Hello, recently my 1985 300D developed an issue where the blower will not turn on when it is hot outside, and the temperature dial is set to cold or MIN. It'll run when the dial is in the white or red (heater), but not blue. EC or AC, doesn't matter.

If the fan is set to high, it'll run for a bit, but then turn off after 30-60 seconds. Auto or Low, it doesn't turn on unless the dial is not in the blue.

Blue/Cold temperatures works when it is mild or cold out, but not when it is hot out.

In all circumstances, whether EC or AC, when set to blue or Min, the air is lukewarm, like the a/c isn't engaging.

Heater/hot settings works under all circumstances. Any insight on what could be causing this would be amazing. Thanks.

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1985 300D Turbo Diesel W123 Federal

Problem with the cabin temp sensor up in the dome light housing??? Bad pushbutton control unit?

Does the compressor engage when it is blowing warm air?

The brushes could be worn on the blower motor.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

I'm not sure if the compressor actually turns on. I'll look into this.

I'm planning on taking a look at the blower motor brushes, but could worn brushes be a cause if the CCU and blower works when it's heating, but not when it's hot outside and the temperature dial is set to blue/cold?

I believe the ambient sensor is on my dashboard. It seems that when it's hot out and the sun is directly on the sensor while the car is parked, is when the blower won't turn on, unless the left CCU button is engaged.

The blower will turn on when the fan is set to high, but turns off after 20-30 seconds.

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1985 300D Turbo Diesel W123 Federal

If the blower can be made to run continually in any mode, the problem isn't the brushes.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

You may have a resistor pack going bad. Also, how old is the blower motor? Try oiling it's bearings.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

99 W210 E300 Turbo Diesel, chipped, DPF/Converter Delete. Still needs EGR Delete, 232K

90 Dodge D250 5.9 Cummins/5 speed. 400K

Gone and still missed...1982 w123 300D, 1991 w124 300D

Hi, thanks for the responses.

I checked the temperature sensor and the foam tube behind the glove compartment, and they appear intact and functional.

The defrost fan operates continually at any temperature setting (no cold air though). Over the weekend when it was hot out (85), the same symptoms occurred, the other buttons and high fan setting works, but turns off after 10-15 seconds. Auto/Low doesn't engage the fan. Lukewarm/uncooled air at Min, and heater settings work.

Today with lower exterior temperature (74 deg), everything seems to work except the air isn't cooled - just lukewarm.

I'll check the blower motor and bearings, I believe it's original.

Could it be possible that there isn't enough freon/refrigerant in the system? A/C worked last year, but not after the winter.

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1985 300D Turbo Diesel W123 Federal

Refrigerant level will affect the temperature of the air coming from the ducts, it will not affect the volume of air nor the amount of time or when the cabin fan runs.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Blower fuse has been closely examined or replaced with a known good one? Cracked fuses can be fun to diagnose. It's usually best to replace or swap them to rule them out.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Thank you for the suggestion, I will swap in a new fuse and clean the contacts to rule that out.

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1985 300D Turbo Diesel W123 Federal

Just to followup, the problem was resolved.

The car was very low on refrigerant. Once that was replenished, the A/C and aux fan still wasn't working. Then was figured out that the fan wasn't getting any signal/electricity. Cleaning the sensor contacts and the fan's power cord connector fixed that. Will have to monitor if vibrations disconnects any electrical signals to the fan.

Now the blower works in very hot exterior temps at any setting.

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1985 300D Turbo Diesel W123 Federal

Great news, glad you got it. When I see an older Benz being driven in hot weather with the windows down I know why, that won't be you!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

show us pics! I cant seemto get my aux fan working consistently

Aux fans seem to cause a lot of trouble for how simple they are to trace. There are the two fuses, two relays and the resistor and pressure switch. Really, the second relay is just for engine overheat and is unlikely to be the problem.

First clean the fuses and their spring contacts, these things are the weakest point of every vintage Euro vehicle. These days, I put "Penetrox" on them which removes and prevents oxide formation. It's designed for aluminum wire but works on most contact materials. It's also super messy so only use a dab and twist the fuse ends in it.

If you turn the key ON and short out the pressure switch, the relay should click and the fan should run at low speed.

If it doesn't even click, check the stupid fuses again and then swap it for a known good relay.

If it clicks and doesn't run, check the resistor and its connections. If you have voltage at both ends of the resistor and the connections are good, it's likely the fan itself.

If everything worked in the test but the fan is still unreliable, check how well your spade terminals are holding on to the switch and replace them if they're wiggly. Polish the terminals on the switch and apply Penetrox there as well.

Final note, 12V jumpered directly onto the resistor should run the fan no matter what. If that doesn't work, the fan or the wire/connector to it is toast. (Or its ground connection... nobody ever seems to check that end of it)

Edit: I linked the gasoline diagram but they are all the same

Attached Thumbnails

 

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1993 300D 2.5L Turbo

It's not totally clear what exactly was the culprit for the fan not turning on. With the engine on and very hot out, we were checking the condenser and fan, and while checking how well the fan was attached, it turned on (caution, don't have your fingers inside the fan). So it likely was bad contact, but not exactly sure if it was a sensor, cable, or power supply cable.

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1985 300D Turbo Diesel W123 Federal

Yes, check the foam hose that goes to the dash temp sensor. They turn to powder over time,

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

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