUT I couldn't implement it so it's not an eye sore. Also the idea of having to turn it on/off manually, don't like it.

So what I did was replaced the coolant/aux fan temp switch with one that have lower temp trigger points.....now clutch/aux fan comes on sooner to cool the coolant/engine & also the A/C condenser. Also it's automatic, no toggle switch.

__________________
85' 190E w/collector plates

Amazing. Like this is an acceptable solution? /\/\/\ Put a hole in the dash with a toggle switch?

How elegant Another dash ruined by backyard butchery.

How about giving the system enough pressure to register it needs to have the AUX fans on? Like it's supposed to?

To the OP. Spend a few bucks and increase the system pressure to the high side of spec- then go from there.

rjp


BTW "jumping" the high pressure switch on the reciever dryer also involves bypassing the pressure cutoff- you know that switch that registers when the system high side pressure exceeds 28 BAR it cuts off the compressor..Does the same when pressure drops below 2 BAR..

Its a safety feature.

Fiddling with that is a good way to blow your car to bits.

__________________
03 540I (GAK!!)
81 280E (W123)
70 LeMans Sport with pissed off Poncho 462"
1989 Silverado Step Side (under the knife- complete restomod)

I believe these are different pressure switches, or at least different pins on the same connector. There's the bi-function pressure switch that cuts off the compressor if the pressure is too low or too high, and there's this pressure switch that turns the aux fan on/off at moderate pressures.

On my late car there is no switch... there's the sensor in the water jacket for the ACC. It turns the fan onto high speed at 110*C and kills the AC at 120*C IIRC.

I'll review the W124 wiring diagrams when I get home.

-J

Why not a low cost kit based on radiator temp?

Derale Cooling Products 16769 - Derale Performance Adjustable Fan Controllers - Overview - SummitRacing.com

Depending on which app, it looks like you place a probe in the fins. I'd think the controller would be able to replace the high-pressure sensor and trigger the slow speed fan via a switched ground. If the controller only puts out a switched hot, then you can re-use the same type relay from the 260.

That's a nice temp switch! You could attach the sensing bulb to the R/D, have it kick on at like 120*F.

-J

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

I just redid my A/C with Envirosafe es-12 (yeah I know it's fuel refrigerant, sue me)

I set the low side pressure @43 PSI at idle, 1500 RPM about 35 PSI at 96 degrees ambient.

Not only does my AC kick ass now, a weird side effect- it also triggered the aux fans to kick on sooner than it did before. Yesterday car said it was 103F and I was still blowing 44F out of the vents according to my thermometer, and the car runs the AUX fan well before the tstat hits 87F.

moral of the story- increase your charge pressures slightly.

rjp

__________________
03 540I (GAK!!)
81 280E (W123)
70 LeMans Sport with pissed off Poncho 462"
1989 Silverado Step Side (under the knife- complete restomod)

The cool harness mod is not what I need. I need to stop it getting to 100C point before the fans kick in. Out here when it hits 120F, its incredibly difficult to cool the engine down, its not a matter of fans turning on and it suddenly drops.

I need to make it so that it slows the rise.

So the cool harness mod is not what I need.

Also raising head pressures is the opposite of what I want. Increased head pressures increases condenser temperatures, which raises coolant temperatures, which raises condenser temperatures which lowers A/C cooling. Its a vicious cycle. Plus, I don't feel the need to contest with MB Engineering on pressures or reduce efficiency and longevity.

__________________
2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

The thing is- increasing the head pressure in relation to the ambient temp (by increasing the charge) causes the AUX fan to kick on earlier. The AUX fan is both temp (by coolant temp) driven, and by high side system pressure. Raise one or the other and the AUX fan comes on. You won't be charging the system beyond spec, but to the maximum allowed.

Bypassing pressure switches isn't a good idea, either your compressor may die one day due to low freon (and you won't know it till it's too late) or you'll run the system till a barrier hose pops.

Read the thread I linked- note the mode of operation, and the fact those systems are properly charged by pros.

rjp

__________________
03 540I (GAK!!)
81 280E (W123)
70 LeMans Sport with pissed off Poncho 462"
1989 Silverado Step Side (under the knife- complete restomod)

I'd recommend checking out the EPC startek site and looking at the wiring diagrams. You'd need to check the Aux fan section (there's a Table of Contents link) and the A/C section (it's not linked but it's in the back).

That'll help with understanding your specific car, since there's 3 or 4 variations in the gas W124s and the diesels are also different for what happens at which temp or pressure.

Then I'd look at the Summit Racing site and see if there's an easy low price kit that lets you replace the function of the pressure-switch for the aux fan with a temp sensor. These fan controller kits vary from mid $20 range to multi-hundred racing applications. The specific link I posted appears to be a cheap (maybe $30 IIRC) temp-programmable option that lets you place a probe in the radiator fins (no drilling or plumbing changes required).

In Arizona, I'd definitely look at an install that is ignition switched, eg, when the underhood temp is 140 F AND the ignition is on, then the Aux fan runs at least at low speed. I suspect that is easily and cheaply do-able with one of the Summit Racing kits. This type of mod should not affect any of the safety cut offs or function of the OE climate control.