I have been watching my a/c slowly go crazy over the past week in my 1991 300D. I know my compressor is good and the blower fan was recently replaced. All worked well and then a short time later, the whole thing went insane. Here are the symptoms (assume temp dial is always at 75):

1) With a/c selected (middle button) at moderate temps (mid-upper 70s), a/c blows correctly out side vents and lightly in center (I think the center control pod may be bad, but lets hold that thought.) Fan speed seems sufficient and will increase if I lower temp dial.

2) When temps get near 80s, the system will suddenly shut off center vents redirecting to windshield, and blows cool (not cold) air on outside vents and to windshield - feels like cold a/c air has been reduced and there is more outside air. Fan reduces speed. Higher temps exhibit same behavior - fan will not increase speed as temp dial is moved - I can get it to turn on heat thought if I spin dial up high enough (do not try this with family in car on 90 degree day )

3) If I choose the switch just left of the middle one, no real change from symptom 2 above. Seems like middle button is acting like one to its left, but only during temps of 80+

Here is why I think my climate control has gone mad (it works on economy):

3) If I choose the economy switch (just right of center) with moderate to hot temps outside (anything above mid 70s) the fan kicks in high with ridiculous amount of air on side vents, windshield vents are shut off and there is some air movement coming from middle vent, but nowhere near the volume at the sides. Apart from a possible "middle vent pod" issue, this scenario appears to have everything functioning correctly (AFAIK.) Fan speed will adjust as temperature dial is moved (fan increases as dial temp moves away from current temperature.)

4) Generally, high fan switch doesn't produce the fastest fan speed - it is decent but not like the wind turbine effect when my economy switch is pressed and its in a hurry to catch up.

Based on this, does a faulty climate control or something other than "just pods" seem reasonable? Any ideas? Is there any other "control" for the system apart from the in-dash unit with the buttons?

I pulled out the climate control module, disassembled it, cleaned what I could and put it back in - no real change. Did notice some dark discoloration below a transistor on the circuit board (kind of a crusty, brown coating) - I am thinking the transistor might have cooked itself.

I am just wondering if I should pony up for the control module before hunting down the pods and the possible overhaul with the dash removal, etc. I haven't tested the pods with a vacuum pump where the 7? lines meet near the glove box - will do that in near future. Trying to decide on control module at the moment.

Thanks,
ThirstyValves

Sure sounds like a goofy PBU. See if there's a forum member near you with a known good unit you can borrow for testing.

Sixto
87 300D

What is a "PBU" ? oh wait, google knows all "Push Button Unit"

Anyone near Charleston, SC with a good PBU (like I know what I am talking about) for a 91 300D?

Push Button Unit.

Putting your location and vehicle in your sig helps forum members to help you.

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Gone to the dark side

- Jeff

just updated, thanks - I'm a newb - figuring it out.

At times I wish I were in Charleston, ... but I'm not.

Unfortunately, it does sould like the PBU (aka: CCU), and the easiest method to find out, is a swap as 62 has mentioned.

__________________

Gone to the dark side

- Jeff

Whats the chance one still in a junk yard is any good? About the same that mine is

I am sure one from another 91-93 300d will work? Will ones from any other benz work? is there a compatibility list somewhere? I am probably asking for too much...

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91 300D 2.5 Turbo, 240K

I have a nice clean type III I pulled from a 91 SD, and I have a 93 parts car, and I will shortly have a 91E parts car, so I'm pretty sure I can supply one to you. also I'll be headed to the beach next weekend, so I'll be about 75miles from you if you wanna meet somewhere.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

It sounds like a goofy PBU, and they certainly do that. But it also sounds like the control unit thinks the temperature in the car is lower than what it actually is. It's been 11 years since I owned a 124, so I'm asking - is it possible that you have an unwanted path from the cold air duct to the temperature sensor?

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References

Always a good idea to vacuum the temp sensor next to the sunroof switch.

__________________

Gone to the dark side

- Jeff

Seriously? I hadn't heard this before, but I guess it makes sense...

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83 300TD (need rear wiper assembly dead or alive)
84 300SD Daily driver
85 300TD almost 400k miles and driven daily.
98 E300D *sold
86 300SDL *sold and made flawless 10 hour journey to new home.

compatibility:
(Looks like it fit lots of W124 s)

Attached Thumbnails

 

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'84 300SD sold
124.128

vstech,

I'd be glad to meet up with you. I'll pm you and we can figure out a plan. Thanks for the help.

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91 300D 2.5 Turbo, 240K

Do the sensors themselves go bad? Can't be too complicated of a device, but one has to ask, should I just replace it to be safe?

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91 300D 2.5 Turbo, 240K