It's just too hot & humid NOT to run R12 in my 1988 300TE wagon (really an E36T AMG wagon).

While I've read that the "red" A/C temperature switch works for BOTH R134 and R12, I don't believe the "red" switch is optimum, as it doesn't match R12 characteristics/temperature curves, and doesn't turn the aux. fan on soon enough.

Can anyone verify this ?

Thanks,
:-) neil

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Also, is there a specific R12 high-pressure switch as well ?

Thanks,
:-) neil

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Those old systems were inadequate, so the best you can do for short $$ is stay with R12 and wire the aux fan using the compressor clutch circuit.
Then you have air-flow when the comp is ON.[ lilke later models]
I also recommend bridging the high fan sensor [ blu ] with a resistor so it kicks in around 100C for coolant.[ vs 105/107C]

__________________
A Dalton

Don't wire the fan directly to the AC clutch. The back-emf generated by the van can cause your compressor to run on the highway. Use a relay.

Anyway, running constantly is a good way to waste energy and reduce the service life of your fan motor.

When we wire the fan to come on wth the comp, we use the relay circuit that is already there ..so it is ISO.
And if one is in hot climates , the fan is on most of time anyway..that is why the new models use fan all the time comp runs..

Another hot climate change in use by many is they rewire the low aux fan circuit to come on with pressure as stock , but they wire it for high speed vs low.
This requires larger gauge wire and moving the Resistor wire to the high side circuit.

__________________
A Dalton

If you've already converted to R134, can you freely go back to using R12?

__________________
I never go far without a little Big Star

1994 E500 199/Gray 82k
84 300D (Salty) Orient Red/Palomino 141k
88 300CE (Ersatzhammer) 904/Java 163k -- Turbo Technics twin turbo kit, AMG Gen I body kit, Sportline steering box and steering wheel, Sportline/Eibach /Bilstein Sport/500E suspension, Quaife LSD in 210 mm diff case, Silver Arrow brakes.
88 300CE Brabus 3.6 199 on Black 44k

Thanks Arthur.

I should be able to use the output from the A/C high-pressure switch to activate the "low-speed" auxiliary fan, correct ?

If that's the case, then a little wiring harness should get this going, correct ?

:-) neil

The red & green are not specific to R-12 vs. R-134a. Rather, they switch the fan on/off at different pressure points. Green requires higher pressures, red is somewhat lower. The climate control manual has the exact pressures, I no longer recall the values.

I'm not sure exactly how Benz determined who got green and which got red. As best I know, diesels got green, gassers red. FWIW I ran a red switch in my old 603 diesel. I suggest a red pressure switch and functioning fan clutch. You'll get maximum airflow at low speeds, and the fans will shut down when airflow is high enough to render them redundant.

- JimY

Yes you can. If PAG oil was used, be sure to get it all out.

Are you sure it's not the other way around: green lower; red higher ?

My 1988 300TE came with a "green" switch, but it was changed to "red" when it was converted to R134a.

Also, aren't these switches "temperature" not "pressure" switches ?

Thanks,
:-) neil

And that's it? No replacing the expansion valve and the receiver/dryer? My coupe was converted, and the R134 is totally inadequate. Of course, the conversion probably did not include the evaporator and I know it didn't include the condenser.

I've looked at getting a parallel flow condenser, but if it's just a case of using different coolant, that would be better!

__________________
I never go far without a little Big Star

1994 E500 199/Gray 82k
84 300D (Salty) Orient Red/Palomino 141k
88 300CE (Ersatzhammer) 904/Java 163k -- Turbo Technics twin turbo kit, AMG Gen I body kit, Sportline steering box and steering wheel, Sportline/Eibach /Bilstein Sport/500E suspension, Quaife LSD in 210 mm diff case, Silver Arrow brakes.
88 300CE Brabus 3.6 199 on Black 44k

If there was any leak in your system, then yes, I would change out the receiver/dryer.

My 1988 300TE is black, and has LOTS of glass, so it get's hot inside, even with a nice tint-job.

