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300TD - Problem with dreaded Servo based Automatic Climate Control System

Hello All,

I have a strange problem with my 1980 300TD ACC with the Servo based system.

First of all, all the major components have been changed (Servo, amp, etc). Servo runs fine through hot/cold, holds temp well, etc.

The problem is the fan speeds. When AC Compressor switch is set to "Off", the fan runs through its settings normally. When the AC is set on, the fan SHOULD significantly increase is speed (to cool the cabin quickly), but on mine it never does. The fan runs at the same speeds (call it "low"), thus it never really cools the car in the summer time. I have changed the blower as well, but nothing. I know there is a resistor bank under the wipers (that I have tested with another unit) and still the same problem.

Perhaps there is there a vacuum switch that need to activate the higher fan speed that I am missing???

Please help! Thanks!!!!

Reply to
w123benzman
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Who says the fan speed should differ between heat and cool? It doesn't.

Fan speed is controlled by the servo based on the temperature difference that needs to be closed i.e. 100 degrees vs. 70 degrees will generate high fan speed but 75 degrees vs. 73 degrees will have a low fan speed. The servo is questionable if your system doesn't do so.

Too bad you replaced all that old junk when a digital retrofit kit eliminates this mickey mouse OEM system. see:

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I installed this retrofit in my '80 300SD and it works well. It comes with a 5 year warranty.

Reply to
T.G. Lambach

Actually, it does. In cooling, the blower has a max speed that is one setting higher than in heat mode. But, I think what he's saying is that the fan doesn't get anywhere near fast enough to work as it should in cooling mode in summer when the car is hot.

This does sound like strange behavior, because the servo controls the blower speeds based on the temp difference. And he says the blower does go to high speed if the AC compressor switch is off, which indicates the servo must be going to the max cooling position. AFAIK, the servo position, which determines the blower speed is only based on temp diff between desired setting and actual and ambient, so, it seems unlikely it's the servo system that is at fault.

What happens if you have a hot cabin, the AC comp switch off, and then move the AC comp switch to ON? Does the blower speed immediately decrease or does it then stay high?

Does air come out the proper vents? With a cold cabin and set to 85, with engine warm, hot air should come out the floor vents, side dash vents by windows, and some bleed air by defroster vents, with blower on speed 4. If you then let the cabin get hot and then move the setting to 65, the blower should gradually slow down, then start to speed up, until it reachs max speed 5. The air should change over, with the floor vents closing, and air now coming from the dash center and side vents, with bleed air at the defroster vents.

To diagnose this, I'd get a schematic and see exactly what all the AC compressor switch is connected to.

Reply to
trader4

Hello all,

Actually, I have noticed on my other w123's with servo systems the fan speeds are the same if I am running heat, or "vent" cool air. BUT, when the AC Compressor switch is set to ON, I SHOULD see a significant increase in blower speed (to cool the cabin quickly) This does happen in my other 2 w123's, but not my wagon. I even tried ANOTHER servo, but still the same. Perhaps there is a fault in the internal wiring that someone "FIXED" at one point, so the blower motor current "path" is being sent to the same resistors no matter the switch position.

Thoughts?

snipped-for-privacy@> > Who says the fan speed should differ between heat and cool? It doesn't. >

Reply to
w123benzman

Hello all,

Actually, I have noticed on my other w123's with servo systems the fan speeds are the same if I am running heat, or "vent" cool air. BUT, when the AC Compressor switch is set to ON, I SHOULD see a significant increase in blower speed (to cool the cabin quickly) This does happen in my other 2 w123's, but not my wagon. I even tried ANOTHER servo, but still the same. Perhaps there is a fault in the internal wiring that someone "FIXED" at one point, so the blower motor current "path" is being sent to the same resistors no matter the switch position.

Thoughts?

snipped-for-privacy@> > Who says the fan speed should differ between heat and cool? It doesn't. >

Reply to
w123benzman

Not clear what it's doing, vs what you expect, vs what it should do. The blower speed is determined by the difference between the set temp and the actual temp. If there is a small diff, the blower is on speed

1, the slowest setting. With a large temp diff, the blower should go to speed 4 in heat mode and 5 in cooling mode.

If the cabin is hot and the temp is set to 65, the blower should be running on 5, the highest speed, whether the AC compressor is set to on or not. Which only makes sense, since it's trying to achieve a big temp drop as quickly as possible, whether using only outside air or the compressor.

Reply to
trader4

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