Great job! It’s really cool when you learn how to evacuate and charge with a manifold. This forum was an inspiration.

So your cycling issue is being caused by power being interrupted to your compressor clutch. This can be caused by the pressure switch, evaporator temperature switch or possibly the temperature switch in your manual climate control in your 240D.

My recollection was the (wonderful) manual system in my old 240D cut the compressor when it hit the target temperature dialed in on the little wheel knob on the dash. This was a really nice feature of the manual CCU because the compressor didn’t run all the time like on my current cars that use heat to balance the cold air for control. So maybe your controller needs to be adjusted?

My center vents get to the high 30s on my 300d before I see the compressor disconnect. I suspect that’s the evaporator temp switch trying to keep the car from icing.

I guess the way to diagnose this is find each switch on the wiring diagram and check each side of the switches with a test light to see which switch is opening. When they’re closed both terminals will be at the same nominal voltage. When open one will be high and the other low. I think…,

To easily test your hypothesis you can jump your compressor relay and see just how cold it goes. Do this at your own risk though. Maybe do it momentarily and stop when you hit 35F. Hmmm…probably better just find out which switch is opening and deal with that. I suspect your system can get much colder if the clutch remains powered.

I’d figure out which switch is shutting down the compressor clutch before adjusting the TXV. Although it seems logical that different gas will require a different setting on the TXV as resago2000 found.

This all reminds me of my wife reaching out and turning on the AC in our 240D while I was merging on a freeway on-ramp. She’d never fail to go for the AC at the absolute worst time. It’s a joke in our turbodiesels now. Makes the Sanden/Duracool setup intriguing. My R4/134a setups reduce my mpg by 20%.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Excellent as always Carl.

Very interesting re: compressor circuits… time to hit the diagrams again.
I reread what Resago had to say about the TXV. Probably my last port of call, as I’d seriously be shooting in the dark there… he mentioned creating a smaller orifice based on the smaller particle of 152. 12a is actually a much larger particle than r12 which is larger than 134. I guess I’d be opening it a touch based on that? Tbh though that’s a level of technical sophistry that makes even me blush.

For the time being, the evaporator temp switch is the most intriguing…

Very funny re: your wife’s AC timing… i’m not sure what kind of hit my fuel economy took on the old r4/134 combo but the power rating was something like treading water with an anvil chained to your ankle.

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1981 240D 4sp manual. Ivory White.

Hey like usual I spoke too soon here. I found this diagram at BW and I cannot find an ETR for the life of me.

https://www.benzworld.org/threads/mercedes-1981-240d-airconditioning-system-diagrams-anyone.1514649/#lg=thread-1514649&slide=1

My 300d and 300sd have an ETR.

Maybe resago2000 is on to something. Do you think due to the fill refrigerant your low side pressure is dropping low enough to open your pressure switch? No idea. Maybe some of the AC gurus can chime in.

My advice is kind of the same. Find out what exactly is interrupting the signal to your AC clutch. Then figure out how to adjust. The 82 240D diagram above definitely shows that the compressor coil signal can be interrupted by the climate control system when the temperature in the car hits the target temperature or when the pressure switch opens. An easy test would be temporarily shorting the pressure switch and seeing if your compressor restarts and your vent temp drops…that seems pretty harmless.

ETA - yeah I’m not too familiar what is inside the TXV. The adjustment may be a spring tension that fights back against a valve to set a pressure? It isn’t so much the molecule size but perhaps the pressure at which the gas goes from liquid to gas. I think it’s kind of a pressure regulation device that’s a step up in sophistication beyond a flow orifice.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Yup, you're right. There's definitely no ETR. Nuts.

I broke out the diagram and moving upstream from the AC compressor, it looks like:

AC compressor clutch --> Pressure Switch --> AC Relay (ice cube) --> At this point it bifurcates, one path to the aux fan relay --> Aux Fan --> temperature switch @ receiver drier(replaced by Rich's relay, so Aux fan runs when compressor runs) --> and back again.

The other path: AC Relay (ice cube) --> Temp Dial --> Temperature control (this thing)

The "pressure switch" (I believe) is the one mounted on the drier. If I jump the two leads going into it, the compressor kicks on. Now I'm not sure if this is a low pressure switch or a high pressure switch, or for that matter how it even works. As the AC doesn't come on without a charge, I'm guessing this switch is open until a certain minimum low pressure is achieved. I don't know enough about AC to know whether something happens during operation that might open this switch, ie, the pressure dropping due to some function I've yet to understand.