My old mechanic converted it from R12 to R134a, and while fine for Santa Barbara, it's inadequate for Florida's heat & humidity.

I converted to ES-12 Industrial refrigerant (which is PAG oil friendly) and has pressure & temperature curves similar to R12, hence to get the optimum performance, I may try the "green" temperature switch, instead of the "red" one.

She gets so cold, the family complains if I keep it down for anything longer then 8-minutes

Once I get the aux. fan going, it's going to be even better in stop-n-go which has no airflow.

:-) neil

I mean, that's it, besides the stuff required to remove the oil. That means a new dryer for sure. Since the TXV must come off, replace it too. And of course, new o-rings all around.

The compressor must be flushed with appropriate oil.

Sure ..but remember , the low fan is accomplished using the Resistor in series.
The high fan circuit simply by-passes the Resistor to give the same fan HIGH speed.
That is how the 2 speeds are gotten from the same motor.
So, if you want low fan with a/c pressure activated , the stock set up does exactly that. The only high fan circuit in that car is Coolant temp sensed.

The a/c sw is listed as a temp sw . but it is , in fact, a pressure activated switch...
One often overlooked problem with a no low fan complaint is if the a/c system is actually slightly LOW on refrigerant..so guys think the fan circuit is bad, when in actuality, the high side never achieves High side pressure sufficient enough to trigger the switch's cut-in specification. So, you have BOTH inadequate cooling due to short refrig level AND no fan for the same reason. But . thwe AC still woks..but not real well....
That is why I have guys jumper the high side sw right off the bat to verify the integrity of that circuit,[ minus the actual sw/freon level.]
Common mistake.
Allways jumper the pigtail leads from the high side sw before doing anything when you have a NO LOW FAN complaint..That will tell you exactly where you stand right off the bat...takes all of 2 seconds....
And if you get the low fan to trigger as it is intended to, you will have air-flow on the condenser when it is required ..and a/f is more inmportant than any other condition when the system has High Termal Load from traffic/idle / or high humid /temps/ambients.. and don't overlook using a high fan for condenser ..guys in Fl do it all the time , including me.

As far as a low fan with comp , I would not worry about it, the high side sw is preferred method. [ Check any new car..they all went w/comp activation]
The main point is, better a/f results in better cooling and freon level is detrimental on performance....R12 is best..and the standard sw that your car came with should be adequate for cut-in if high side pressures are correct.
Another trick guys use is to simply run 2 wires across the high pressure sw and bring them in the cabin to a simple toggle sw..this gives the driver full control BEFORE ambients start to creep...you know when you are coming into traffic , or towing a boat, or it's real hot out..way before the sensors do..so you can get a jump on it..but that is just for guys who understand..the Wifie does not want to hear about it !!!!!
The advantage here is this is in parellel with the stock set-up, so if you do not use it, the system stays stock, electrically..but you have the OPTION of Manually by-passing the stock set-up with the flick of a switch...also nice if you ever get into an overheat condition..you have some control over the radiator airflow manually..kinds nice, and a very popular addition for BenzHeads from way back ....
I will send you the schamatic if you want ..it is simple and very basic...

__________________
A Dalton

Here are the specs you wanted:

Original sensor for your chassis was Red w/R12.

Sensor S/32- Aux Fan -High Side Pressure

Red :
Cut-in- 16 Bar +/- .5 Bar
Cut-out- 12 bar +/- .5 Bar
Green:
Cut-in- 20 Bar +/- .5 Bar
Cut-out- 15 Bar +/- .5 Bar

So , Red S/32 will give you the earliest and longest low fan with R12.
..and you can still do a cabin toggle across those two pigtail terminals if you want a manual over-ride w/o affecting the stock circuitry.
Tapping into the circuit there will eliminate the problems of any additional relays or fuses b/c you are simply using the original circuitry [ relay , resistor, and fuse ] that are already there.
Slick..........

__________________
A Dalton