W124 A/C - new pressure switch, auxillary fan worked twice, now nothing... - PeachParts Mercedes-Benz Forum

renaissanceman · #1 08-09-2015, 06:09 PM

I posted in another thread about not getting any aux fan function in my W124 300D a few months ago. I would not get any fan when the A/C compressor was engeged. I've since tested the fan (it is fine) and changed the resistor. It still would not kick on, so I replaced my green pressure switch with the red one (which supposedly kicks on at a lower 15.5 bar (225psi) and off at 10.5 bar (per this thread: aux radiator fan - Page 2 - Mercedes-Benz Forum).

After evacuating and recharging (with enviro-safe), I got the aux fan to kick on and run...once right after filling the system. It ran intermittently the next day. Now it no longer runs at all. I hooked my gauges up, and the pressures were above 225 psi, as before. I added an additional 1/2 can of refrigerant until the sight glass ran clear. Still nothing. Pressures are approximately 35psi/250psi.

I'm quite frustrated and at a loss of what to look at or what to do. I don't like running the A/C without a functional aux fan, but at the same time it's 95-100 degrees here in central CA and not using A/C is far from fun. I get great cooling at speed, at least.

leathermang · #2 08-09-2015, 06:50 PM

http://www.epa.gov/ozone/snap/download/MailFOV_EnviroSafe_06112013.pdf

Notice starting on page for number 17 to 28.

Detailed Questions About HC-12a ®, OZ-12 ®, DURACOOL 12a ®, EC-12a, and other Flammable Hydrocarbon Refrigerants | Alternatives / SNAP | US EPA

" May hydrocarbon refrigerants be used to replace CFC-12, commonly referred to as "Freon® ," in cars?
No. It is illegal to use hydrocarbon refrigerants like HC-12a® and DURACOOL 12a® as substitutes for CFC-12 in automobile or truck air conditioning under any circumstances."

The FSM specifically forbids the use of the sight glass as a method for determining the charge of the AC... since there are conditions where it will still have bubbles at full charge... causing people to overcharge if they continue until no bubbles appear.

There is a sticky AC thread which has a lot of good information...
and lots of non stickied also....
AC is not simple...and it is very tedious... but much reading needs to be done to make the best decisions when working on your own AC...

jay_bob · #3 08-09-2015, 06:51 PM

There are two control modes for the aux fan in the 87 124.

(Note, they changed this in 88+ 124s, this does not apply to any 124 that is 88 or newer. They changed the temperature switch in the head to a thermal sensor, and the climate control PBU regulates the fan relays.)

Anyway, for those of you with an 87 124, keep reading.

There are two ways to control the aux fan in this model. The first is low speed, which is regulated by refrigerant pressure. The second is the high speed, which is regulated by the coolant temperature at the upper radiator hose connection to the head.

If you short the two pigtail leads at the dryer - for the pressure switch - the fan will be engaged on low speed.

The electrical low speed control path is as follows:
Fuse 7 -> K10 pin 86 (coil +) -> K10 pin 85 (coil -) -> S32 (dryer pressure switch -> ground.

The low speed power path is:
Fuse B -> K9 pin 30 (common) -> jump over to K10 pin 30 (common) -> K10 pin 87 (normally open) -> into the resistor -> out of the resistor -> motor -> ground.

On high speed this is regulated by temperature.
Fuse 7 -> K10 pin 86 (coil +) -> jump over to K9 pin 86 (coil +) -> K9 pin 85 (coil -) -> S25/5:X1 pin 2 (normally open) -> S25/5:X1 pin 1 (common) -> ground.

The high speed power path is:
Fuse B -> K9 pin 30 (common) -> K9 pin 87 (normally open) -> the bottom of the resistor -> the motor -> ground.

Long story short, if you jump the two pigtails at the dryer, you should get K10 to pick up (you should be able to feel it with your thumb) and the fan should run on low speed via the resistor.

If you jump the two-position connector at S25/5 on the water neck, then K9 should pick up and the fan should run at high speed without the resistor in the circuit.

You will need to take the cover off the rear part of the fuse box to find the relays.

There are 3 rows of relays. The most inboard row is, from back to front:
K5 - power seat relay
K10 - low speed fan relay
K9 - high speed fan relay
K2 - headlamp washer relay
F14 - power seat fuses

renaissanceman · #4 08-09-2015, 07:54 PM

I will troubleshoot the electrical side tomorrow. Looking at the A/C thread, the pressure chart for R12 shows that the system should not even get up to the pressure needed to activate the fan until 90 degrees ambient temperature...

I'm quite familiar with R134a systems and have done a lot of work on those. But those systems have a fan that runs full speed and full time during operation.

I'm starting to wonder if the system is just a poor design and if the aux fan is unreliable...

In regards to the legality, I think it's a grey area. Once I put the 134a adapters in place, the system became legal according to my reading (legal for converted systems). If I could get real R12 and knew 100% that the system does not leak, I would convert the connectors back and use that.

leathermang · #5 08-09-2015, 08:33 PM

Ask at your local auto parts shops if with a proper license.... you can buy R12.
609 perhaps....

The EPA reference was meant to be directed towards the Flammability issue...