Hmm...

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1981 240D 4sp manual. Ivory White.

The BW diagram says the switch closes when the pressure is above 29 psi. That explains the startup during charging.

Not sure what to say at this point. Maybe you need to tweak the TXV?

Or try shorting over the various switches and see how low the temps go? If you still have your manifold you can check the ranges of pressures you see during operation. I doubt though that you’re dropping below 29 psi at the drier.

Just curious how did you come to two cans of Duracool? My foggy recollection was it takes less than r134a…but this is just from reading other’s stories online. I’ve never tried it.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Had a chat with heffe who’s in a similar boat, and it sounds like there is indeed a thermistor on the manual climate box. In the wiring diagram I believe it’s the obliquely named “temperature control.

On a longer drive today, I watched as the vent temp plummeted to 46 and then shot up to 62, then back to 46 then up to 62 again and again and again.

It’s certainly going to take more digging as I’ve found zero references to the manual airvox thermistor.

__________________
1981 240D 4sp manual. Ivory White.

That could be the climate control doing its job. 46-62 may be the off and on switch points for its control band when you have the temperature wheel set to max. There is going to be some hysteresis designed into the thermostat so it doesn’t turn on and off often - referred to as “chatter” in control speak. It overshoots below the set point before it turns off and then overshoots high before it turns on. Just like a home HVAC thermostat.

Bang Bang control or ON-OFF control as opposed to proportional control on the 300D ACC. It’s more crude but it works okay if looser control is permissible.

Just saying it might be working. The only thing that wouldn’t be right is that the 46 seems a little high. If you’re on MAX the circuit diagram looks like the controller is bypassed and the clutch relay should be on all the time. That’s my interpretation of the potentiometer on the “Temperature Selector” box in the diagram above.

Perhaps the potentiometer has some extra resistance in it from age or dirt on the wiper and it cannot reach 0. The little offset error is making the controller think you don’t want Max cold. Or maybe the thermistor has drifted out of spec over its very long life and thinks it’s colder than it really is.

So yes, there is a thermistor, a potentiometer and a difference amplifier in the system - functionally you dial in a desired temperature represented by a voltage on the potentiometer. It is subtracted from the measured temperature represented by a voltage across a thermistor. If that value is positive it activates the relay and alternatively if it’s negative it deactivates the relay. Exactly like the system in a fridge. There’ll be some overshoot programmed in so it doesn’t act spastic.

H3ffe asked about the thermistor in another thread yesterday but I’m not sure the 240D uses the same one as a 300D. Likely does because 10k NTC type are very common.

This is all speculation of course. The best way to test the limits of the system is just short over the pressure switch with a 8A or up blade fuse. And jump the relay so that it turns the compressor all the time. Then see what the temperature does. This will isolate the problem on the refrigeration system or on the control system.

If it is on the control system then it’s time to find out why the set point is so high when you want max cold. If it’s swinging between 46-62 that would suggest its trying to control to some average temperature of 54F which is too high for max cool. My thinking is on max cool it should hold the clutch relay on all the time - till the cabin turns into a fridge.

A further fly in the ointment is I started looking online for an evaporator temp switch for a 240D and this part keeps coming up…so does one exist and it’s not on the diagram?

I got rid of my 240D back in 2014 but I do recall it would turn on and off the compressor between some wide extremes. It’s a compromise by the engineers not to have the thing go on and off all the time vs. accuracy of control. It did seem todo a good job of keeping the cabin comfortable. I’m wondering what you’re seeing is normal and we are over thinking it. As usual some ground truthing with a thermometer and meter will verify if it is actually operating as designed. Perhaps there is an FSM diagnostic procedure?

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

It has crossed my mind, but there are so few data points... The compressor "chatter" average temp concept totally tracks with me.

If I knew what else to jump (other than low pressure switch), I might be able to circumvent the entire HVAC control. Though with the knowledge and diagram I have, the only point I truly understand is in fact, the low pressure switch. Just to make sure I've got you, you're saying I could add some sort of blade fuse socket to the two low pressure switch wires and take the car for a spin? How long would you guess that'd be safe to run with? I suppose if the vent temps dip too low I could always switch off the blower...

Moving back up the chain, I've previously disassembled the AC dial wheel for cleaning. The one thing I didn't do, however, was polish the circuit path. I think that'll be my next move. While I have the wheel out, I'll see if I can follow the wires to this mystery thermistor.

Now here's an oddity which H3ffe pointed out to me. 240Ds don't ship with an in cabin temp sensor. That little mini grill in the center of the dash? I have one. I've looked at photos of over a dozen 240Ds with AC and I have not seen a single one with that sensor. H3ffe thought the dash may have been pulled at some point and replaced with a 300D dash. That's a reasonable guess, but based on the service history I have (fairly extensive until about 2010, I bought the car in 2015) I think it unlikely. Time to pull the glove box liner and see if the thing is rigged. It's getting interesting here.

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1981 240D 4sp manual. Ivory White.

The thumb wheel is adjustable on many, will lower set temp.

On a manual airbox? Adjustable how exactly?

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1981 240D 4sp manual. Ivory White.

If electronic controlled system, the thumb wheel is a potentiometer, many are adjustable.
By holding the inner shaft and moving the external thumb part. To do so a thin tool can
be made,12mm wide,75mm long with a u shape cut out on one end about 8 mm wide,4mm deep. Slide the tool through the face plate on the right side of the pot ,it will contact the the pot shaft ,hold the shaft by pressing inward ,spin the thumb back a small amount ,retest system, good luck. Have done this many times.

I'm not sure what you mean by "electronic controlled system."

The thumbwheel is indeed a potentiometer and here's a photo.

I assume by "pot shaft" you're referring to the "axle" of the AC wheel?

__________________
1981 240D 4sp manual. Ivory White.

Yes. Run it for a few minutes so you can get a temperature measurement. I don’t think you’ll damage anything. It’s a low pressure switch. It is supposed to save you by shutting down the compressor when you don’t have enough pressure. It’s not a high pressure switch which is supposed to save you from some kind of slugging event that’ll blow up your compressor. The car is already closing it for a few minutes at a time, you just want to go a little further just to see if you can make things colder by keeping it closed. If so then you’ll learn that the pressure is dropping too low for whatever reason…HC refrigerant, low fill. Incidentally I thought two cans was a lot of Duracool but I forgot Duracool comes in 6oz cans.

The pressure switch is in series with the controller. Those are two switches wired in series that interrupt your clutch current. If you short the pressure switch that pulls it out of the equation. From there if it keeps cycling you know it’s your controller that’s doing it. Probably intentionally. The controller switches the current using a relay. In the diagram above it’s labeled AC Relay (Code 12). On the diagram you can jump the relay momentarily to see if you can get your temperatures down. This will possibly allow you to past your wall of 46F. This will tell you if the issue is your controller is not holding the switch closed long enough. Again, a short test. Doing this a long time will likely freeze your evaporator into a block of ice. Once you know your refrigeration system can get to some low desired temperature like 38F at the vent then you can start thinking about how to make the controller trigger the relay appropriately to get you there without any intervention.

Alternatively you can use a test light or voltmeter and check which of the two switches is opening when your clutch is cycling. This way you don’t Jerry rig anything. You just passively observe what is shutting down your compressor and the decide how to go from there.

Sounds good with the wheel. If there is dirt on the contact you’ll add some series resistance. I think you’ve done enough. Cleaning the circuit traces won’t do anything. The thermistor won’t be on that circuit. It’ll be another input to the amplifier electrically. It’ll be likely wired into the board nearby but electrically speaking the thermistor and thumb wheel create two voltages that are subtracted (Reality - desired temp) on the controller electronics. The job of the controller is to minimize this difference.

Now that’s interesting. I am trying to recall where my 240d measured temperature. It was not in the middle of the dash. But there is certainly a thermistor somewhere. It’s in that diagram a few posts ago. It’s the variable resistor labeled Ambient Temperature Sensor. The system has to know when to stop cooling. I just cannot recall where the little grill is. Near the mirror up high? (Or is that my 126?). Too long ago. There is going to be a sensor someplace where the system knows the exact temperature. Look for a little grill. It’ll be placed where the designer wanted a constant temperature - like somewhere on the centerline of the cabin up front. I suspect yours is working though. Otherwise your system would stay on or stay off all the time. It sounds like the control system is just out of calibration. Either the thermistor is reading too high or your dial is indicating low.

@hercules is on to something. I didn’t know there was a calibration in there. Is that something you could have tweaked while cleaning the innards of the knob? From his description it sounds like a way to rotate the knob position relative to the potentiometer. That can definitely cause the offset you are seeing. Maybe the knob is internally set so it thinks you want 54F air instead of Max cool?

This should be checked first before trying any of the jumping stuff I mentioned. It’s screaming to be looked at simply because you went in there and worked on it in the past. What you want to do is find out if it’s adjusted so that the pot has the freedom to dial in the coldest possible temperature.

Since your car was owned by a former HP engineer I’ll share a flow chart we used to throw at each other over there when something stopped working. An HP tradition. .

https://tayloredge.com/bits-n-pieces/humor/problemsolver.pdf

ETA- like usual I spoke too soon about cleaning up the circuit traces under the knob. If there is some dirt on the switch contacts and spring fingers on the knob or board traces under it, this could add a little resistance in series with the potentiometer. On MAX it should just give you a direct short across the potentiometer but some dirt may make it think there’s some resistance in there. It may be worth looking at the circuit traces and wiping them off with a q-tip and alcohol.

Some interesting stuff I dug up. See the round thing next to the green wire? I’m 90% sure it’s the thermistor on a manual HVAC for w123.



Looking up this part number 123 830 30 58 I found this parts list and diagram. Note item 26. I think the thermistor is mounted in the airbox. Just saying because it may be a red herring and you’ll waste hours searching for it. It’s likely good.

https://www.ilcats.ru/mercedes/?function=getParts&class=1&catalog=68K&aggtype=FG&model=123123&group=83&subgroup=080&language=en

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Thanks for all this Carl -it was incredibly helpful. And productive!

Pulled the kick panel today and followed the thermistor wire from the evaporator (you were right) up behind the switch panel. I pulled the AC wheel again for cleaning and found the connection point for the thermistor. Guess I never noticed pulling two harnesses in the past… anyway, dove back into the AC wheel and nothing in there is even remotely customizable. There’s really only one way to assemble the the thing. At the max setting, three copper prongs from the wheel itself complete a circuit on a board in the switch, which I assume is “max cool.”

I first tried unplugging the thermistor harness and quickly found the compressor wouldn’t even fire. Next I took some copper wire and jumped the two naked prongs on the back of the AC wheel. Compressor fired as usual, only this time no cycling.

Measured 37f at the vent. Stable, no cycling.

Hmm, so what to do now. I’d prefer not to bypass the switch… seems unlikely considering the temp I recorded, but it’s still theoretically possible to freeze my evaporator.

Saw there’s a 240 in the local yard. Think I’ll try pulling the thermistor lead and seeing what that’s like. Provided I can pull it, seems reasonable to assume it’s rebuildable with a generic thermistor. I suppose?

Edit: thanks for the flowchart. Think I’ll laminate a copy for the engine bay
Oh, and had a poke at the in-dash cabin temp sensor and found it’s connected to absolutely nothing. It appears someone at some point did in fact replace the dash… strange thing finding that…

__________________
1981 240D 4sp manual. Ivory White.

Still noodling with the evaporator temp module.

Went to the yard today and pulled one from a 240D with another manual airbox.
It unplugs from the box pretty easily. Just a rubber grommet sealing it in there. The cord on the other hand is almost impossible to remove without some disassembly that I simply wasn't up to. The takeaway however, is that the module is not rebuildable... the thermistor is encased in plastic like a laminated popsicle stick. Bummer.

So note to future enthusiasts, you'll likely have to snip the cord on the old one. The new one can be re-routed in a much simpler fashion.

I checked the resistance values of the harness that plugs into the AC wheel. Based on the chart Ykobayashi posted in another thread (on replacing thermistors), I seem to be waaaay out of spec.

At 15C ambient, the resistance should be 15.7 +/- 0.5k ohms.
At 25C ambient, the resistance should be 10.0 +/- 0.5k ohms.

I took my measurements at about 22C ambient and recorded a resistance of 18.5... womp womp. So that seems bad.

I had to disassemble the harness to get the probes on something and found a diode (?). Guess this could also be a fail point but I'm out of my depth Donnie.



In the meantime, jumping the switch at the back of the AC dial is providing some pretty sweet vent temps, I suppose how MAX AC should be operating when it's operating.
The bad news, however, is that it essentially turns the AC wheel into a button: MAX.

__________________
1981 240D 4sp manual. Ivory White.